JPP invites you to join us for the JPP Frontiers of Plasma Physics Colloquium
Organisers: Cary Forest and Alex Schekochihin
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For details of upcoming talks, please see here
Speaker: Young Dae Yoon, Asia Pacific Center for Theoretical Physics - Chaired by: Nuno Loureiro, Editor, JPP
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Date/Time: Thursday 7th August 2025, 4PM BST/11AM EDT
Title: Interaction between coherent whistler waves and energetic particles in the magnetosphere and its exploitation for runaway electron suppression in tokamaks
Abstract: Recent spacecraft observations indicate that whistler mode chorus subelements are extremely coherent, i.e., are monochromatic and phase-aligned. In such cases, popular diffusion mechanisms such as the quasi-linear mechanism that requires a broadband wave spectrum are invalid, and coherent wave-particle interactions must be examined. It is shown that the vector relativistic equation of motion of a particle under a circularly polarized electromagnetic wave can be reduced to a scalar conservative equation of motion of a resonance mismatch parameter. This equation of motion involves a conserved pseudo-energy in a pseudo-potential, whose shape if double-welled can lead to a rapid, extreme scattering, which is often observed in the magnetosphere. This mechanism can then be leveraged to construct a "momentum firewall" for hazardous runaway electrons in tokamaks, wherein the accelerating electrons are prevented from gaining parallel momentum upon hitting the firewall and are instantaneously scattered in momentum space. This method is further verified by self-consistent particle-in-cell simulations.
Speaker: Alexandra Dudkovskaia, York Plasma Institute, School of Physics, Engineering and Technology, University of York, Heslington, York, YO10 5DD, UK.
Chaired by: Nuno Loureiro, Editor, JPP
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Date/Time: Thursday 24th July 2025, 4PM BST/11AM EDT
Title: Dynamic mode decomposition for gyrokinetic eigenmode analysis
Abstract: Plasma confinement in tokamaks is generally degraded by turbulence. This turbulence is associated with drift instabilities with short wavelengths across the magnetic field lines (typically comparable to Larmor radii). Various branches of such drift instabilities often coexist, which significantly complicates microstability analysis of electromagnetic fusion plasmas, particularly, in relevance to scenario predictions in the presence of energetic particles and impurities. Drift instabilities are typically described by gyrokinetic theory and simulations. Gyrokinetic initial value solvers, while being well-suited to identifying dominant branches of drift wave eigenmodes, are not able to detect subdominant or stable branches (including branches only slightly less unstable than a dominant instability). To resolve subdominant modes, in the past, two types of gyrokinetic eigenvalue solvers have been developed: computationally expensive distribution function eigenvalue solvers and more efficient numerically gyrokinetic Maxwell dispersion relation solvers. While the latter can allow one to explore various gyrokinetic models and/or drift instabilities (e.g. thresholds of kinetic ballooning modes, closely spaced hybrid eigenmodes, global effects etc.), it is mathematically challenging to retain the majority of these effects simultaneously within one eigensolver. In this talk, we will present a different approach to compute dominant, subdominant and stable gyrokinetic eigenmodes in fusion plasmas without setting any restrictions on the plasma shape, beta (ratio of the plasma pressure to the magnetic field pressure), collisionality, number and type of plasma species. The approach is based on a dimensionality reduction algorithm called dynamic mode decomposition (DMD). DMD is a data-driven technique for performing spectral analysis of time series data via constructing a lower dimensional, linear dynamical model. The linear model is characterised by a set of spatial and temporal modes, to be obtained from eigenvalues and eigenvectors of a linear operator. In this talk, DMD is applied to the gyrokinetic-Maxwell system of the CGYRO gyrokinetic code via a newly developed CGYRO-DMD post-processor. The approach is numerically efficient and adds no cost to a typical gyrokinetic initial value problem. The CGYRO-DMD performance is illustrated for conventional, core tokamak plasmas; spherical tokamak (ST) like plasmas and for the high confinement, H-mode pedestal plasmas.
Speaker: Richard Fitzpatrick, University of Texas, Austin - Chaired by: Nuno Loureiro, Editor, JPP
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Date/Time: Thursday 17th July 2025, 4PM BST/11AM EDT
Title: Investigation of Neoclassical Tearing Mode Detection by ECE Radiometry in Tokamak Reactors via Asymptotic Matching Techniques
Abstract: Neoclassical tearing modes (NTMs) are the leading cause of disruptions in conventional fusion-relevant tokamak discharges. NTMs can be stabilized by driving a toroidal current that is localized at the magnetic flux-surface in the plasma at which the mode reconnects magnetic flux by means of externally injected electron cyclotron waves. The keys to the success of this method are the early detection of the NTM, and an accurate determination of the location of the reconnecting surface. The most promising method of mode detection and accurate location of the reconnecting surfaces are by means of electron cyclotron emission (ECE) from the plasma. Existing calculations of the ECE signals likely to be produced by an NTM are surprisingly crude. This talk will describe how an asymptotic matching code can be used to produce a much more realistic synthetic ECE signal. The code works by splitting the plasma into two regions. In the outer region, the NTM is basically an ideal mode that displaces magnetic flux-surfaces without changing their topology. In the inner region, which is localized in the vicinity of the reconnecting surface, the NTM generates a radially asymmetric magnetic island chain. The solutions in the inner and outer regions must be asymptotically matched to one another to produce a global solution.
Speaker: Sam Frank, Realta Fusion, USA - Chaired by: Cary Forest, Associate Editor, JPP
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Date/Time: Thursday 10th July 2025, 4PM BST/11AM EDT
Title: Advancing the Magnetic Mirror with Theory and Integrated Modeling at Realta Fusion
Abstract: The magnetic mirror is a simple, open-ended, fusion concept that could potentially form the basis of an attractive fusion power plant. While the magnetic mirror saw substantial development throughout the 20th century, development of the concept slowed significantly after the 1980s. Realta Fusion is a private fusion company working to commercialize the mirror and was spun out of the Wisconsin High-Temperature-Superconducting (HTS) Axisymmetric Mirror (WHAM) project at the University of Wisconsin-Madison. Realta Fusion is developing the axisymmetric magnetic mirror concept with HTS magnets, new stability control mechanisms, and advanced computing. In this talk, we will discuss the theory and modelling activities underway at Realta Fusion. This will include integrated heating, transport, and equilibrium modeling in support of the WHAM experiment and for the design of the next step device Anvil as well as confinement performance predictions for the fusion pilot plant concept Hammir.
Speaker: Jason Parisi, Princeton Plasma Physics Laboratory, USA - Chaired by: Troy Carter, Associate Editor, JPP
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Date/Time: Thursday 3rd July 2025, 4PM BST/11AM EDT
Title: Turbulent Transport-Limited Pedestals in Tokamaks
Abstract: H-mode operation of tokamak fusion plasmas free of dangerous Type 1 edge-localized-modes (ELMs) requires a non-ELM mechanism for saturating the edge pedestal growth. One possible mechanism is turbulent transport. We introduce a transport threshold model to find pedestal width-height scalings for turbulent transport-limited pedestals. The model is applied to electron heat transport resulting from electron-temperature-gradient (ETG) turbulence. The width-height scalings are highly sensitive to the relative contribution of density and temperature to the pedestal pressure. Pressure that builds up mainly through temperature is more likely to be transport-limited, and hence ELM-free. A relative radial inward shift of the temperature to density pedestal location is also more likely to transport-limit the pedestal. A second constraint such as flow shear is required to saturate pedestal growth. We also calculate width-height transport scalings resulting from particle and heat transport arising from ETG and kinetic-ballooning-mode turbulence. Comparisons are performed for ELMy and ELM-free experiments in MAST-U, NSTX, and DIII-D. This is a first step towards a pedestal width-height scaling for transport-limited ELM-free pedestals.
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Speaker: Stepan Bulanov, Lawrence Berkeley National Laboratory, USA - Chaired by: Louise Willingale, Associate Editor, JPP
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Date/Time: Thursday 26th June 2025, 4PM BST/11AM EDT
Title: Laser-plasma-based sources of positron beams
Abstract: It is widely accepted that the next lepton collider beyond a Higgs factory would require center-of-mass energy of the order of up to 15 TeV. Plasma based acceleration is considered a promising concept for the next generation of linear electron-positron colliders. Despite the great progress achieved over the last twenty years in laser technology, laser- and beam-driven particle acceleration, and special target availability, positron acceleration remains significantly underdeveloped if compared to electron acceleration. This is due to both the specifics of the plasma-based acceleration, and the lack of adequate positron sources tailored for the subsequent plasma based acceleration. Here several designs for laser plasma based positron sources are discussed. First, a set of compact, two-stage plasma-based positron sources is presented. In the first stage the positrons are created by a multi GeV electron beam produced by a laser-plasma accelerator interacting with a solid density foil. In the second stage the positrons are captured and accelerated in a plasma wave driven by either an electron beam or a laser pulse. Second, a positron source based on the collision of a high energy electron beam with a high intensity laser pulse is proposed. The source relies on the subsequent multi-photon Compton and Breit-Wheeleer processes to generate an electron-positron pair out of a high energy photon emitted by an electron. The remarkable property of this source is that low divergence positron beams can be generated by employing frequency doubled and quadrupled laser pulses. The resulting low emittance in the sub-micron range potentially makes such source interesting for collider applications.
Speaker: Anton Sudnikov, Budker Institute of Nuclear Physics, Russia - Chaired by: Cary Forest, Associate Editor, JPP
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Date/Time: Thursday 19th June 2025, 4PM BST/11AM EDT
Title: Open magnetic trap with a helically symmetric multiple-mirror plugging
Abstract: Magnetic mirrors for plasma confinement are being considered as a possible way for fusion again. One of the central questions in mirror research is how to suppress particle and energy losses along the magnetic field lines. There are several possible solutions of this problem. Here, we discuss one specific approach to multiple-mirror confinement. The multiple-mirror confinement reduces axial losses due to the momentum transfer between the periodic magnetic field and the trapped ions. Moving multiple mirrors improve the confinement; this configuration can be achieved by the plasma rotation in the helically corrugated magnetic field. The concept of the helical mirror has been experimentally validated in a laboratory-scale experiment SMOLA at the Budker INP. Direct evidence of the helical confinement has been obtained, with the plasma density in the confinement region increasing more than threefold under optimal experimental conditions compared to the solenoidal magnetic configuration. Remarkably, enhanced confinement persists even at low densities where the ion mean free path relative to binary collisions exceeds the device length, and the multiple-mirror confinement is possible only in case of anomalous scattering. In this talk, we summarize the experimental findings and discuss the possibility to bridge the gap between the laboratory-scale device and fusion-grade ones.
Speaker: Andreas Döpp, LMU - Chaired by: Luís O. Silva, Associate Editor, JPP
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Date/Time: Thursday 12th June 2025, 4PM BST/11AM EDT
Title: Bayesian approaches to measurement and optimization
Abstract: This work explores the application of Bayesian methods to enhance measurement and optimization in experimental physics, with a focus on laser-plasma interactions. Bayesian updates enable the integration of prior knowledge with new data, facilitating refined parameter estimation and uncertainty quantification. These methods have been employed to achieve the first single-shot measurement of complete spatio-temporal vector fields, providing a comprehensive characterization of petawatt laser pulses. Additionally, Bayesian Autocorrelation Spectroscopy (BAS) is introduced as an innovative technique for spectral measurements, leveraging prior information for rapid convergence. Finally, Bayesian optimization demonstrates exceptional efficiency in tuning laser wakefield accelerators, enabling precise control over electron beam properties through systematic exploration of parameter space.
Speaker: Karl Zeil, HZDR, Germany - Chaired by: Antoine Bret, Associate Editor, JPP
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Date/Time: Thursday 5th June 2025, 4PM BST/11AM EDT
Title: Laser-driven proton acceleration experiments at HZDR: Relativistically induced transparency for high-energy beams and applications
Abstract: Ion acceleration via compact laser-plasma sources holds great potential for applications from radiation therapy research to fusion research. Achieving the desired beam quality requires a deep understanding and precise control of laser-plasma interactions. Our collaborative research at the DRACO PW (HZDR) and J-KAREN-P (KPSI) laser systems investigates the promising regime of Relativistically Induced Transparency (RIT). Previous studies achieved high-performance proton beams (>60 MeV) in an expanded foil configuration, identifying an optimum at target transparency onset. Later experiments recorded proton energies over 100 MeV, emphasizing transparency onset timing in optimizing beam parameters. Using particle and laser diagnostics, we explore the correlation between transparency onset and acceleration performance. This contribution details our recent investigations into spectral components of transmission and emission from laser-plasma interactions. Building on established methodologies, we apply spectral interferometry, using the unperturbed laser beam as a reference, and correlate findings with proton acceleration. Our results suggest a promising direction for analyzing spectral and spatial distribution, offering deeper insights into laser-plasma interactions and optimizing beam quality parameters necessary for applications like secondary neutron generation or radiobiology experiments .
Speaker: Sara Vaz Mendes, IPP, Greifswald - Chaired by: Peter Catto, Associate Editor, JPP
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Date/Time: Thursday 29th May 2025, 4PM BST/11AM EDT
Title: ITG-turbulence-driven Alfvénic modes in the Wendelstein 7-X stellarator
Abstract: Throughout several operational phases (2018–2025) of Wendelstein 7-X (W7-X), an optimized stellarator in Greifswald, Germany, magnetic and density fluctuation measurements show that shear Alfvén wave (SAW) excitation correlates with the broadband turbulence spectrum. The typical role of SAWs in magnetic confinement fusion (MCF) research relates to their excitation by energetic particles. In recent years, the interaction between SAWs and turbulence has revealed important numerical results in the presence of energetic particles. In W7-X, despite the absence of conventional energetic particle sources, SAWs persist in most experiments. The measured Alfvénic fluctuations resemble observations in ohmic tokamak scenarios, such as those at TFTR, ASDEX Upgrade, and TCV. Across various machines, common characteristics are evident for Alfvénic fluctuations correlated with the turbulence spectrum, in contrast to those driven by energetic particles. Alfvén eigenmodes (AEs) detected through Mirnov coil measurements are consistent with ellipticity, toroidicity, and non-circularity induced AEs. Phase Contrast Imaging (PCI) measurements indicate dominant ion-temperature-gradient (ITG) driven turbulence in these plasmas. We observe a correlation between the amplitudes of AEs and density fluctuations across different magnetic field configurations. Nonlinear gyrokinetic simulations using the EUTERPE code show that ITG turbulence can simultaneously drive zonal flows and generate AEs.
Speaker: Ida Ekmark, Chalmers University of Technology, Sweden - Chaired by: Tunde Fulop, Associate Editor, JPP
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Date/Time: Thursday 22nd May 2025, 4PM BST/11AM EDT
Title: Optimization of runaway electron mitigation by massive material injection in ITER and SPARC
Abstract: During tokamak disruptions, strong electric fields can arise which are sufficient to cause electron runaway, whereby electrons are accelerated continuously. In future large-current tokamaks, such as ITER and SPARC, significant runaway electron generation is expected, due to the runaway generation being exponentially sensitive to the pre-disruption plasma current. Should the beam of runaway electrons come in contact with the tokamak wall, its energy can be almost instantly deposited deep into the wall. During disruptions, high heat loads can also arise from heat being transported into the wall when the magnetic surfaces are broken up during the thermal quench. Additionally, if the current decay is too fast or too slow, electromechanical forces caused by eddy or halo currents can cause forces and torques on the tokamak structure. While all of these unwanted aspects of a disruption can individually be addressed by massive material injection, they pose conflicting requirements on the injected material quantity and composition. In this talk, we investigate disruptions mitigated with combined deuterium and noble gas injection in ITER and SPARC. We use multi-objective Bayesian optimization of the densities of the injected material, taking into account limits on the maximum runaway current, the transported fraction of the heat loss, and the current quench time. Regions in the injected material density space corresponding to successful mitigation are found for both machines when optimizing pure deuterium plasma scenarios. When optimizing deuterium-tritium plasma scenarios, on the other hand, simultaneous mitigation of runaway current, transported heat loss and electromechanical forces appear more challenging for ITER than for SPARC.
Speaker: Eduardo Rodríguez, Max Planck Institute for Plasma Physics, Greifswald - Chaired by: Per Helander, Associate Editor, JPP
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Date/Time: Thursday 15th May 2025, 4PM BST/11AM EDT
Title: Shaping the Ion-Temperature-Gradient driven instability: a kinetic view on its localisation
Abstract: In a stellarator, the toroidal magnetic field can be shaped with a high degree of freedom, significantly influencing the behaviour of the confined plasma. Different aspects of plasma dynamics respond uniquely to these geometric variations, requiring a careful understanding of the underlying physics—often in regimes far from the well-studied tokamak case. In this talk, we examine how ion-temperature-gradient (ITG) driven instabilities respond to changes in magnetic geometry. Specifically, we analyse the linear kinetic response of ions, emphasizing the crucial role of localisation along the field line. We present a simple semi-analytical formula for the mode spectrum that captures key physical effects: long-wavelength Landau damping, ion-scale Larmor radius stabilisation, the weakening of Larmor radius effects at short wavelengths, and stabilisation due to magnetic-drift resonance. This model provides a clear qualitative framework for understanding typical stellarator linear gyrokinetic spectra, including the emergence of multiple maxima or transitions to extended modes, while acknowledging its limitations for precise quantitative predictions. These insights also inform a broader discussion on the trade-offs between ITG-driven turbulence and other phenomena such as MHD stability, offering new perspectives on stellarator optimisation.
Speaker: Georgia Acton, Oxford University, UK - Chaired by: Peter Catto, Associate Editor, JPP
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Date/Time: Thursday 8th May 2025, 4PM BST/11AM EDT
Title: Optimisation of Gyrokinetic Microstability Using Adjoint Methods
Abstract: Microinstabilities drive turbulent fluctuations in inhomogeneous, magnetized plasmas. In the context of magnetic confinement fusion devices, this leads to an enhanced transport of particles, momentum, and energy, thereby degrading confinement. In this work, we elaborate on the application of the adjoint method to efficiently determine the variation of linear growth rates for plasma microstabilities concerning a general set of external parameters within the local δf-gyrokinetic model. We then offer numerical verification of this approach. When coupled with gradient-based techniques, this methodology can facilitate the optimization process for the microstability of the confined plasmas across a high-dimensional parameter space. We present a numerical demonstration wherein the ion-temperature gradient (ITG) instability growth rate in a tokamak plasma is minimized with respect to flux surface shaping parameters. The adjoint method approach demonstrates a significant computational speed-up compared to a finite-difference gradient calculation.
Speaker: Simon Hooker, University of Oxford, UK - Chaired by: Tunde Fulop, Associate Editor, JPP
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Date/Time: Thursday 1st May 2025, 4PM BST/11AM EDT
Title: Progress towards high-repetition-rate laser-plasma accelerators
Abstract: In a laser-plasma accelerator (LPA) the ponderomotive force of a short (~ 50 fs), intense (~1018 W / cm2) laser pulse drives a trailing plasma wave. The spatial variation of the electron density within this wave generates electric fields of order 100 GV / m, around three orders of magnitude larger than those used in conventional, radio-frequency accelerators. This electric field structure, the plasma wakefield, propagates at the speed of the laser, and hence can be used to accelerate particles to relativistic energies. Laser-plasma accelerators have generated electron bunches with energies as high as ~ 10 GeV, in accelerator stages only a few centimetres long. Their compact size, and the desirable properties of the bunches they generate – such as low emittance and femtosecond duration – would seem to make laser-driven accelerators ideally suited to applications such as driving compact light sources or enabling experiments in novel high-field science. However, many of these applications would require the LPA to operate at pulse repetition rates substantially higher than the few pulses per second typically achieved today. In this colloquium I will briefly review the operation of laser-plasma accelerators and the state of the field, before discussing recent work by my group aimed at enabling GeV-scale LPAs to operate at kilohertz repetition rates: (i) the development of hydrodynamic optical-field-ionized (HOFI) plasma channels, and (ii) plasma-modulated plasma accelerators (P-MoPAs).
Speaker: Peter Bruggeman, University of Minnesota, USA - Chaired by: Ed Thomas, Associate Editor, JPP
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Date/Time: Thursday 17th April 2025, 4PM BST/11AM EDT
Title: Plasma-surface interactions in non-equilibrium plasmas: a new scientific frontier.
Abstract: Non-equilibrium low temperature plasmas are an abundant source of reactive species at ambient temperatures and pressures. This unique capability enables several emerging applications including health, materials synthesis and energy security. All these applications are governed by interactions of non-equilibrium plasmas with complex interfaces. The strong nonlinear coupling between the plasma and the complex interface leads to complex interfacial interactions which remain not well understood and have a major impact on the plasma properties, impinging species fluxes and can even lead to self-organization. Atmospheric pressure sheaths and boundary layers have length scales of 100 μm or less leading to significant diagnostics challenges. These plasmas are also prone to instabilities introducing high spatial and temporal variations in plasma properties and dynamics. This presentation will highlight three important case studies of plasma interactions with complex interfaces: nanoparticles, viruses and plasma-liquid interactions. Insights in these areas rely on quantitative studies involving modeling and in situ diagnostics that allow identifying mechanisms.
Speaker: Marco Velli, UCLA, USA - Chaired by: Francesco Califano, Associate Editor, JPP
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Date/Time: Thursday 10th April 2025, 4PM BST/11AM EDT
Title: Parker Solar Probe in the inner heliosphere: insights and problems.
Abstract: The solar magnetic field plays a dual role in the generation of the Heliosphere. On the one hand it creates the corona by storing and transmitting, via a Poynting flux crossing the photosphere and transition region, the energy provided by the dynamo and convective motions; on the other hand, it provides the confinement, or magnetic cage, through which the heated coronal plasma must break through - everywhere except solar polar regions around solar minimum - to produce the supersonic solar wind. Solar wind streams are characterized by different plasma properties, from overall speed, to temperature and temperature anisotropies, composition, as well as different properties for the embedded turbulence. While it has been established that the source of fast solar wind streams at solar minimum are the polar coronal holes, slower solar wind streams have contributions from different sources. The larger than expected filling factor of slow solar wind has been attributed to flows coming from coronal hole boundaries, i.e., regions with large expansion factors, or from the complex mapping of the magnetic field from the photosphere into the heliosphere. The observation by Parker Solar Probe that much of the solar wind, independently of speed, is dominated by Alfvénic fluctuations, and the frequent observation of slow Alfvénic solar wind, previously observed relatively rarely in Helios and Wind data, provide evidence for a modified picture of solar wind origins. I will discuss the properties, origin and acceleration of different types of solar wind streams as observed by Parker Solar Probe in the inner heliosphere.
Speaker: Francois Waelbroek, IFS, University of Texas, Austin - Chaired by: Peter Catto, Associate Editor, JPP
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Date/Time: Thursday 3rd April 2025, 4PM BST/11AM EDT
Title: Magnetic islands and wake fields in fusion reactors
Abstract: Leading magnetic fusion strategies aim to create configurations where the magnetic field winds around simply-nested flux surfaces enclosing a single, closed field line called the magnetic axis. In practice such configurations unavoidably contain a multitude of additional closed magnetic field lines that each want to act as the magnetic axis of their own flux tubes. The resulting subsidiary flux tubes, called magnetic islands, impair the confinement properties and are generally undesirable except at the edge of the volume where a single island chain can be used to tailor the exhaust of heat and particles. Controlling magnetic islands requires an understanding of the drift-acoustic wakes excited by their interaction with diamagnetic flows. These wakes affect the island by radiating momentum, thereby exerting an azimuthal force on the island and inducing a radial electric field and zonal flows. They also induce a polarization current that can cause the island to either grow or heal depending on the velocity of the plasma with respect to the island.
Speaker: Chris Hegna, Type One Energy, USA - Chaired by: Alex Schekochihin , Editor, JPP
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Date/Time: Thursday 27th March 2025, 3pm GMT/11am EDT
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Title: The Infinity Two Stellarator Fusion Pilot Plant baseline plasma physics design
Abstract: In this set of talks, we provide an assessment of the Infinity Two Fusion Pilot Plant (FPP) baseline plasma physics design. Infinity Two is a four-field period, aspect ratio A = 10, quasi-isodynamic stellarator with improved confinement appealing to a max-J approach, elevated plasma density and high magnetic field ( = 9 T). At the envisioned operating point (800 MW DT fusion), the configuration has robust magnetic surfaces based on MHD equilibrium calculations and is stable to both local and global MHD instabilities. The configuration has excellent confinement properties with small neoclassical transport and low bootstrap current (Ibootstrap ~ 2 kA). Calculations of collisional alpha particle confinement show small energy losses to the first wall (< 1.5%) and stable energetic particle/Alfven eigenmodes at high density. Low turbulent transport is produced using a combination of density profile control consistent with pellet fueling and reduced stiffness to turbulent transport via three-dimensional shaping. Transport simulations with the T3D-GX-SFINCS code suite with self-consistent turbulent and neoclassical transport predict that the Pfusion = 800 MW operating point is attainable with high fusion gain (Q = 40) at volume averaged electron densities of
Speaker: Louis Richard, Swedish Institute of Space Physics, Uppsala U - Chaired by: Thierry Passot, Associate Editor, JPP
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Date/Time: Thursday 20th March 2025, 3pm GMT/11am EDT
Title: Particle Energization in Magnetic Reconnection Jets
Abstract: Magnetic reconnection is a fundamental plasma process that converts electromagnetic energy into particle acceleration and bulk plasma motion, driving some of the most energetic events in the Universe. In this study, we investigate how reconnection outflows transfer energy to the surrounding plasma using data from NASA’s Magnetospheric Multiscale (MMS) spacecraft in Earth’s magnetotail. Our analysis shows that turbulence generated within the reconnection outflow plays a key role in energy dissipation. Specifically, thermal ions are rapidly scattered and heated due to their interaction with strongly curved magnetic fields, while the convective electric field further accelerates higher-energy ions. Additionally, we find that electrons are efficiently heated by a magnetic field-aligned electric field, which arises to maintain charge neutrality. These findings provide new insights into energy dissipation and particle energization mechanisms in magnetic reconnection, improving our understanding of plasma dynamics in space and astrophysical environments.
Speaker: Arnas Volcokas, EPFL - Chaired by: Alex Schekochihin, Editor, JPP
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Date/Time: Thursday 13th March 2025, 3pm GMT/11am EDT
Title: Turbulence-Modified Safety Factor Profile in Tokamaks with Low Magnetic Shear
Abstract: In this work, we investigate the impact of electromagnetic effects on plasma turbulence self-organization at low magnetic shear using nonlinear gyrokinetics simulations. Previous electrostatic studies showed that turbulent eddies can extend along magnetic field lines for hundreds of poloidal turns when the magnetic shear $\hat{s}$ is low or zero. Under these conditions, parallel self-interaction induces strong corrugations in plasma profiles at multiple low-order rational surfaces, including the formation of stationary current layers. When magnetic shear is low and electromagnetic effects are considered, these turbulence-generated currents locally flatten the safety factor profile to form staircase structures. This feedback mechanism between turbulence and the imposed safety factor profile results in a significant reduction of turbulent transport. To further explore this interaction, we employed a novel extension of the flux tube model, allowing simulations of non-uniform magnetic shear profiles, including $q$ profiles with reversed magnetic shear relevant to Internal Transport Barrier (ITB) formation. Our findings indicate that turbulence-generated current layers “pull” the safety factor profile toward rational values, thereby broadening the safety factor profile minimum and enhancing the turbulent stabilization. We believe these results are key in ITB formation, can inform long-standing experimental observations, and are important for the design of low magnetic shear fusion devices.
Speaker: Pablo Rodriguez-Fernandez, MIT, USA - Chaired by: Eleonora Viezzer , Associate Editor, JPP
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Date/Time: Thursday 06th March 2025, 4PM GMT/11AM EST
Title: Core transport predictions of SPARC tokamak plasmas with flux-matched nonlinear gyrokinetic simulations
Abstract: Predicting core gradients in tokamak plasmas using first-principles based models has long been the goal of transport research, but it has largely been intractable due to the large computational expense of nonlinear turbulent simulations. Given this high computational cost, the tokamak transport community has relied on a set of tools that span from empirical methods to quasilinear models of turbulence to scope and study future devices, while only using full nonlinear gyrokinetic simulations as standalone, spot-check validation studies. This talk will present the PORTALS workflow that reduces the computational cost of first-principles, multi-channel, non-linear gyrokinetic profile predictions of the plasma core. Thanks to this, the core of SPARC—a burning-plasma device under construction by Commonwealth Fusion Systems— has been extensively studied and predicted with nonlinear CGYRO. This talk will introduce the need of flux-matched simulations to enable profile predictions, will present the fundamentals of the PORTALS technique and will discuss the findings simulating the core of SPARC, from the impact of edge pressure to impurity effects on turbulence stabilization.
Speaker: Alan Goodman, Max Planck Institute for Plasma Physics, Greifswald - Chaired by: Paolo Ricci, Associate Editor, JPP
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Date/Time: Thursday 27th February 2025, 4PM GMT/11AM EST
Title: Searching for SQuIDs: Stable Quasi-Isodynamic Designs as Fusion Reactor Candidates
Abstract: Quasi-isodynamic (QI) stellarators are uniquely attractive fusion reactor candidates due to their good fast ion confinement and small toroidal currents, ensuring disruption-free operation and a plasma geometry that remains roughly fixed regardless of the plasma’s pressure profile. In addition to being QI, a stellarator reactor configuration must be resilient to magnetohydrodynamic (MHD) instabilities and have reduced turbulent transport, the latter of which has limited the performance of Wendelstein 7-X, itself an approximately QI stellarator. Recent developments in stellarator optimisation have uncovered a new class of stellarators known as “SQuIDs” (Stable Quasi-Isodynamic Designs) which enjoy excellent QI quality (and all the benefits thereof), alongside MHD stability and considerably lower turbulence than W7-X. In this work, we expand on the original SQuID design methodology in two ways. First, we include the findings from Kappel et al., which allow for these SQuIDs to be realized by a set of relatively simple electromagnetic coils. Second, we find that SQuIDs can be designed to have an electron root — a region of outward-pointing radial electric field within the plasma core — which provides an elegant solution to the problem of impurity accumulation. Their small net toroidal currents, along with their optimised rotational transform profiles, grant compatibility with an edge island divertor. As such, these designs are alluring candidates for future stellarator experiments and reactors.
Speaker: Aaron Tran, University of Wisconsin, Madison - Chaired by: Edward Thomas , Associate Editor, JPP
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Date/Time: Thursday 20th February 2025, 4PM GMT/11AM EST
Title: Drift-cyclotron loss cone instability in 3D kinetic-ion simulations of WHAM
Abstract: The Wisconsin High-temperature superconductor Axisymmetric Mirror (WHAM) experiment's beam-ion plasma may induce drift-cyclotron loss-cone (DCLC) instability: a coupled ion Bernstein / drift wave excited by the plasma’s radial density gradient and loss-cone velocity distribution. We present 3D plasma simulations, using the kinetic-ion/fluid-electron code Hybrid-VPIC, of various WHAM configurations with sloshing (45 deg. pitch angle) ion distributions from the collisional Fokker-Planck code CQL3D-m as an initial condition. Edge-localized electrostatic waves grow and saturate in ~1–10 μs with ω ~ 1–2× the ion cyclotron frequency. Wave properties can be explained by linear theory of DCLC in a planar slab. DCLC scatters ions to a marginally-stable distribution with a filled loss cone and hence gas-dynamic rather than classical-mirror confinement. Sloshing ions can trap cool (low-energy) ions in an electrostatic potential well to stabilize DCLC, but DCLC itself does not scatter sloshing beam-ions into said well. Instead, cool ions must come from external sources such as charge-exchange collisions with a low-density neutral population. Manually adding cool ~1 keV ions to the plasma edge improves beam-ion confinement by ~2–5× in our simulations. I will also briefly comment on (i) other ways to stabilize DCLC, (ii) how DCLC fits into a broader landscape of instabilities in mirrors, and (iii) the effect of externally-driven shear flows.
Speaker: Pablo Oyola, PPPL, Princeton, USA - Chaired by: Troy Carter, Associate Editor, JPP
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Date/Time: Thursday 13th February 2025, 4PM GMT/11AM EST
Title: Mitigation of the toroidal Alfvén Eigenmodes in Negative Triangularity plasmas at the TCV tokamak
Abstract: Fast ion confinement, and particularly alpha particles, is key for the development of a successful fusion power plant (FPP), as they should be the main heating and current drive mechanism. Instabilities in plasmas, like Alfvén eigenmodes, can be excited by large fast ion populations and can result in an untamable loss of power, as well as damage to the reactor vessel[1-3]. 3D Nonlinear hybrid kinetic simulations performed with the MEGA[4] code have recently revealed a systematic mitigation of the Toroidicity-induced Alfvén Eigenmodes (TAEs) in TCV plasma when operating in negative triangularity (NT) plasmas[5-7], compared to positive triangularity (PT). The full kinetic treatment of the fast ions is key in reproducing the underlying wave-particle resonance structures in the phase space. About 30% less energy is found to be exchanged in the NT case. The change in particle orbit topology due to the triangularity significantly changes the energy exchange between the modes and the particles, damping it in the NT case, highlighting the need for kinetic treatment. A 3-fold reduction of the AE-induced fast ion losses has been observed in the selfconsistent MEGA simulation in the NT case, in comparison to the PT plasma. This trend is further confirmed with purely kinetic simulations using the ASCOT5 code[8], which shows that for even more marked negative triangularities, fast ion losses reduce even further. This poses the NT scenario as a more promising candidate for the future FPP, which would naturally have better alpha confinement.
Speaker: Iryna Butsky, Stanford University, USA - Chaired by: Roger Blandford , Associate Editor, JPP
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Date/Time: Thursday 6th February 2025, 4PM GMT/11AM EST
Title: What in the Galaxy is Scattering Cosmic Rays?
Abstract: Cosmic rays with energies << TeV affect galaxy evolution on all scales, from ionizing protoplanetary disks and molecular clouds to driving galactic outflows that alter the gas phase hundreds of kiloparsecs from the galactic disk. All models of cosmic-ray physics on “macro” scales (> pc) are sensitive to the assumed models of cosmic-ray scattering on “micro” scales (~ au), which are observationally and theoretically unconstrained. These effects are amplified in the circumgalactic medium, where models that fit existing data can vary by many orders of magnitude in their predictions for the cosmic-ray transport rate. Traditional first-principles models, which assume these magnetic fluctuations are weak and uniformly scatter CRs in a homogeneous ISM, struggle to reproduce basic observables such as the dependence of CR residence times and scattering rates on rigidity. In this talk, I will explore a new category of 'patchy' CR scattering models, wherein CRs are predominantly scattered by intermittent strong scattering structures with small volume-filling factors. These models produce the observed rigidity dependence with a simple size distribution constraint, such that larger scattering structures are rarer but can scatter a wider range of CR energies. I will present the physical properties any such structures would need to successfully scatter low-energy CRs and discuss future observational constraints that could test these models.
Speaker: Evan Yerger, University of New Hampshire, USA - Chaired by: Thierry Passot, Associate Editor, JPP
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Date/Time: Thursday 30th January 2025, 4PM GMT/11AM EST
Title: Collisionless conduction in a high-beta plasma: a collision operator for whistler turbulence
Abstract: In this talk, I will discuss the regulation of electron heat transport in high-beta, weakly collisional, magnetized plasma. A temperature gradient oriented along a mean magnetic field can induce a kinetic heat-flux-driven whistler instability (HWI), which back-reacts on the transport by scattering electrons. Previous analytical and numerical studies have shown that the heat flux for the saturated HWI scales as the inverse of the electron plasma beta. These numerical studies, however, had limited scale separation and consequently large fluctuation amplitudes, which calls into question their relevance at astrophysical scales. To this end, I will present a series of particle-in-cell simulations of the HWI across a range of electron plasma beta and temperature-gradient length scales under two different physical setups. The saturated heat flux in all of the simulations follows the expected electron-beta scaling, supporting the robustness of the result. I will also present several methods used to construct an effective collision operator for whistler turbulence from the simulation results. The collision operators obtained point to an issue with the standard quasi-linear explanation of HWI saturation, which is analogous to the well-known 90-degree scattering problem in the cosmic ray community. Despite this limitation, the methods developed here can serve as a blueprint for future work seeking to characterize the effective collisionality caused by kinetic instabilities.
Speaker: Rebecca Diesing, Institute for Advanced Study, Princeton, USA - Chaired by: Dmitri Uzdensky, Associate Editor, JPP
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Date/Time: Thursday 23rd January 2025, 4PM GMT/11AM EST
Title: Diffusive Shock Acceleration from Plasma to Astrophysical Scales
Abstract: Interpreting observations of extreme astrophysical phenomena requires a detailed understanding of the microphysical processes responsible for charged particle acceleration. In the standard picture, supernova remnants and other astrophysical shocks accelerate these particles, known as cosmic rays, via diffusive shock acceleration (DSA), an efficient mechanism that produces power-law distributions in momentum. However, both the multi-wavelength emission from astrophysical shocks—in particular, supernova remnants—and the populations of CRs detected at Earth reveal discrepancies between this standard theory and observations. To address these discrepancies, I will introduce a fast, semi-analytic modeling framework that self-consistently incorporates findings from state-of-the-art kinetic simulations. I will show how we can use this model to bridge the gap between plasma and astrophysical scales, and will predict the multi-messenger emission from astrophysical objects including supernova remnants, novae, and black hole winds.
Speaker: Swadesh M Mahajan, IFS, University of Texas Austin, USA - Chaired by: Francesco Califano, Associate Editor, JPP
Date/Time: Thursday 16th January 2025, 4PM GMT/11AM EST
Title: Revisiting simple Alfven waves in Hall (extended) MHD Emergence of a Singular Branch
Abstract: It is well appreciated that the Hall term constitutes a singular perturbation to magnetohydrodynamics (MHD). Although, singular solutions and their importance has found much attention in equilibrium HMHD, it has not been appreciated, at least explicitly, that the elementary Alfvenic motions also undergo propound transformations in HMHD (extended MHD). Apart from modifying the conventional MHD spectrum, the Hall current induces a distinct and new branch consisting of purely circularly polarized waves that may become the representative shear wave. This branch - Alfven H- with no MHD counterpart, is the direct expression of the singular perturbation. Alfven H has profound effects on both coherent -nonlinear and turbulent evolution of the system. The robustness of Alfven H is tested by examining its fate in more realistic systems- when, for instance, the equilibrium magnetic field is sheared.
Speaker: Alessandro Biancalani, Ecole Supérieure d'Ingénieurs Léonard de Vinci, France - Chaired by: Alex Schekochihin, Editor, JPP
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Date/Time: Thursday 9th January 2025, 4PM GMT/11AM EST
Title: The role of zonal structures as mediators of energetic particles and turbulence
Abstract: Spatial gradients of density and temperature are present in tokamak plasmas. Microinstabilities grow due to these gradients, and nonlinearly interact, forming turbulence. Zonal (i.e. axisymmetric) structures (ZS) take an important role in the nonlinear saturation of turbulence. Energetic particle populations are also present in tokamak plasmas due to fusion reactions, and in part due to external heating mechanisms. In the recent years, the interaction of EPs and turbulence has taken a particular attention of the magnetic-confinement fusion community. A global gyrokinetic model is necessary, if one wants to address such an intrinsically multiscale kinetic problem. In this work, we investigate the nonlinear dynamics of ZS as mediators of turbulence and EPs. The main theoretical tool is the global particle-in-cell code ORB5, originally developed for electrostatic turbulence studies, and later on extended to its multispecies electromagnetic version. The effect of the EPs on the ZS will be shown, and the role of macroscopic instabilities as mediators will be considered.
Speaker: Sebastien Le Pape, LULI, Ecole Polytechnique, France - Chaired by: Luís O.Silva, Associate editor, JPP
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Date/Time: Thursday 19th December 2024, 4PM GMT/11AM EST
Title: Foams as a Pathway to Energy from Inertial Fusion
Abstract: The perspective of reaching ignition using high power lasers such as the National Ignition Facility (NIF) in the USA or the Laser MégaJoule (LMJ) in France is significantly closer today thanks to technical advances (e.g. the ability to manufacture high-density carbon shells doped with high-Z elements) and to an increased knowledge of the underlying fundamental physics. Following the same logic, a eurofusion project (FOPIFE) has been funded to study the physics of nano-structured medium/foams in a fusion framework. Foams are envisioned for various applications: increase laser to matter coupling, reduce laser imprint, enable high repetition rate implosion at reduced cost. In this presentation, I will go over the various challenges and benefits associated to the use of foams for Inertial Fusion Energy and the effort in Europe to answer these questions.
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Speaker: Jonathan Graves, EPFL & University of York, UK - Chaired by: Peter Catto, Associate Editor, JPP
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Date/Time: Thursday 12th December 2024, 4PM GMT/11AM EST
Title: Identification of new MHD mechanisms responsible for plasma performance-setting in tokamaks and the use of analytic theory for informing and improving numerical models
Abstract: Tokamak plasma performance is significantly limited by long-wavelength MHD instabilities. Despite these modes being a necessary condition for good plasma operation, they are subtle, leading to mistakes being made in both analytical and numerical approaches to their modelling. For pressure driven instabilities, the stabilising role of ordinarily dominant effects such as magnetic field line bending and compression are typically very small, and as such, many apparently weak effects are critical. It is for this reason that realistic geometry, as defined by the boundary conditions of the laboratory plasma, finite beta effects, equilibrium flows, and kinetic corrections typically determines plasma stability. This presentation documents the role that analytic theory plays in understanding linear and nonlinear MHD instabilities, and in informing codes of their errors, and advising improved models. Alternative calculations of nonlinearly saturated instabilities are presented, and their virtues and applications are compared. Showcased are the effects of parallel magnetic field fluctuations, centrifugal corrections associated with strong toroidal flows, and the continuum damping of internal kink modes and Kelvin Helmholtz-like modes due to respectively zonal and geodesic acoustic modes. Finally, after the review of the aforementioned topics, a model will be presented which is consistent with tokamak hybrid scenarios where the plasma ceases to diffuse over resistive confinement timescales. An ideal nonlinear MHD model is offered as an alternative to the flux pump hypothesis which has recently excited some in the magnetic fusion community for enabling high-performance sawtooth-avoiding plasma operation.
Speaker: Juan Ruiz Ruiz, University of Oxford, UK - Chaired by: Peter Catto, Associate Editor, JPP
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Date/Time: Thursday 5th December 2024, 4PM GMT/11AM EST
Title: Measurement and gyrokinetic simulations of zero-frequency fluctuations generated by coupling between Alfvén modes in the JET tokamak
Abstract: Energetic particles will be ubiquitous in next-generation burning plasmas, most prominently in the form of 3.5 MeV alpha particles issued from the DT reaction. Understanding how they will impact the overall plasma confinement is therefore of dire importance for predicting fusion performance. For decades we have learned that energetic particles can linearly destabilize certain types of Alfvén modes, which drive large transport of the energetic particles. More recently, theory and numerical simulations have suggested that Alfvén modes could have a beneficial effect on the thermal plasma by generating a stationary zonal perturbation, which could tear apart the turbulent eddies and result in improved confinement. This zero-frequency zonal perturbation, however, has remained without experimental confirmation until now. In this work, we report the first experimental detection of a zero-frequency fluctuation that is generated by Alfvén modes in a magnetically confined plasma. Experiments in the JET tokamak have shown that Alfvén modes destabilized by energetic particles exhibit three-wave coupling interactions with a zero-frequency fluctuation, and its presence is correlated with (otherwise unexplained) increased ion temperature and improved confinement. Nonlinear gyrokinetic simulations confirm the three-wave coupling between zero-frequency zonal modes and Alfvén modes, resulting in reduced turbulence and transport levels. This suggests that the zero-frequency perturbation that we detect is a zonal mode that is responsible for the improvement in confinement observed in the JET experiment. The implications from this work and prospects for next-generation burning plasmas will be discussed.
Speaker: Francesco Porcelli, LPolytechnic University of Turin, Italy - Chaired by: Francesco Califano , Associate editor, JPP
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Date/Time: Thursday 21st Nov 2024, 4PM GMT/11AM EST
Title: Resonant axisymmetric modes in tokamak fusion plasmas
Abstract: The impact of divertor X-points on axisymmetric modes (toroidal mode number n=0) in tokamak plasmas is discussed. Axisymmetric perturbations, such as vertical displacements, can lead to current sheets localized in the vicinity of magnetic X-points and extending along the magnetic divertor separatrix, likely to have a profound impact on disruptions and MHD edge stability. When a nearby resistive wall is present, the vertical mode can still grow on the relatively slow resistive wall time scale. Active feedback control is then required for complete stabilization. However, it is shown that the resistive growth rate can be significantly faster, scaling with fractional powers of wall resistivity, if the wall position satisfies the criterion for ideal-MHD marginal stability, thus posing more stringent conditions for active feedback stabilization. Furthermore, it is shown that so-called Vertical Displacements Oscillatory Modes (VDOM) can be excited. These modes have a frequency of oscillation scaling with the poloidal Alfvén frequency. They are normally damped by wall resistivity, but can be driven unstable by the resonant interaction with fast ions, leading to a new type of fast ion driven mode. Instability requires a fast ion distribution with a positive slope, or with a significant anisotropy, in velocity space. Scenarios for the required fast ion population are discussed. Numerical simulations of VDOM using the NIMROD extended-MHD code are presented. This theory may explain the observation of saturated n=0 fluctuations in recent JET experiments.
Speaker: Richard Nies, Princeton University, USA - Chaired by: Nuno F. G. Loureiro, Associate Editor, JPP
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Date/Time: Thursday 14th November 2024, 4PM GMT/11AM EST
Title: Propagating zonal flows and turbulence saturation in magnetised fusion plasmas
Abstract: Toroidal fusion devices are plagued by drift-wave turbulence causing large heat fluxes and limiting the confinement time. It is therefore crucial to understand the mechanisms for turbulence saturation, which typically involve the interplay between drift waves and zonal flows shearing apart turbulent eddies. Toroidal geometry has long been known to affect the linear zonal flow physics, weakening stationary zonal flows and introducing the rapidly oscillating geodesic acoustic modes. In this talk, I will demonstrate that the toroidicity also plays a pivotal role for the nonlinear zonal flow dynamics, giving rise to the toroidal secondary, a new mode of radially propagating zonal flows. The theory and physical mechanism of the toroidal secondary will be presented and validated using gyrokinetic simulations. Furthermore, the toroidal secondary will be shown to set the saturation level of ion-temperature-gradient turbulence. By combining the new zonal flow physics with the critical balance conjecture, scaling laws are derived for the turbulent heat flux, the eddy scale lengths, and the zonal flow amplitude, which are in agreement both with turbulence simulations and with past experimental observations.
Speaker: Pablo Bilbao, IST, Lisbon Chaired by: Dmitri Uzdensky, Associate Editor, JPP
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Date/Time: Thursday 07th November 2024, 4PM GMT/11AM EST
Title: Radiative Dynamics and Instabilities in Strongly Magnetized Relativistic Plasmas
Abstract: Radiative Dynamics and Instabilities in Strongly Magnetized Relativistic Plasmas Relativistic plasmas in strong electromagnetic fields exhibit distinct dynamics compared to classical plasmas, with radiative and quantum electrodynamical (QED) effects fundamentally altering their collective properties and dynamics. Here, we investigate strongly magnetized plasmas for which synchrotron cooling timescales are comparable to the collective dynamical timescales, causing particles to radiate their energy, and surprisingly part of their collective entropy. This radiative cooling drives the plasma out of kinetic equilibrium, triggering kinetic instabilities. Our theory and simulations show that strongly magnetized plasmas spontaneously generate coherent, linearly polarized radiation through the interplay between radiative cooling and the electron cyclotron maser instability. These findings showcase the unique dynamics that radiative mechanisms introduce in relativistic plasmas, offering key insights into environments with intense electromagnetic fields such as neutron stars and magnetars. With laboratory experiments now able to recreate these conditions, we propose potential experimental platforms to probe these dynamics and link theory with astrophysical observations.
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Date/Time: Thursday 31st October 2024, 3PM GMT/11AM EDT
Title: The surprising effectiveness of cosmic ray acceleration
Abstract: If I did not know better from observations I would say that cosmic rays could not be accelerated to multi-PeV energies in the Milky Way Galaxy or to energies of 100EeV beyond the Milky Way. Cosmic ray acceleration is basically a plasma process. I will examine the limits imposed by plasma physics and consider ways in which acceleration might be more effective than I at least would have expected.
Date/Time: Thursday 24th October 2024, 4PM BST/11AM EDT
Title: Deployable fusion plasmas?
Abstract: The enormous and urgent challenges to provide the world with sufficient and suitable low carbon energy has given new impetus to fusion research. Some very substantial programmes aimed at pilot or developmental fusion power plants are now underway, usually at a very rapid pace. This pace can challenge the traditional scientific method (decisions based on empirical evidence supported by scientific understanding), so these programmes accept significant or even large uncertainties in the plasma and other systems. However, it seems likely that the uncertainty appetite will be reduced for deployable fusion plants, e.g. those built and operated for energy utilities perhaps part of national energy policies. The pace may still be high with such plants potentially designed in parallel with the pilot plants yet with significant differences. It is worth considering the impact on plasma R&D by asking a few questions about how one might arrive at a plasma which has the desired performance and can be adopted and integrated with sufficient confidence into a deployable plant. Some transitions in thinking and approach may be needed on the way..
Speaker: Yajie Yuan, Washington University, St Louis, USA - Chaired by: Roger Blandford, Associate Editor, JPP
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Date/Time: Thursday 17th October 2024, 4PM BST/11AM EDT
Title: Nonlinear evolution of Alfven waves and fast waves in magnetar magnetospheres
Abstract: Young and active neutron stars like magnetars may experience star quakes that can launch kHz Alfven waves into the magnetosphere. An Alfven wave packet propagating along the closed field lines undergoes several interesting nonlinear processes: (1) Alfven waves propagating to the outer magnetosphere may break and form an ejecta that accelerates away from the star. This process may be a viable mechanism to produce the simultaneous X-ray bursts and fast radio bursts from the galactic magnetar SGR 1935+2154. (2) The Alfven waves bouncing back and forth in the inner magnetosphere can convert to fast magnetosonic waves that spherically expands out. The fast waves can also become nonlinear before escaping the magnetosphere, producing shocks or regions with E>B that efficiently dissipate the fast wave energy, powering observable emission. In this talk, I’ll present our systematic study of all these processes and discuss their observational consequences.
Speaker: Alec Thomas, University of Michigan, USA - Chaired by: Luís O.Silva, Associate editor, JPP
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Date/Time: Thursday 26th Sep 2024, 4PM BST/11AM EDT
Title: Plasma physics in strong laser fields
Abstract: Plasma behavior in extremely strong background electromagnetic fields is substantially modified from that of classical plasma owing to the onset of relativistic quantum electrodynamics (QED) processes. These processes include the stochastic emission of photons by electrons and the decay of a photon into an electron-positron pair. They are important because electron kinetic behavior is affected by momentum loss during the emission of energetic photons (known as “radiation reaction”), and plasma density may continuously vary over time through pair production. Such plasma behavior may be observed in extreme astrophysical environments and in focused laser light at the highest intensities. The coupling of quantum emission processes and relativistic collective particle dynamics results in dramatically new plasma physics phenomena, such as the generation of dense electron-positron pair plasma from near vacuum, complete laser energy absorption through QED processes, modified plasma dispersion properties, and anomalous particle trapping phenomena. This talk will review the fundamental physics of QED in strong fields and address open questions, provide an overview of pioneering experiments conducted in the laboratory, discuss theoretical and computational advances aimed at understanding plasma in strong fields, and explore upcoming and future research involving ultra-high-intensity lasers and high-energy beam facilities worldwide.
Speaker: Mel Abler, Space Science Institute, USA - Chaired by: Edward Thomas, Associate Editor, JPP
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Date/Time: Thursday 19th Sep 2024, 4PM BST/11AM EDT
Title: First Laboratory Observations of Residual Energy Generation in Strong Alfvén Wave Interactions
Abstract: In the MHD inertial range (scales larger than ion-kinetic scales) turbulent fluctuations in the solar wind are often Alfvénic in character, meaning that their magnetic and flow velocity fluctuations are proportional to each other and predominantly perpendicular to the background magnetic field. However, observations of the solar wind have shown that there is a significant difference in the energy in velocity fluctuations and normalized magnetic fluctuations. This difference, called the residual energy, should be zero for linear Alfvén waves, but is consistently observed to be negative in the solar wind, with magnetic fluctuations dominating. This work investigates the energy partition in strong three-wave interactions through an experimental campaign on the LArge Plasma Device (LAPD) in an MHD-like regime. Primary (driven) modes are launched from antennas, and secondary modes generated by the strong three-wave interaction are observed. The primary modes are shown to have no residual energy, while the secondary modes have significant residual energy - negative in the “sum” mode and positive in the “difference” mode. These results constitute the first laboratory demonstration that residual energy can indeed be generated by nonlinear mode coupling.
Speaker: Dennis Whyte, Massachusetts Institute of Technology, USA - Chaired by: Nuno Loureiro, Associate Editor, JPP
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Date/Time: Thursday 12th Sep 2024, 4PM BST/11AM EDT
Title: Fusion economics: power density, materials and maintenance
Abstract: A review of prior magnetic d-t fusion power plant (FPP) studies reveals consistent patterns of high fusion power density and effective materials replacement. In order to assess the relative impact of plasma and component design a set of governing relationships that generically solve for optimizing FPP economics are developed, with component usage driven by cumulative neutron damage. Scans of the controlling parameters reveal minimum requirements in power density that are in general agreement with prior studies. Yet the transparent economic model reveals further surprising insights as to the relative importance of power density, neutron tolerance, energy conversion efficiency and financing costs.
Speaker: Gabriel Xu, University of Alabama, USA Chaired by: Edward Thomas, Associate Editor, JPP
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Date/Time: Thursday 5th September 2024, 4PM BST/11AM EDT
Title: Progress in Laboratory 3D Torisional Magnetic Reconnection
Abstract: Magnetic reconnection is a well known phenomenon in astrophysical plasmas and occurs in the solar corona and Earth's magnetotail. The most commonly studied type of reconnection in the lab is the 2D X-type. However, astrophysical plasmas have complex 3D magnetic structures which result in 3D reconnection events. This work is focused on a particular type of 3D reconnection called the torsional magnetic reconnection (TMR). TMR is one potential mechanism for coronal mass ejections and has been investigated theoretically and computationally since the early 2000s. Besides astrophysical cases, 3D reconnction also has potential application to space propulsion and inertial fusion, if the phenomenon can be produced in the lab. This talk presents the background on TMR and the current efforts to produce laboratory TMR between the University of Alabama in Huntsville and SpaceWave LLC under funding from the US Department of Energy.
Speaker: Charlotte Palmer, Queen’s University Belfast, UK - Chaired by: Louise Willingale, Associate Editor, JPP
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Date/Time: Thursday 29th AUGUST 2024, 4PM BST/11AM EDT
Title: Exploiting novel liquid sheet targets for the generation of bright MeV proton beams
Abstract: Laser-plasma acceleration has enormous potential to provide compact sources of ultra-short ion beams. However, several factors hamper their wider adoption, such as the low shot-to-shot stability, large beam divergence and the difficulty of high-repetition rate operation. In this talk I will outline an approach for overcoming these challenges by using an automated experimental setup incorporating a novel liquid sheet target, developed by collaborators at the SLAC National Accelerator Laboratory. I will report on recent experiments at the GEMINI TA2 laser facility (10 TW, 5 Hz) which demonstrated stable acceleration of few MeV proton beams with high flux and low-divergence proton beams in comparison to typical laser-accelerated ion beams. Supporting PIC simulations have shown that the presence of background vapour around the target plays an important role in the observed collimation of the proton beam. The measured proton beams are already suitable for applications requiring high proton flux and the platform can be extended to kHz repetition rates or higher laser energies extending the utility of the source to a wide range of applications in radiobiology, materials science and fundamental physics.
Speaker: Rony Keppens, KU Leuven - Chaired by: Steve Tobias, Associate Editor, JPP
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Date/Time: Thursday 22nd August 2024, 4PM BST/11AM EDT
Title: New insights in old problems: waves in ion-electron plasmas
Abstract:I will review our recent findings on waves in ion-electron plasmas, where we introduced a novel convenient labeling scheme that uniquely identifies each of the six wave pairs: S-A-F-M-O-X. In contrast to textbook treatments, our labeling does not start from the specific behavior at exactly parallel or exactly perpendicular orientation between wave vector and magnetic field, but recognizes the clear frequency ordering obeyed by the six wave pairs at all other (oblique) orientations. Our treatment is completely general for any two-fluid view on ion-electron plasmas, and the dispersion relation is conveniently expressed in polynomial expressions. The effect of collisions can be included and quantified for every wave type at all wavelengths. Important parameters include the magnetization and make our findings valid for extreme plasma conditions where the magnetohydrodynamic slow-alfven-fast (S-A-F) waves are given by their relativistically correct expressions. Special cases (cold plasmas, pair plasmas, …) are recovered and findings on Faraday rotation of electromagnetic (O-X) waves are easily generalized.
Speaker: Robert Ewart, University of Oxford, UK - Chaired by: Luis Silva, Associate Editor, JPP
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Date/Time: Thursday 15th August 2024, 4PM BST/11AM EDT
Title: Relaxation to universal non-Maxwellian equilibria in a collisionless plasma
Abstract: The question of how the particle distribution of a turbulent collisionless plasma will evolve, and what it may eventually relax to, is of both fundamental theoretical interest and observational consequence. In quiescent collisional plasmas, where the chaos is provided by inter-particle collisions, the answer is provided by thermodynamics and the universal maximum-entropy state is the Maxwellian distribution. However, in collisionless systems, the chaos is typically provided by the turbulent mean-field dynamics which nominally conserve an infinite set of invariants (conservation of phase volume) giving the system some memory of its initial conditions and making relaxation to a Maxwellian impossible. We will argue theoretically, and show numerically, that such plasmas will still relax towards universal equilibria, which are non-Maxwellian and feature a power-law distribution of particle energies. These equilibria will still be obtained by a maximum-entropy principle, while taking into account the conservation of these collisionless invariants on short time scales. Of course, there is no such thing as a truly collisionless system. On longer timescales these invariants are subtly broken by a turbulent cascade giving the system a kind of 'turbulent amnesia': relaxation, driven by the turbulence, to equilibrium is controlled by the collisionless invariants (the system's memory) but this memory is made imprecise by that same turbulence. We demonstrate that the invariants are driven to a universal form which results in a distribution function with a universal power-law tail in energy, with exponent -2
Speaker: Vincent David, University of New Hampshire, USA - Chaired by: Francesco Califano, Associate Editor, JPP
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Date/Time: Thursday 8th AUGUST 2024, 4PM BST/11AM EDT
Title: Monofractality in the Solar Wind at Electron Scales: Insights from Kinetic Alfvén Waves Turbulence
Abstract: The breakdown of scale invariance in turbulent flows, known as multifractal scaling, is considered a cornerstone of turbulence. In solar wind turbulence, a monofractal behavior can be observed at electron scales, in contrast to larger scales where multifractality always prevails. Why scale invariance appears at electron scales is a challenging theoretical puzzle with important implications for understanding solar wind heating and acceleration. We investigate this long-standing problem using direct numerical simulations of three-dimensional electron reduced magnetohydrodynamics. Both weak and strong kinetic Alfvén waves turbulence regimes are studied in the balanced case. After recovering the expected theoretical predictions for the magnetic spectra, a higher-order multiscale statistical analysis is performed. This study reveals a striking difference between the two regimes, with the emergence of monofractality only in weak turbulence, whereas strong turbulence is multifractal. This result, combined with recent studies, shows the relevance of collisionless weak KAW turbulence to describe the solar wind at electron scales.
Speaker: Jena Meinecke, Gettysburg College, USA Chaired by: Dmitri Uzdensky, Associate Editor, JPP
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Date/Time: Thursday 1st August 2024, 4PM BST/11AM EDT
Title: Strong suppression of heat conduction in a laboratory replica of galaxy-cluster turbulent plasmas
Abstract: Radio synchrotron emission and Faraday rotation measurements have all revealed that the universe is ubiquitously magnetized—from clusters to filaments to voids—implying that magnetic fields are essential players in the dynamics of luminous matter. Furthermore, understanding heat transport is important in astrophysical plasmas such as those in galaxy clusters. In conventional gases and plasmas, heat fluxes are proportional to temperature gradients, with collisions between particles mediating energy flow from hotter to colder regions and the coefficient of thermal conduction given by Spitzer’s theory. However, this theory breaks down in a magnetized, turbulent, weakly collisional plasma. We present experimental efforts underway at the National Ignition Facility (NIF) to generate laboratory replicas of galaxy-cluster turbulent plasmas where the generated magnetic energy reaches equipartition with kinetic energy. Our data indicates a reduction of heat transport by two orders of magnitude or more, leading to large temperature variations on small spatial scales, consistent with observations of cluster plasmas.
Speaker: Fulvio Zonca, ENEA, Frascati, Italy - Chaired by: Alex Schekochihin, Editor, JPP
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Date/Time: Thursday 25th July 2024, 4PM BST/11AM EDT
Title: Structure formation and transport in magnetized fusion plasmas
Abstract: Transport in magnetized fusion plasmas provides an interesting example of how collective behaviors in complex systems lead to structure formation and self-organization. This occurs because transport is dominated by fluctuations that are excited at the system microscales, characterized by the typical width of charged particle orbits in the strong equilibrium magnetic field. These fluctuations can provide nonlinear feedback on the plasma equilibrium profiles, i.e., on the system macroscales. Most importantly, however, fluctuation spectra and transport phenomena also yield to structure formation on the mesoscales, which are intermediate between the micro- and the macro-scales and can be due to both linear wave packet propagation in the slowly evolving and weakly non-uniform plasma medium as well as to nonlinear couplings. Meanwhile, high temperature, weakly collisional, reactor-relevant fusion plasmas can have velocity space distributions, which deviate from local thermodynamic equilibrium. Thus, transport and structure formation in magnetized fusion plasmas must be necessarily described in the phase space, and studying their nonlinear dynamics requires advanced kinetic treatments of thermal as well as supra-thermal components, including realistic geometries and plasma non-uniformities. This work presents the main findings of a comprehensive approach that enables predicting and describing the long time scale behavior of reactor relevant magnetized fusion plasmas as nonlinear equilibria, which evolve self-consistently with fluctuation spectra as well as sources/collisions. Examples of applications to cases of practical interest are also given, focusing on simplified paradigmatic cases for illustrating the workflow of the Advanced Transport model for Energetic Particle (ATEP) code. In particular, the case of neutral beam injected particles in ITER will be shown, which are redistributed under the action of Coulomb collisions and a fixed amplitude toroidal Alfv´en eigenmode.
SPEAKER: MOHAMAD SHALABY, LEIBNIZ INSTITUTE FOR ASTROPHYSICS POTSDAM, GERMANY - CHAIRED BY: ANTOINE BRET, ASSOCIATE EDITOR, JPP
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DATE/TIME: THURSDAY 18TH JULY 2024, 4PM BST/11AM EDT
Title: Cosmic-Ray Choreography: How Ion-Driven Instabilities Propel Electron Acceleration and Dictate Ion Transport in Astrophysical Plasma
Abstract: Cosmic-ray-driven instabilities are pivotal in accelerating particles at shocks and during the propagation of GeV cosmic rays in galaxies and galaxy clusters. Within the self-confinement picture of cosmic ray (CR) transport, these instabilities amplify magnetic fields, which scatter cosmic rays and self-regulate their movement. This creates a strong coupling between the collisionless cosmic ray population and the thermal background plasma, suggesting potentially significant dynamic feedback. In this talk, I will discuss the inconsistencies between the self-confinement picture in Alfvénic turbulence and certain observations. Attempting to reconcile these inconsistencies by incorporating fast-magnetosonic turbulence cascades requires very fine-tuning to align with the data. I will then introduce a new discovery: a cosmic ray-driven instability termed the intermediate-scale instability. This instability triggers comoving ion-cyclotron electromagnetic waves at sub-ion skin-depth scales and grows notably faster than the resonant ion gyro-scale (streaming) instability, previously considered the dominant instability in the self-confinement model. This breakthrough suggests that the intermediate-scale instability could be crucial in cosmic ray transport in galactic and stellar environments. Next, I will demonstrate how these resonant instabilities saturate in isolation, challenging the fundamental assumptions about saturation in the self-confinement model. This points to a necessary fundamental revision of the theory of CR transport in galaxies and galaxy clusters. Through Particle-in-Cell (PIC) simulations, I will also show that the intermediate-scale instability is the key mechanism for efficient electron injection at parallel electron-ion shocks, addressing the long-standing challenge of electron injection at these shocks. Additionally, the simulations reveal that using reduced ion-to-electron mass ratios in shock simulations, which artificially suppresses the intermediate-scale instability, not only impedes electron acceleration but also leads to incorrect electron and ion heating in downstream and shock transition areas.
Speaker: Ellie Tubman, Imperial College, UK - Chaired by: Louise Willingale, Associate editor, JPP
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Date/Time: Thursday 11th July 2024, 4PM BST/11AM EDT
Title: Observations of magnetised, collisionless shocks in laser-driven experiments
Abstract: Magnetized, collisionless shocks are observed throughout the universe, including within supernova remnants and at the Earth’s magnetopause. These shocks have the potential to accelerate particles to far greater energies than other astrophysical processes and may provide a source of high-energy cosmic rays. One challenge, yet to be addressed, is determining the exact mechanism of the acceleration, and resulting energy dissipation by these shock waves. Designing a laboratory-based experiment that can create and investigate these shocks is advantageous for testing different theories and developing our knowledge of this phenomena. I will present results from an experiment performed at the Omega laser facility where we use gas jet and MIFED assembly provide a pre-ionized, pre-magnetized background plasma, through which a shock wave is launched. We diagnose the effect of the background magnetic fields on the shock formation using both temporally and spatially resolved Thomson scattering. This allow us to measure the plasma conditions, as well as identify where the background and piston material are spatially located. We are also able to measure the evolving electromagnetic field structures using proton probing. The results assist in benchmarking particle-in-cell codes and hydrodynamic models as well as interpreting measurements from spacecrafts, to gain a better understanding of the underlying physics.
Speaker: Kelli Humbird, Lawrence Livermore National Laboratory, USA - Chaired by: Louise Willingale, Associate Editor, JPP
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Date/Time: Thursday 06th June 2024, 4PM BST/11AM EDT
Title: Predicting Fusion Ignition at the National Ignition Facility with Machine Learning
Abstract: On December 5, 2022, 192 laser beams at the National Ignition Facility focused 2.05 megajoules of energy onto a capsule of frozen hydrogen fuel. In less time than it takes light to travel 10 feet, the laser crushed the capsule to smaller than the width of a human hair, vaulting the fuel to temperatures and densities exceeding those found in the sun. Under these extreme conditions, the fuel ignited and produced 3.15 megajoules of energy, making it the first experiment to ever achieve net energy gain from nuclear fusion. After several decades of research, fusion breakeven at NIF brings humanity one step closer to the dream of limitless carbon-free, safe, clean energy. Yet, the shot that finally ushered in the Fusion Age was not actually that surprising. Prior to the experiment, our physics team used a machine learning model to predict the outcome of the experiment. The model, which blends supercomputer simulations with experimental data, indicated that ignition was the most likely outcome for this shot. In this talk, we discuss the breakthrough experiment, nuclear fusion, and how we used machine learning to predict that this experiment would demonstrate one of the biggest scientific achievements of our lifetime. LLNL-ABS-844080.
Speaker: Toby Adkins, University of Otago, New Zealand - Chaired by: Alex Schekochihin, Editor, JPP
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Date/Time: Thursday 30th May 2024, 4PM BST/11AM EDT
Title: The effects of finite electron inertia on helicity-barrier-mediated turbulence
Abstract: Understanding the partitioning of turbulent energy between ions and electrons in weakly collisional plasmas is crucial for the accurate interpretation of observations and modelling of various astrophysical phenomena. Many such plasmas are “imbalanced”, wherein the large-scale energy input is dominated by Alfvénic fluctuations propagating in a single direction. In this talk, we demonstrate that when strongly-magnetised plasma turbulence is imbalanced, nonlinear conservation laws imply the existence of a critical value of the electron plasma beta (the ratio of the thermal to magnetic pressures) that separates two dramatically different types of turbulence in parameter space. For betas below the critical value, the free energy injected on the largest scales is able to undergo a familiar Kolmogorov-type cascade to small scales where it is dissipated, heating electrons. For betas above the critical value, the system forms a “helicity barrier” that prevents the cascade from proceeding past the ion Larmor radius, causing the majority of the injected free energy to be deposited into ion heating. Physically, the helicity barrier results from the inability of the system to adjust to the disparity between the perpendicular-wavenumber scalings of the free energy and generalised helicity below the ion Larmor radius; restoring finite electron inertia can annul, or even reverse, this disparity, giving rise to the aforementioned critical beta. We relate this physics to the “dynamic phase alignment” mechanism (that operates under yet lower beta conditions and in pair plasmas), and characterise various other important features of the helicity barrier, including the nature of the nonlinear wavenumber-space fluxes, dissipation rates, and energy spectra. The existence of such a critical beta has important implications for heating, as it suggests that the dominant recipient of the turbulent energy — ions or electrons — can depend sensitively on the characteristics of the plasma at large scales.
Speaker: Wonho Choe, Korea Advanced Institute of Science and Technology, Korea - Chaired by: Cary Forest, Associate Editor, JPP
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Date/Time: Thursday 23rd May 2024, 4PM BST/11AM EDT
Title: Stabilization of liquid instabilities with plasmas at the gas-liquid boundary
Abstract: Historically, boundary physics has been crucial in understanding a diverse range of phenomena prevalent in nature and in creating highly unusual processes. A good example is the cavity produced by blowing air through a straw directly above a cup of water. One longstanding issue is the hydrodynamic stability of such gas–liquid systems, where some attention has been paid to the formation of gas-blown cavities. However, little attention has been given to the stability of gas-blown cavities, i.e., how the interface is deformed and destabilized remains unknown. In our recent work (Nature 592 (2021), 49), the stabilization of such instabilities by weakly ionized plasma for the case of a gas jet impinging on water is demonstrated. We focus on the interfacial dynamics relevant to electrohydrodynamic (EHD) gas flow, so-called electric wind, occurring inside the plasma which is induced by the momentum transfer from accelerated charged particles to neutral gas under an electric field (Nature Comm. 9 (2018) 371). A weakly ionized plasma consisting of periodic pulsed ionization waves, called plasma bullets, exerts more force via electrohydrodynamic flow on the water surface than a neutral gas jet alone, resulting in cavity expansion without destabilization. Furthermore, both the bidirectional electrohydrodynamic gas flow and electric field parallel to the gas–water interface produced by plasma interacting ‘in the cavity’ render the surface more stable. This case study demonstrates the dynamics of liquids subjected to a plasma-induced force, offering insights into physical processes and revealing an interdependence between weakly ionized plasma and deformable dielectric matter, including plasma–liquid systems.
Speaker: Hideo Sugama, National institute for fusion science, Japan - Chaired by: Peter Catto, Associate Editor, JPP
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Date/Time: Thursday 16th May 2024, 4PM BST/11AM EDT
Title: Gyrokinetic studies on momentum transport and energy exchange
Abstract: Governing equations of electromagnetic turbulent plasma systems are derived from the variational principle for the action integral of the Lagrangian based on gyrokinetics. It is shown in the first part of this colloquium that the invariance of the Lagrangian of the system under an arbitrary spatial coordinate transformation can be used to derive the local momentum balance equation. The symmetric pressure tensor representing the local momentum transport is directly derived from the variational derivative of the Lagrangian with respect to the metric tensor. The derived symmetric pressure tensor is useful to treat momentum conservation in symmetric magnetic configurations. In the second part, gyrokinetic simulation results of turbulent energy exchange between electrons and ions in ITG turbulence are presented. In low collisional plasmas, turbulent effects dominate over energy exchange. Energy transfer from ions to electrons occurs due to the ITG turbulence even when electrons are hotter than ions. In addition, quasilinear modeling of the turbulent energy exchange is discussed based on comparison between linear and nonlinear simulation results.
Speaker: Stephen Majeski, Princeton University, USA - Chaired by: Alex Schekochihin, Editor, JPP
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Date/Time: Thursday 9th May 2024, 4PM BST/11AM EDT
Title: Turbulence in collisionless, high-beta plasmas
Abstract: Owing to their simplicity, the MHD equations have enjoyed a long tenure as the go-to model for describing turbulence in astrophysical environments ranging from the solar wind to the intracluster medium of galaxy clusters (ICM). Unfortunately, the vast majority of these astrophysical plasmas can only be considered weakly collisional at best, meaning significant deviations from the MHD assumption of local thermodynamic equilibrium are not only possible but common. Additionally, recent studies have increasingly found that the various nonequilibrium- and micro-physics of the weakly collisional regime can fundamentally transform the global evolution of such astro-plasmas, especially when the plasma beta is of order unity or larger. In this talk, I will explore the consequences of micro-instabilities and anisotropic pressure-stress on collisionless, high-beta turbulence to distinguish what essential physics of the MHD model remains, and what must be done away with. This will be accomplished through analytical and numerical means, beginning with an examination of the individual waves that form the foundation of collisionless plasma turbulence, and ending with the collective, self-organization effect known as ‘magneto-immutability’. Some of the novel findings that will be covered include modifications to the interaction of hydromagnetic waves, a surprisingly low but highly intermittent volume filling fraction of turbulence-driven microinstabilities, and inertial-range suppression of turbulent perturbations to the magnetic-field strength. The application of these results to black hole accretion flows and the ICM will be discussed, with special attention given to observations of suppressed viscosity in ICM turbulence.
Speaker: Sara Ferry, Massachusetts Institute of Technology, USA - Chaired by: Hartmut Zohm, Associate Editor, JPP
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Date/Time: Thursday 2nd May 2024, 4PM BST/11AM EDT
Title: Fueling the emerging fusion industry: fusion fuel cycle modeling and tritium breeder blanket technology development at the MIT PSFC
Abstract: Decades of global work on fusion technology and plasma physics, coupled with a burgeoning private fusion industry, have led to a renewed optimism in the potential of magnetically confined, deuterium-tritium (DT) fueled fusion power plants (FPP). However, to unlock this potential, we must rapidly accelerate development of tritium breeding blanket technology. Due to the scarcity of tritium, it is imperative that DT-fueled FPPs breed their own tritium in lithium-containing blankets that surround the plasma. The first part of the talk will discuss the broad challenges associated with developing breeder blankets and fuel cycle systems capable of attaining tritium self-sufficiency in an FPP, as identified by the recently culminated U.S. Fusion Fuel Cycles and Breeding Blankets workshop and roadmapping process. These challenges are heightened by the fact that experimental data regarding tritium production and extraction in breeder materials is very limited, but the development timelines of private companies are quite accelerated. The second part of the talk will focus on recent results from the tritium breeding research program at the MIT Plasma Science and Fusion Center. First, I will discuss our efforts to build improved models for understanding the tritium needs of a given FPP design, including updated definitions of tritium burn efficiency and more accurate methods of incorporating tritium loss mechanisms. These models help us understand key performance targets for an FPP, predict on-site inventory, and determine whether a given design is likely to be able to sustain sufficient levels of tritium production to ensure its continued operation and the eventual startup of new plants. Second, I will provide an overview of the MIT LIBRA project, which aims to demonstrate global measurements of tritium breeding ratios approaching 1 in molten FLiBe salt exposed to a 14 MeV neutron environment. I will present the key advantages and drawbacks of a molten-salt liquid immersion breeding blanket, present results from our most recent tritium breeding campaigns, and discuss the practical experimental challenges - both past and ongoing - that the LIBRA project team is tackling as we carry out this work.
Speaker: Emilia Solano, Centre for Research on Energy, Environment and Technology, Spain - Chaired by: Per Helander, Associate Editor, JPP
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Date/Time: Thursday 25th April 2024, 4PM BST/11AM EDT
Title: L-H transition studies at JET, presented by E.R. Solano, for the JET L-H Team
Abstract: One of the most beautiful phenomena we observe in tokamaks is the transition between L and H-mode, a clear phase transition. Here we present and discuss results from dedicated L-H transition experiments at JET-ILW [ER Solano et al, Nuclear Fusion 63 (11), 112011, 2023]. Uniquely, we studied the power threshold in plasmas composed of pure Tritium, to be compared with Hydrogen and Deuterium plasmas, D+T mixtures, and H+T mixtures [G. Birkenmeier et al, PPCF 65 (5), 054001, 2023]. We show that critical pressure profiles are required for the L-H transition to occur, and such critical profiles are independent of plasma content, but the power threshold itself depends strongly on isotopic content. PLH is in fact determined by plasma transport characteristics in L-mode. Additionally, in Deuterium plasmas, an analysis of Doppler reflectometer measurements of the edge perpendicular velocity in D and He plasmas shows that there is no critical radial electric field value or critical vExB rotation before the transition [C. Silva et al, Nuclear Fusion 61 (12), 126006, 2021]. Instead, there appears to a vExB profile that is characteristic of the L-mode. The diamagnetic velocity, proportional to ∇p, is a better indicator of proximity to the L-H transition. Together, these results enable us to challenge the widely accepted model of the L-H transition being associated to the stabilisation of electrostatic turbulence by sufficient vExB shear. An alternate model of the L-H transition is presented, based on a magnetisation transition of the plasma.
Speaker: Michael Nastac, University of Oxford, UK - Chaired by: Bill Dorland, Associate Editor, JPP
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Date/Time: Thursday 18th April 2024, 4PM BST/11AM EDT
Title: Forced Vlasov-Poisson turbulence
Abstract: A theory is proposed, and confirmed by numerical simulations, of Vlasov-Poisson kinetic plasma turbulence at Debye and sub-Debye scales. This theory describes what is likely to be the “final cascade”—a universal regime to be encountered at the extreme small-scale end of any turbulent cascade in a nearly collisionless plasma. The cascaded invariant is the generalized (negative) entropy C_2, which is the quadratic Casimir invariant of the distribution function. C_2 cascades to small scales in both position and velocity space via both linear and nonlinear phase mixing, in such a way that the time scales associated with the two processes are “critically balanced” at every scale. We derive the scalings of the wavenumber spectra of C_2 and the electric field. The electric-field spectrum is sufficiently steep for the mixing to be controlled by the largest scales of the electric field, and so the C_2 cascade resembles the Batchelor cascade of a passive scalar field, albeit in the kinetic phase space. Our theory’s conclusions are corroborated by direct numerical simulations of a forced 1D-1V plasma. The phase-space cascade enables the irreversibility of particle heating to be achieved on collisionless time scales. The mean distribution function arising from this heating process is shown to be derivable from quasilinear considerations and is not a Maxwellian.
Speaker: Lucia Armillotta, Princeton University, USA - Chaired by: Antoine Bret, Associate Editor, JPP
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Date/Time: Thursday 11th April 2024, 4PM BST/11AM EDT
Title: Cosmic-ray transport and feedback in star-forming environments
Abstract: In recent years, there has been a growing interest in understanding the role of cosmic rays (CRs) in shaping galaxy evolution. MHD galaxy simulations including CRs have demonstrated that CRs can significantly contribute to the dynamics of the interstellar medium (ISM), aiding in the internal support against gravity and helping to launch galactic outflows. However, the degree to which CRs influence these phenomena is highly contingent upon the chosen CR transport prescription within the model. To address this issue, we have recently developed a transport prescription that is grounded in physical principles rather than empirical considerations, aiming for a more realistic representation of CR transport in ISM simulations. In my talk, I will outline the novelty of our model and discuss the implications of adopting this more realistic formulation for CR transport. Furthermore, I will present the application of our novel scheme in MHD simulations of galactic outflows. These simulations reveal that CRs efficiently accelerate extra-planar material if the latter is mostly warm gas, while they have very little effect on fast, hot outflows.
Speaker: Silvia Trinczek, Princeton University, USA - Chaired by: Peter Catto, Associate Editor, JPP
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Date/Time: Thursday 4th April 2024, 4PM BST/11AM EDT
Title: Neoclassical transport in strong gradient regions in tokamaks
Abstract: Theoretical descriptions of neoclassical transport in tokamaks are usually restricted to the core where the profiles of density, temperature, and radial electric field are relatively flat, compared to those in transport barriers. For example, in the pedestal, density and temperature can fall off on very short length scales and as a result a deep well forms in the profile of the radial electric field. In transport barriers, the gradient scale lengths can be of the order of the ion poloidal gyro radius. If standard, ”weak gradient”, neoclassical theory is applied to understand the properties of transport barriers, important strong gradient modifications to transport and other neoclassical quantities such as the bootstrap current are missed. Using a large aspect ratio and low collisionality expansion, we can capture gradient length scales as small as the ion poloidal gyro radius and calculate these strong gradient effects. One such effect is the poloidal variation in the electric potential, which contributes to trapping and causes transport modifications. We present a model describing the neoclassical transport of particles, momentum and energy for ions and electrons including strong gradients in the banana regime. We study the changes in the transport and the bootstrap current for some example pedestal profiles. The modifications show nonlinear dependence on the gradient strength and can enhance or decrease transport in comparison to the weak gradient neoclassical predictions.
Speaker: Andre Melzer, University of Greifswald, Germany - Chaired by: Ed Thomas, Associate Editor, JPP
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Date/Time: Thursday 28th March 2024, 3pm GMT/11am EDT
Title: Dusty plasmas under weightlessness
Abstract: Dusty (complex) plasmas provide a fascinating system to study fundamental processes in many-particle systems since the (dust) particles can be imaged and followed on the kinetic individual-particle level. Gravity is often a major problem when experimenting with dusty plasmas since, on Earth, larger dust clouds are compressed to the lower edge of the plasma. We have performed experiments on parabolic flights without the influence of the disturbing force of gravity where we have measured the dust motion in three dimensions by stereoscopic techniques. This makes new properties of the dust-plasma system accessible for measurements, such as self-excited waves, forces at the dust boundary or the dynamics of phase separation. Moreover, I will present the planned multi-user facility for complex plasma experiments COMPACT on the ISS.
Speaker: Romain Meyrand, University of Otago, New Zealand - Chaired by: Francesco Califano, Associate Editor, JPP
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Date/Time: Thursday 21st March 2024, 3pm GMT/11am EDT
Title: Reflection-driven turbulence in the super-Alfv\'enic solar wind
Abstract: In magnetized, stratified environments such as the Sun's corona and solar wind, Alfv\'enic fluctuations ``reflect'' from background gradients, enabling nonlinear interactions that allow their energy to dissipate into heat. This process, termed ``reflection-driven turbulence,'' likely plays a key role in coronal heating and solar-wind acceleration, explaining a range of detailed observational correlations and constraints. During this talk, I will present the basic physics of reflection-driven turbulence using reduced magnetohydrodynamics in an expanding box---the simplest model that can capture local turbulent plasma dynamics in the super-Alfv\'enic solar wind. Although idealized, our high-resolution simulations and simple theory reveal a rich phenomenology that is consistent with a diverse range of observations. Outwards-propagating fluctuations, which initially have high imbalance (high cross helicity), decay nonlinearly to heat the plasma, becoming more balanced and magnetically dominated. Despite the high imbalance, the turbulence is strong because Els\"asser collisions are suppressed by reflection, leading to ``anomalous coherence'' between the two Els\"asser fields. This coherence, together with linear effects, causes the growth of ``anastrophy'' (squared magnetic potential) as the turbulence decays, forcing the energy to rush to larger scales and forming a ``$1/f$-range'' energy spectrum in the process. Eventually, expansion overcomes the nonlinear and Alfv\'enic physics, forming isolated, magnetically dominated ``Alfv\'en vortices'' with minimal nonlinear dissipation. I will discuss, how these results can plausibly explain the observed radial and wind-speed dependence of turbulence imbalance (cross helicity), residual energy, fluctuation amplitudes, plasma heating, and fluctuation spectra, as well as making a variety of testable predictions for future observations.
Speaker: Gary Zank, University of Alabama, Huntsville - Chaired by: Thierry Passot, Associate Editor, JPP
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Date/Time: Thursday 14th March 2024, 3pm GMT/11am EDT
Title: Turbulent heating of the solar corona and acceleration of the solar wind: Insights, theory, and modeling from NASA Parker Solar Probe and ESA Solar Orbiter missions
Abstract: The problem is easily stated and yet has endured for longer than the space age: the surface temperature of the Sun is a mere 6,500 degrees Kelvin yet the temperature of the solar corona exceeds 1 million degrees, hot enough to create a supersonically expanding wind that fills interplanetary space. What heats the solar corona? Sixty five years after the discovery of the solar wind, this remains one of the most outstanding unanswered question in space physics. The launches of ESA’s Solar Orbiter and NASA’s Parker Solar Probe spacecraft were designed to answer this question, venturing closer to the surface of the Sun than any previous missions. Parker Solar Probe has now made multiple excursions below the Alfven surface, during encounters 8 – 14, which for the first time has allowed us to observe the properties of the sub-Alfvenic solar wind that in principle is in direct contact with the surface of the Sun. The Parker Solar Probe and Solar Orbiter missions have motivated considerable development in theory and modeling to explain the heating of the solar corona. An emerging consensus is that the solar corona is heated via the transport and dissipation of low-frequency magnetohydrodynamic turbulence. Recent measurements from Parker Solar Probe and observations by the Metis instrument on the Solar Orbiter spacecraft suggest that interchange reconnection of magnetic field lines in the lower solar corona generates turbulence that can yield sufficient energy, likely in the form of advected nonlinear magnetic structures, to heat the solar corona and thus drive the solar wind. Current solar turbulence theory and modeling will be presented in the context of relevant Parker Solar Probe and Solar Orbiter observations to suggest that we may have a solution to the 65-year-old question of the origin of the supersonic solar wind.
Speaker: Peter Norreys, University of Oxford, USA - Chaired by: Luis Silva, Associate Editor, JPP
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Date/Time: Thursday 7th March 2024, 4pm GMT/11am EST
Title: Fundamental Physics Opportunities with Multi-PetaWatt- and Multi-MegaJoule-class Facilities
Abstract: In my talk, I will present some recent highlights from my research group in the Clarendon Laboratory, obtained working in partnership with many outstanding international collaborators. These fall under the three broad themes. The first is novel laser-plasma interactions, which include those resulting from twisted light, ionisation injection for laser- and beam-driven wakefield accelerators, along with frequency domain holographic and compressive sensing diagnostic developments relevant to future colliders. The second theme is that of extreme field physics using multi-petawatt laser facilities. These include first principles ionisation processes, absorption processes and ultra-bright attosecond X-rays from high harmonic generation, photon-photon scattering in the non-linear QED regime and, most recently, gravitational wave generation using petaWatt- and/or megaJoule-class facilities. We are now extending these studies to those of gravity-wave detection, with promising preliminary results. The third theme is that of inertial fusion studies. All of these studies indicate that an international, dual-use, 20-MJ ICF/IFE facility, with the first 2-MJ at high repetition rate supplying single-shot high energy amplifiers, will open many new exciting avenues for both fundamental physics and high energy density science in the decades ahead.
Speaker: Arianna Gleason, Stanford University, USA - Chaired by: Louise Willingale, Associate Editor, JPP
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Date/Time: Thursday 29th February 2024, 4pm GMT/11am EST
Title: From planetary interiors to harnessing star power: exploring the frontier matter at extreme conditions.
Abstract: The study of matter under extreme conditions is a highly interdisciplinary subject with broad applications to materials science, plasma physics, geophysics and astrophysics. Understanding the processes which dictate physical properties in warm dense plasmas and condensed matter, requires studies at the relevant length-scales (e.g., interatomic spacing) and time-scales (e.g., phonon period). Experiments performed at XFEL lightsources across the world, combined with dynamic compression, provide ever-improving spatial- and temporal-fidelity to push the frontier. This talk will cover a very broad range of conditions, intended to present an overview of important recent developments in how we generate extreme environments and then how we characterize and probe matter at extremes conditions– providing an atom-eye view of transformations and the fundamental physics dictating plasma and materials properties. Examples of case-studies closely related to planetary sciences and inertial fusion energy, as enabled by ultrafast X-ray imaging, diffraction and spectroscopy, will be discussed.
Speaker: Vinícius Duarte, Princeton Plasma Physics Laboratory, USA - Chaired by: Dmitri Uzdensky, Associate Editor, JPP
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Date/Time: Thursday 15th February 2024, 4pm GMT/11am EST
Title: Formulation of a self-consistent reduced transport theory for discrete modes-with applications to resonant dynamics in plasmas and galaxies
Abstract: Resonances, though delicate fine structures in phase space, can mediate massive transport of particles in plasmas. In this colloquium, it will be shown that a quasilinear plasma transport theory that incorporates Fokker-Planck dynamical friction (drag) and pitch angle scattering self-consistently emerges from first principles for an isolated, marginally unstable mode resonating with an energetic minority species. It is found that drag fundamentally changes the structure of the wave-particle resonance, breaking its symmetry and leading to the shifting and splitting of resonance lines. In contrast, scattering broadens the resonance in a symmetric fashion. Comparison with fully nonlinear simulations shows that the emergent quasilinear system preserves the exact instability saturation amplitude and the corresponding particle redistribution of the fully nonlinear theory. Even in situations in which drag leads to a relatively small resonance shift, it still underpins major changes in the redistribution of resonant particles. This novel influence of drag is equally important in plasmas and self-gravitating systems. In fusion plasmas, the effects are especially pronounced for fast-ion-driven instabilities in tokamaks with low aspect ratio or negative triangularity, as evidenced by past observations. The same theory directly maps to the resonant dynamics of the rotating galactic bar and massive bodies in its orbit, providing new techniques for analyzing galactic dynamics.
Speaker: Michael Fitzgerald, United Kingdom Atomic Energy Authority, UK - Chaired by: Dmitri Uzdensky, Associate Editor, JPP
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Date/Time: Thursday 8th February 2024, 4pm GMT/11am EST
Title: Burning plasma physics studies on JET
Abstract: The era of burning magnetically confined plasmas is on the horizon. The optimization of plasma scenarios for stability and performance will have to be re-learned when a large population of alpha particles drives kinetic instabilities and produces the majority of the heating. A primary mission of the JET tokamak was to successfully confine alpha particles to allow the study of their behaviour, with JET being one of only two tokamaks to have ever operated with deuterium-tritium (DT) fuelling. In this talk, we discuss recently published results from JET experiments that give glimpses into future alpha particle effects, such as destabilization of Alfven eigenmodes, electron heating, and losses.
Speaker: Siyao Xu, University of Florida, USA - Chaired by: Roger Blandford, Associate Editor, JPP
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Date/Time: Thursday 1st February 2024, 4pm GMT/11am EST
Title: Mirror diffusion of cosmic rays in MHD turbulence
Abstract: Diffusion of cosmic rays (CRs) in magnetized and turbulent plasmas is a long-standing and challenging problem. The recent explosive interest in understanding CR diffusion arises from its broad impacts on plasma physics, space physics, and astrophysics. However, the existing CR diffusion theories in general face severe difficulties when explaining new-generation CR observations, making it more urgent to reexamine the CR diffusion mechanisms. Very recently, the progresses in understanding and modeling MHD turbulence lead to intense research effort toward a paradigm shift of CR diffusion. In this talk, I will focus on one of the new diffusion mechanisms, the mirror diffusion, which is induced by large-scale magnetic compressions, i.e., magnetic mirrors, in MHD turbulence. I will present its basic physical picture, our theoretical predictions on the diffusion behavior, and our numerical testing with test particle simulations. I will also briefly discuss its implications on astrophysical observations and the IBEX Ribbon.
Speaker: Robin Greif, University of Oxford, UK - Chaired by: Nuno Loureiro, Associate Editor, JPP
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Date/Time: Thursday 25th January 2024, 4pm GMT/11am EST
Title: Physics-preserving AI-Accelerated simulations of plasma turbulence
Abstract: Turbulence in fluids, gases, and plasmas remains an open problem of both practical and fundamental importance. Its irreducible complexity usually cannot be tackled computationally in a brute-force style. Combining Large Eddy Simulation (LES) techniques with Machine Learning (ML) allows us to only retain the largest dynamics explicitly, while small-scale dynamics are described by an ML-based sub-grid-scale model. Applying this novel approach to self-driven plasma turbulence allows us to remove large parts of the inertial range, reducing the computational effort by about three orders of magnitude, all while retaining the full statistical description of the turbulent systems' physical properties.
Speaker: Thomas White, University of Nevada, Reno, USA - Chaired by: Luis Silva, Associate Editor, JPP
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Date/Time: Thursday 18th January 2024, 4pm GMT/11am EST
Title: Dynamic and transport properties in warm dense matter
Abstract: The concept of warm dense matter (WDM) emerged over two decades ago, delineating a state of matter existing under extreme conditions—typically at pressures exceeding 100 GPa and concurrently at temperatures ranging between 104–106 K. WDM lies at the intersection of solids and plasmas, and displays a hybrid set of characteristics from both states. In this unique region of phase space, matter attains a state of partial degeneracy and ionization, fostering intricate correlations among charged particles that also exhibit considerable quantum mechanical behaviors. Its intricate nature challenges conventional perturbative techniques, resulting in considerable disparities between theoretical predictions and computational models. In particular, ion transport properties, which play a crucial important for planetary physics, show some of the largest variation with few experimental results. Scattering experiments, utilizing high-intensity beams of penetrating radiation to probe micro-structure and dynamics, represent a pivotal tool for investigating particle dynamics in this regime. Our team recently showcased a groundbreaking inelastic X-ray scattering (IXS) technique for use at X-ray free electron lasers, offering energy resolutions down to 50 meV. This high-resolution technique enables unprecedented access to ion dynamics in plasmas, where the ions typically exhibit much smaller energy shifts than the electrons. In the collective regime, we have measured acoustic waves in warm dense methane, directly determining the sound speed at planetary interior conditions. In the single-particle regime, we have pioneered a direct measurement of ion temperatures in dense plasmas, unveiling new insights into electron-ion temperature exchange. In this presentation, I will provide an overview of our current understanding of transport and dynamics in WDM, spotlighting recent experimental results and discussing future directions.
Speaker: Alfred Mallet, Space Sciences Laboratory, University of California, Berkeley - Chaired by: Alex Schekochihin, Editor, JPP
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Date/Time: Thursday 11th January 2024, 4pm GMT/11am EST
Title: Breaking the Alfvén wave: harder than it looks
Abstract: The Alfvén wave is ubiquitous in the solar wind. In particular, NASA’s Parker Solar Probe, currently exploring the outer edges of the corona, has revealed frequent patches of extremely large-amplitude waves (∆B/B ~1), often even locally reversing the direction of the background magnetic field - these structures have therefore been dubbed “switchbacks”. Often, switchbacks also exhibit extremely sharp boundaries, with large magnetic field rotations over only a few ion inertial lengths. I will attempt to answer several questions suggested by these new observations: How do these waves attain such large amplitudes while maintaining their Alfvénic character? How do they develop such sharp discontinuities? And how are these discontinuities maintained? I will show that the Alfvén wave is surprisingly resilient, and is able to survive unscathed at very large amplitude and even at relatively small scales. These results have interesting implications for the heating of the corona and solar wind by imbalanced Alfvénic turbulence.
Speaker: Mickael Grech, Ecole Polytechnique, Paris - Chaired by: Antoine Bret, Associate Editor, JPP
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Date/Time: Thursday 21st December 2023, 4pm GMT/11am EST
Title: Electromagnetic QED cascades: showers, avalanches, electron-positron pair plasmas and future experiments
Abstract: Several facilities worldwide aim to harness the unique properties of multi-petawatt laser and/or conventional accelerators to probe strong-field quantum electrodynamics (SF-QED). Among the most important and exotic predictions of SF-QED, prolific electron-positron pair generation via so-called electromagnetic cascades has attracted much attention as it may lead to the production of pair plasmas in the laboratory. In this work, we discuss two types of QED cascades that may be achieved in a foreseeable future on multi-petawatt laser facilities. Shower-type cascades can develop in the collision of an ultra-intense laser pulse with a relativistic electron or photon beam, and draw their energy from the incident (seeding) particles. They can develop under conditions that will soon be achievable at laser facilities such as Apollon, KoReLS or on facilities coupling a high-energy electron beam with a moderately intense laser (see e.g. the FACET-II or LUXE experiments). If seeded with high-energy photon beams, such cascades could lead to the first signature of pure light-by-light pair production. In contrast, avalanche-type (or self-sustained) cascades require that the produced electrons and positrons keep being accelerated by an external (background) field so that the loop of high-energy photon emission (nonlinear Compton scattering) and pair production (Breit-Wheeler) can be sustained. This requires quite large intensities (typically beyond 1024 W/cm2 for an optical background field) as well as special configurations of the electromagnetic field. The rotating field configuration is the typical toy model configuration for self-sustained cascades. These cascades draw energy from the background field and can lead to prolific pair production as the number of pairs increases exponentially with time. This process provides the most likely path toward the production of pair plasmas of astrophysics relevance. This talk will present analytical models for both type of cascades and their careful benchmarking against Particle-In-Cell simulations performed with the open-source, high-performance code Smilei. Particular attention will be paid to predicting the number of generated pairs in either photon- or electron-seeded showers, and the onset and growth rate of self-sustained cascades. Guidelines for future experiments will be given, focusing in particular on the Apollon and ELI laser facilities.
Speaker: Suying Jin, Princeton University, USA - Chaired by: Troy Carter, Associate Editor, JPP
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Date/Time: Thursday 14th December 2023, 4pm GMT/11am EST
Title: Structure formation in magnetohydrodynamic turbulence as a modulational instability
Abstract: Structure formation in magnetohydrodynamic (MHD) turbulence is a problem that has long been of interest, for example, in the context of turbulent dynamo and other types of self-organization of astrophysical and laboratory plasmas. Its onset can be approached as an instability of a statistical equilibrium with respect to a small perturbation. Such modulational instabilities (MIs) can be described much like familiar linear plasma instabilities in kinetic theory, except the role of mean fields there is played by coherent structures (such as an average magnetic field), and the role of plasma particles is played by the turbulent background fluctuations. Although local closures for the electromotive force are the current standard in mean-field dynamo theories, scale separation is far from guaranteed and quantumlike wave-kinetic theory, generally known as Wigner-Moyal kinetics, is needed instead. In the past, this framework has proven fruitful for studies of drift-wave and other types of turbulence, even within the quasilinear approximation (QLA), or collisionless-wave approximation. We show that although quasilinear Wigner-Moyal kinetics yields quantitatively accurate predictions for MHD in some regimes, remarkably, in adjacent regimes QLA can be inaccurate even qualitatively. This raises the question: what is the cause of such dramatic variations? We explore this question within two-dimensional incompressible MHD assuming simple nonlinear backgrounds that allow for a tractable model without truncating the modulational spectrum. We find that, although MI modes are typically assumed to be exponentially localized in the spectral space, i.e. have the form of evanescent spectral waves, this assumption is violated for some parameters. Then, MI modes become propagating spectral waves (PSWs) instead. PSWs have a constant amplitude and thus provide efficient ballistic energy transport down the modulational spectrum, something that is not captured by a typical wave-wave collision operator. Once transients have dissipated, PSWs are self-maintained as global modes with specific real frequencies and drain energy from the primary structure at a constant rate until the primary structure is depleted. The presence of PSWs thereby introduces dissipation to almost any nonlinear MHD wave even at arbitrarily small viscosity; this suppresses MIs in regimes where QLA would predict instabilities. Although PSWs cannot be captured by naive truncations, it may be possible to accommodate them with a proper closure. Finally, we find that introducing dispersion or dissipation constrains the scaling of modulational modes, suppressing PSWs and reinstating the accuracy of QLA. In this sense, ideal MHD is a "singular" system that is particularly sensitive to the accuracy of the closure within mean-field models.
Speaker: David Hatch, Institute for Fusion Studies, University of Texas at Austin - Chaired by: Troy Carter, Associate Editor, JPP
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Date/Time: Thursday 7th December 2023, 4pm GMT/11am EST
Title: New Strategies for Dramatically Increasing Confinement in Magnetic Fusion Energy
Abstract: The projected size and cost of a fusion power plant is strongly dependent on plasma confinement. In most magnetic fusion devices, the confinement time is determined by small scale turbulent transport of heat and particles. This turbulence can be suppressed under certain conditions, resulting in enhanced confinement regimes with enormous gradients of temperature and density. I will present a fundamental picture of the physical mechanisms by which such extreme conditions are thermodynamically viable. This fundamental understanding points to strategies by which transport barriers can be controlled and optimized. I will also describe numerical investigations interpreting wide-ranging experimental observations of transport barriers in tokamaks. One of the most pressing challenges for a fusion power plant is the conflicting demands of heat exhaust and confinement. I will introduce a new solution to this conundrum: a divertor concept that works in synergy with transport barriers to unlock even greater gains in confinement. If this can be achieved, it would enable much smaller and cheaper fusion devices than typically envisioned.
Speaker: Daniel Kennedy, United Kingdom Atomic Energy Authority, UK - Chaired by: Alex Schekochihin, Editor, JPP
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Date/Time: Thursday 30th November 2023, 4pm GMT/11am EST
Title: Electromagnetic Instabilities in Spherical Tokamaks
Abstract: Electromagnetic microinstabilities are likely to dominate transport in high β next generation spherical tokamaks (STs) such as STEP. While gyrokinetic (GK) simulations have thus far proven to be a very accurate tool in modelling turbulent transport in predominantly electrostatic regimes at low β, obtaining saturated nonlinear simulations of plasmas with unstable kinetic ballooning modes (KBMs) and microtearing modes (MTMs) has proven computationally and conceptually challenging. However, recent investigations that retain only MTMs and exclude KBMs (by neglecting compressional perturbations) reveal strong sensitivity of the heat flux to parallel dissipation and velocity space resolution. With sufficient resolution or dissipation to avoid numerical instabilities, recent MTM-only simulations saturate cleanly at much reduced electron heat flux. We find that including compressibility can unleash a KBM-like instability which drives very large heat fluxes (orders of magnitude greater than the available heating power). The saturated fluxes are sensitive to equilibrium flow shear, and at mid-radius diamagnetic levels of flow shear can reduce the fluxes to much lower values.
Speaker: Thomas Grismayer, Higher Technical Institute, Lisbon - Chaired by: Antoine Bret, Associate Editor, JPP
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Date/Time: Thursday 16th November 2023, 4pm GMT/11am EST
Title: Plasma Physics at the higher energy and QED frontier
Abstract: Plasma physics generally involves well-known physics of electromagnetism, special relativity and statistical mechanics. Various descriptions of the plasma state (kinetic, fluid, MHD) have been developed and each of them have a specific scope that depends on time/length scales, chemical composition, density, and temperature. Whereas these descriptions are well established, we could wonder when quantum electrodynamics (QED) comes into play. The answer lies in the threshold of QED cross sections which become non negligible for certain critical lengths, energies and fields. The presence of ultra-strong electromagnetic field and relativistic temperatures modifies the underlying basic physics to such a great extent that relying on classical plasma physics is often not justified. The zoo of QED processes implies emission of hard photons, photon-lepton collisions, non-constant number of particles (creation and annihilation), stochastic particle orbits, which have stimulated the community for constructing a QED-based plasma physics. While the state of development of QED-based plasma physics theory is still far from being as mature as that of the classical plasma theory, lots of progresses in this area has been achieved in the past few decades. These advances have implications in many astrophysical and laboratory scenarios (associated to new PetaWatt class lasers), that share common underlying microphysics and collective plasma effects associated with intense fields. This new physics is highly nonlinear and multi scale, and require a combination of theory and large scale numerical simulations. The main challenges in this area with be presented and the recent progresses triggered by large scale simulations of compact objects and of multi dimensional laser/beam-plasma interactions in the presence of fields close to the critical Schwinger field will be discussed, emphasising the interplay between collective plasma dynamics and QED processes.
Speaker: Lia Hankla, University of Colorado, Boulder - Chaired by: Dmitri Uzdensky, Associate Editor, JPP
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Date/Time: Thursday 9th November 2023, 4pm GMT/11am EST
Title: Kinetic and Two-Temperature Physics of Black Hole Accretion Disks and X-ray Coronae
Abstract: Understanding the plasma physics of accretion disks and coronae around black holes is crucial for interpreting the radiation observed from these systems. However, these plasmas span several different physical regimes. They can be highly collisional and well-described by a single temperature, or collisionless with nonthermal particles that have been accelerated to high energies. My work brings small-scale kinetic and two-temperature physics into the global setting of the accretion disk/corona system. I first use particle-in-cell (PIC) simulations to understand turbulence and particle acceleration in a collisionless, magnetized plasma. Next, I build an analytic model using prescriptions from PIC simulations to demonstrate an observable power-law from within the plunging region of a black hole. Finally, I use general relativistic magnetohydrodynamic (GRMHD) simulations to examine the impact of two-temperature physics on the radial structure of the full accretion disk.
Speaker: Pasquale Blasi, Gran Sasso Science Institute, Italy - Chaired by: Edward Thomas, Associate Editor, JPP
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Date/Time: Thursday 26th October 2023, 4PM BST/11AM EDT
Title: Cosmic ray self-confinement around their sources
Abstract: The large density and density gradients that cosmic rays have around their sources suggest that they may have a dynamical role in such regions and may be able to excite instabilities capable of self-confining such particles in the circus-source regions. I will discuss the important implications that this phenomenon has in the surroundings of supernovae and pulsar wind nebulae, where we can potentially observe the effects of self-confinement, in terms of suppressed diffusivity. I will also discuss self-confinement in the context of Galactic Cosmic Rays escaping our Galaxy and ultra high energy cosmic rays escaping their sources.
Speaker: Guilhem Dif-Pradalier, CEA, Cadarache, France - Chaired by: Per Helander, Associate Editor, JPP
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Date/Time: Thursday 19th October 2023, 4PM BST/11AM EDT
Title: Shear dynamics at the edge of magnetised fusion plasmas
Abstract: Transport bifurcations are essential to fusion performance and mostly originate in a narrow, peripheral region of the confined plasma, the ‘edge’. This region however is especially challenging to model and to understand. The plasma edge is home to large, concentrated shear and to steep gradients. As turbulent scales become comparable to free energy gradient scales, oft-assumed scale separations in turbulence models break down, promoting the use of more computationally-intensive ‘flux-driven’ frameworks. Sources and boundary conditions play an important role as the plasma edge sits at the confluence of conflicting dynamics, notably outgoing heat and particle fluxes from the confined core and incoming particle or momentum fluxes from the unconfined Scrape-Off Layer (SOL). Through magnetic connection to the material boundaries, sheath physics also strongly influences SOL dynamics, which in turn constrains electric field (and thus shear) dynamics in the plasma edge. We propose to review some of the key features above, some recent results and passage points that have been obtained in view of a consistent flux-driven description of the plasma edge. In particular, we will discuss evidence that magnetic connection with the material boundaries deeply modifies the turbulence drive in the edge, leading to a global reorganisation of fluctuations and the onset of a modest yet stable transport barrier close to the magnetic separatrix. We will briefly touch upon the mechanisms and causality behind electric field and shear (vorticity) production in the edge, highlighting the special importance of diamagnetic fluctuations. We will question the electric field dependence on plasma current at the edge and confront calculations to experimental measurements. The last part of the talk will be concerned with describing sheath physics within our kinetic framework and steps that have been undertaken in view of incorporating this physics into a comprehensive (gyro)kinetic turbulence model.
Speaker: Justin Ball, EPFL, Switzerland - Chaired by: Hartmut Zohm, Associate Editor, JPP
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Date/Time: Thursday 12th October 2023, 4PM BST/11AM EDT
Title: Theoretical studies towards a negative triangularity tokamak power plant
Abstract: Experimental observations show that negative triangularity plasma shaping can significantly improve the energy confinement time of tokamaks. Moreover, unlike the standard positive triangularity shape, negative triangularity plasmas typically cannot access H-mode. Together these two facts may enable an attractive power plant design – the plasma can be heated to reactor-relevant conditions while remaining in L-mode to avoid the material survivability concerns associated with ELMs, yet still achieve sufficiently good confinement for high fusion gain. This potential has motivated the creation of EUROfusion’s Theory, Simulation, Verification, and Validation (TSVV) project on negative triangularity, which is investigating the feasibility of a negative triangularity power plant. In this talk, we will synthesize the most important results including the physical reasons behind the confinement time improvement, how performance scales to new parameter regimes (like spherical tokamaks), insights from reduced transport modeling, the scrape-off layer width, and more. We will connect these results to recent experiments and comment on the prospects for a negative triangularity power plant.
Speaker: Oreste Pezzi, Institute for Plasma Science and Technology, Italy - Chaired by: Tunde Fulop, Associate Editor, JPP
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Date/Time: Thursday 5th October 2023, 4PM BST/11AM EDT
Title: The role of large-scale plasma turbulence in influencing particle transport and energization in astrophysical contexts
Abstract: Populations of energetic particles, ranging from solar energetic particles to incredibly high-energy cosmic rays, are ubiquitous in space and astrophysical plasmas. Several intertwined phenomena, including shocks, magnetic reconnection, and turbulence, are responsible for the efficient acceleration of particles and for determining their transport properties for efficient particle energization. Plasma turbulence produces patchy coherent structures, such as reconnecting current sheets, plasmoids, and vortices across a vast range of spatial scales. Under some circumstances, these structures can entrap particles, thus providing fast energization through, for example, drift acceleration. In this talk, I will review some of these mechanisms and outline recent numerical efforts aimed at investigating how the large-scale complexity of the turbulent magnetic field influences particle transport and how large-scale coherent structures impact particle energization. I will also comment on the applicability of these results in space and astrophysical contexts.
Speaker: Antonino Di Piazza, University of Rochester, USA - Chaired by: Luis Silva, Associate Editor, JPP
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Date/Time: Thursday 28th September 2023, 4PM BST/11AM EDT
Title: Flying focus beams as a tool to investigate strong-field physics
Abstract: In a flying focus beam (FFB) the velocity of the focus can be “programmed” and it is independent of the group and the phase velocity of the beam itself. Recent experiments have demonstrated a moving focus over centimeter lengths, i.e., much longer than the Rayleigh length. Scaling this technology to higher power laser pulses would allow one to employ FFBs as a tool for fundamen[1]tal high-field physics, especially to investigate effects that accumulate with the interaction length. Specifically, by considering an ultrarelativistic electron beam counterpropagating with respect to a FFB, whose focus copropagates with the electrons at the speed of light, we show that the effects of the so-called transverse formation length of radiation on the radiation itself can be enhanced as compared to the case of a conventional Gaussian beam. Analogously, radiation-reaction and vacuum-polarization effects can be rendered measurable at much lower intensities than conventionally required in a similar setup. Finally, we show how FF beams with angular momentum can be an efficient tool to transport ultrarelativistic electron beams over macroscopic distances without significantly spreading on the transverse plane.
Speaker: Eric Sonnendrucker, IPP Garching, Germany - Chaired by: Bill Dorland, Editor, JPP
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Date/Time: Thursday 21st September 2023, 4PM BST/11AM EDT
Title: Geometric numerical methods
Abstract: Neglecting collisions and other dissipative effects, many models of plasma physics including kinetic, fluid, MHD and hybrid models have been shown to possess a noncanonical hamiltonian structure. This comes with some important properties in particular conservation of energy and some Casimir invariants, typically div B = 0 and also Gauss’ law for the Vlasov-Maxwell equations. Adequate conservation of these quantities has been proven to be essential for well behaving numerical solutions. Many numerical methods have been devised to this aim. Geometric numerical methods achieve this by discretizing the Hamiltonian structure, i.e. Poisson bracket and Hamiltonian, rather than the resulting PDEs. This approach approximates the infinite dimensional original hamiltonian system by a Finite Dimensional hamiltonian system and in this way guarantees the conservation of the appropriate discretized invariants. After introducing the concept of Geometric discretization, we will illustrate it on a hamiltonian formulation of the Vlasov-Maxwell model and hybrid fluid-kinetic models. We will also extend geometric numerical methods to dissipative physical systems by adding to the hamiltonian part of the model a dissipative part as for example the Landau collision operator to the Vlasov-Maxwell model in the form of a dissipative symmetric bracket yielding altogether a so-called metriplectic formulation.
Speaker: Arno Vanthieghem, Princeton University, USA - Chaired By: Dmitri Uzdensky, Associate Editor, JPP
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Date/Time: Thursday 14th September 2023, 4PM BST/11AM EDT
Title: Towards a unified picture of energy partition in unmagnetized collisionless shock waves
Abstract: Collisionless shock waves shape the nonthermal emission in a wide range of environments, including modern laboratory experiments and astrophysical outflows. In weakly magnetized plasma flows, self-generated nonlinear electromagnetic plasma processes are inferred to heat and accelerate electrons and ions. Understanding the mechanisms that underpin the energy transfer between plasma species and the downstream temperature ratio between electrons and ions constitutes a fundamental challenge in modeling such blast waves. In this talk, I will outline recent theoretical efforts to model the transport of electrons in Weibel-mediated shocks. I will introduce a new model accounting for electron heating in an ambipolar-type process through the interplay between pitch-angle scattering in the microturbulence and the coherent electrostatic field induced by the difference in inertia between species. Via analytical kinetic and fluid estimates, a semi-analytical Monte Carlo-Poisson method, and large-scale ab-initio Particle-In-Cell simulations, I will discuss the electron-ion energy partition in the downstream of high Alfvén Mach numbers shocks relevant to supernova remnants and laboratory experiments. I will then present the extension of this model to the relativistic regime to demonstrate equipartition inferred from the afterglow emission of gamma-ray bursts. Finally, I will explore the implications of this model on electron injection in nonthermal distributions.
Speaker: Mervi Mantsinen, Barcelona Supercomputing Center, Spain - Chaired by: Tunde Fulop, Associate Editor, JPP
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Date/Time: Thursday 7th September 2023, 4PM BST/11AM EDT
Title: ICRF heating and ICRF-accelerated fast ions in tokamaks: Modelling and theory-to-experiment comparisons
Abstract: Heating with waves in the ion cyclotron range of frequencies (ICRF) is a well-established method on present-day tokamaks and is envisaged as one of the main heating techniques for ITER. The reference ICRF heating scheme for ITER deuterium-tritium plasmas at the full magnetic field of 5.3 T is the second harmonic heating of tritium. Its heating mechanism is a finite Larmor radius effect, i.e. it takes place when the Larmor radius of the resonant tritons are finite with respect to the wave length of the launched wave. The physics of such a scheme can be studied on present-day fusion devices using second or higher-harmonic ICRF heating scenarios together with available diagnostics to compare experimental results with the predictions of ICRF modelling codes. In addition to providing challenging benchmarks between experiments and modelling, such schemes can also be used for fast ion physics studies and for testing alpha particle diagnostics in preparation of ITER. Representative examples from present-day experiments and the underlying physics are reviewed.
Speaker: Meirielen Caetano De Sousa, École Polytechnique, France - Chaired By: Nuno F. G. Loureiro, Associate Editor, JPP
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Date/Time: Thursday 31st August 2023, 4PM BST/11AM EDT
Title: Electron acceleration by a two stage laser-plasma interaction
Abstract: Ultraintense, short pulsed lasers with high contrast allow the exploration of new mechanisms for particle acceleration. In particular, when such lasers irradiate an overdense plasma, electromagnetic waves appear at the vacuum-plasma interface, enabling local field confinement and enhancement. Since these mechanisms involve an overdense plasma, high values of total electron beam charge (~nC) can be achieved, which is important for applications in the fields of plasma-based accelerators, electron sources, among others. Recent Particle-in-Cell (PIC) simulations analyzed a laser pulse interacting with the wedge of an overdense plasma, and showed that it produces a diffracted electromagnetic wave with an intense longitudinal electric field that efficiently accelerates collimated electron bunches with high charge (~nC) and relativistic energies up to ~130 MeV. In this talk, I will present a new scheme which couples electron acceleration in an overdense plasma with the wakefield produced by the laser propagating through an underdense plasma. Highly charged electron bunches are emitted from the overdense plasma, and trapped by the wakefield in which they are accelerated even further. Such a scheme enables us to obtain highly charged and highly energetic electron bunches at the end of the second acceleration stage.
Speaker: Philip Snyder, Oak Ridge National Laboratory, USA - Chaired by: Troy Carter, Associate Editor, JPP
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Date/Time: Thursday 24th August 2023, 4PM BST/11AM EDT
Title: Physics of the Tokamak Pedestal, and Implications for Magnetic Fusion Energy
Abstract: Fusion, the process that powers the stars, offers the potential to provide plentiful clean energy here on earth. Magnetic confinement of high temperature plasmas in toroidal devices known as tokamaks is a promising approach to fusion, one that has successfully produced millions of watts of fusion power. High performance in tokamaks is achieved via the spontaneous formation of a transport barrier in the outer few percent of the confined plasma. This narrow insulating layer, referred to as a “pedestal,” typically results in a >30x increase in pressure across a 0.4-5cm layer. The overlap of drift orbit, turbulence, and equilibrium scales across this narrow layer leads to rich and complex physics, and challenges traditional analytic and computational approaches. Development of high resolution diagnostics, and coordinated experiments on several tokamaks, have validated understanding of important aspects of the physics, while highlighting open issues. A predictive model (EPED) has proven capable of predicting the pedestal height and width to ~20-25% accuracy in large statistical studies. This model was used to predict a new, high pedestal “Super H-Mode” regime, which was subsequently discovered on DIII-D, leading to high fusion performance, and motivated experiments on Alcator C-Mod which achieved world record, reactor relevant pedestal pressure. Coupling this new understanding of pedestal physics to models of the core plasma and open field line region enables extensive optimization of global fusion performance in concert with desired exhaust conditions, including development of attractive scenarios for a fusion pilot plant.
Speaker: William Matthaeus, University of Delaware, Delaware, United States - Chaired By: Alex Schekochihin, Editor, JPP
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Date/Time: Thursday 17th August 2023, 4PM BST/11AM EDT
Title: 1/f noise in the solar wind: connections to dynamo, inverse cascade and causality
Abstract: I review the observations of 1/f noise in the heliosphere and discuss the theoretical background of generic 1/f noise models as they may be relevant to the solar wind. The planning for Parker Solar Probe emphasized that understanding 1/f “noise” in the solar wind may inform central problems in heliospheric physics such as the solar dynamo, coronal heating, the origin of the solar wind, and the nature of interplanetary turbulence [1]. It is of significance that 1/f noise can occur in a great variety of physical systems ranging from Johnson noise in vacuum tubes and semiconductors, fluctuations in heart rates, brain waves sandpiles [2], and solids [3], the spectrum of music and economic indicators, and many other examples. An appealing generic mechanism that can explain many of these phenomena is the superposition principle [4,5] in which a composite signal is formed from a collection of individual signals that are characterized by scale-invariant correlation times. In the solar wind context, it was proposed [4] that such a superposition might occur in the corona due to scale-invariant reconnections, thus explaining the observed 1/f signal in the interplanetary magnetic field that spans the approximate frequency range 2 x 10^-6 Hz to several times 10^-4 Hz. at 1 au. Subsequent observational analyses showed that signals in the photosphere and the corona [5,6] exhibit 1/f signatures at frequency ranges compatible with the 1 au observations. This suggests an even lower altitude origin in the dynamo itself and this possibility is supported by both dynamo experiments and simulations [7]. There is also empirical evidence that inverse cascade systems (including dynamo) generically exhibit 1/f noise, possibly due to nonlocality of interactions at the largest scales. Recent interest in employing Parker Solar Probe observations to address whether interplanetary 1/f noise can be generated by local dynamics [as proposed e.g., in 8, 9,10] is also reviewed. Here we present theoretical arguments that due to causality issues, interplanetary 1/f noise extending to very low frequencies is not explained by the recent observations.
Speaker: Yohei Kawazura, Tohoku University, Japan - Chaired by: Alex Schekochihin, Editor, JPP
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Date/Time: Thursday 10th August 2023, 4PM BST/11AM EDT
Title: Inertial range of magnetorotational turbulence
Abstract: Accretion of matter onto a compact object is triggered by turbulence driven by magnetorotational instability (MRI). While there have been numerous theoretical and computational studies on MRI turbulence in the past few decades, the inertial range of MRI turbulence remains poorly understood. For instance, both the magnetic and kinetic energy spectra deviate significantly from those predicted by theories of MHD turbulence and those observed in simulations of externally driven MHD turbulence. This discrepancy arises due to the fact that the energy injection via MRI happens broadly in a Fourier space, posing challenges for numerically resolving the inertial range. In this colloquium, I will present the numerically resolved inertial range of MRI turbulence using Rotational Reduced MHD (RRMHD) model. This model assumes the presence of a near-azimuthal mean magnetic field and considers small amplitude turbulent fluctuations elongated along the mean magnetic field. By making these assumptions, RRMHD allows us to reach the inertial range with minimal computational resources. Our findings include: (1) both magnetic and kinetic energy exhibit -3/2 spectra, (2) Alfvenic fluctuations and slow-mode-like compressive fluctuations are decoupled at small scales, and (3) compressive fluctuations are approximately twice as strong as Alfvenic fluctuations. Furthermore, we will show our ultra-high resolution simulation of MRI turbulence using standard MHD and demonstrate that the small-scale spectra closely resemble those of RRMHD, thereby validating the RRMHD approach.
Speaker: Subir Sarkar, University of Oxford, UK - Chaired by: Alex Schekochihin, Editor, JPP
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Date/Time: Thursday 3rd August 2023, 4PM BST/11AM EDT
Title: Testing cosmic ray acceleration in the laboratory
Abstract: Petawatt lasers can be used to recreate interesting astrophysical processes, e.g. amplification of magnetic fields by plasma turbulence generated in colliding laser-produced plasma flows. MHD instabilities in such turbulent, magnetized plasmas are seen to energise electrons above the thermal background, thus demonstrating an injection mechanism for cosmic ray acceleration. Ongoing experiments are studying whether stochastic acceleration by the Fermi process indeed occurs in such astrophysical environments, e.g. supernova remnants or the Galactic Fermi bubbles or radio galaxies.
Speaker: Virginia Bresci, Leibniz Institute for Astrophysics Potsdam, Germany - Chaired By: Antoine C Bret, Associate Editor, JPP
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Date/Time: Thursday 27th July 2023, 4PM BST/11AM EDT
Title: PIC simulations of particle acceleration at relativistic magnetized shocks
Abstract: The efficiency of particle acceleration at shock waves in relativistic, magnetized astrophysical outflows is a debated topic with far-reaching implications. Most of previous numerical studies based on fully PIC simulations consider laminar in-flow conditions, i.e., nonturbulent, homogeneous background plasma of uniform magnetization. In this talk, based on a recent study, I will show how the presence of a well-developed turbulence upstream of a fast shock may change the picture. In particular, we carried out PIC simulations of a mildly relativistic magnetized pair shock (Lorentz factor γsh ≃ 2.7, magnetization level σ ≃ 0.01), and we found that strong turbulence can revive particle acceleration in a superluminal configuration that otherwise prohibits it. By the analysis of tracked particles we investigated the acceleration process and could conclude that, depending on the initial plasma temperature and magnetization, shock-drift or diffusive-type acceleration governs particle energization, producing power-law spectra dN/dγ ∝ γ^−s with s ≈ 2.5–3.5. At larger magnetization levels, stochastic acceleration within the preshock turbulence becomes competitive and can even take over shock acceleration.
Speaker: Camille Granier, Observatoire De La Cote D’azur, France - Chaired By: Thierry Passot,
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Date/Time: Thursday 20th July 2023, 4PM BST/11AM EDT
Title: Tearing and secondary instabilities in collisionless plasmas based on gyrofluid modeling
Abstract: This talk presents the application of a Hamiltonian gyrofluid approach allowing to account for finite Larmor radius corrections, parallel magnetic field fluctuations, and electron inertia, in the study of magnetic reconnection and related secondary instabilities. Non-collisional current sheets that form during the nonlinear development of spontaneous magnetic reconnection are characterized by a small thickness comparable to the electron skin depth and can become unstable, leading to the formation of plasmoids and facilitating high reconnection rates. In the context of cold ions with negligible parallel velocity, we investigate the marginal stability conditions for plasmoid formation in collisionless plasma, considering the impact of finite but moderate βe, a regime that has seen little investigation before. These results are tested against gyrokinetic simulations, showing a very good agreement. The gyrofluid approach also enables the study of the transition from the cold to the hot-ion regime. By considering a regime with small βe and hot ions, we present an analysis of the turbulent regime arising from Kelvin-Helmholtz-like secondary instabilities outside magnetic islands, resulting in the formation of magnetic vortices. Turbulence develops at scales smaller than the electron skin depth, with a magnetic energy spectrum exponent close to -11/3 as predicted by strong-turbulence phenomenology.
Speaker: Prakriti Pal Choudhury, University of Cambridge, UK - Chaired by: Alex Schekochihin, Editor, JPP
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Date/Time: Thursday 13th July 2023, 4PM BST/11AM EDT
Title: Radiative cooling-driven (thermal) instabilities in weakly magnetized, diffuse, hot plasmas
Abstract: Astrophysical environments are diverse in characteristic density, temperature, and magnetic field. For example, intracluster and circumgalactic medium (ICM/CGM) are large-scale, mildly magnetized, weakly collisional, hot, and diffuse environments. On the other hand, solar and planetary coronae have strong magnetic fields in hot and relatively tighter gravitationally bound atmospheres. A common feature in many of these distinctly different environments is the multiphase nature of the medium, or in other words, the local coexistence of a range of temperatures and the lack of monolithic cold or hot phase. This feature suggests (i) maintenance of an approximate thermal equilibrium in a time-averaged sense and (ii) a mechanism to produce multiphase condensation despite an equilibrium. In this talk, I will discuss the physical mechanisms responsible for (i) and (ii) in the ICM/CGM using analytic models and numerical experiments. I will include a discussion on the permitted length scales in a multiphase medium. Further, I will highlight the role of the weak magnetic field in condensation (specifically, large-scale mode along the field) and maintenance of the hot background medium (energy transport problem). Some of these ideas can also be relevant to speculate on such instabilities and interpret solar prominence, clumpy AGN winds, and condensation in planetary coronae.
Speaker: Francesco Valentini, University of Calabria, Italy - Chaired by: Alex Schekochihin, Editor, JPP
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Date/Time: Thursday 6th July 2023, 4PM BST/11AM EDT
Title: Plasma Observatory ESA M7 candidate mission: science and numerical support
Abstract: Plasma Observatory (PO) multi‐scale space mission is one of the five ESA M7 candidates for a launch in 2037 and is currently undergoing a competitive Phase 0 at ESA, for further downselection to Phase A at the end of 2023. The mission concept is tailored to study plasma energization and energy transport in the Earth's Magnetospheric System, simultaneously at both fluid and ion scales, at which the largest amount of electromagnetic energy is converted into energized particles and energy is transported. PO targets the two ESA Voyage 2050 themes for ESA-led M Mission: Magnetospheric Systems and Plasma Cross-scale Coupling. Baseline mission includes one mothercraft and six identical smallsat daughtercraft, covering all the key regions of the Magnetospheric System. Plasma energization and energy transport are central in space plasma physics research, with important implications for space weather science as well as for the understanding of distant astrophysical plasmas, and are strictly interconnected with important processes such as shocks, magnetic reconnection, turbulence and waves, plasma jets and their combination. The Earth's Magnetospheric System is the complex and highly dynamic plasma environment where the strongest particle energization and energy transport occur in the near‐Earth space. PO Numerical Simulation Working Group (NSWG) provides support to the development of the mission concept during all phases of mission design, not only from the scientific point of view but also for the definition of the payload, as well as for the optimization of the configuration of the spacecraft constellation.
Here, we discuss the concept of the PO space mission and the role of the NSWG, focusing specifically on the complex interaction between shocks and plasma turbulence and presenting a novel combination of magnetohydrodynamic (MHD) and small‐scale, hybrid kinetic simulations, where a shock is propagating in a turbulent medium. Finally, we put our modelling effort in the context of spacecraft observations, elucidating the role of cross‐scale, multi‐spacecraft simultaneous measurements in resolving shock front irregularities at different scales.
Speaker: Julia Tjus, Ruhr Universitaet Bochum, Germany - Chaired by: Antoine C Bret, Associate Editor, JPP
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Date/Time: Thursday 29th June 2023, 4PM BST/11AM EDT
Title: High-energy cosmic-ray propagation and interaction in magnetized plasmas
Abstract: Already detected more than 100 years ago, cosmic rays and their origins are still among the biggest riddles in astrophysics and physics. The reason is that they are transported in a highly magnetized plasma, thus interacting with the magnetic field and even with the ionized plasma background. While the back-reaction with the plasma is of highest relevance at low energies (below 10-100GeV), the highest energy cosmic rays can typically be considered to be transported in static magnetic field configurations. Here the biggest challenge is to properly describe the interaction with the magnetic field. In this talk, I will discuss this high-energy cosmic-ray regime, including the challenges in the modeling of the particle-wave interactions and compare the theoretical description and propagation models with data. I will discuss the possibility of contributions of magnetic mirrors to the propagation as well as the transition to the quasi-ballistic propagation regime and how it is imprinted in the measured particle spectra. I show that even cosmic-ray secondaries iike gamma-rays and neutrinos, that are produced in inelastic, hadronic interactions are useful diagnostic tools to understand cosmic-ray propagation in the magnetized plasma. Gamma-ray data will be used to test the models and an outlook will be given on what we will be able to learn from neutrino astronomy once it has come of age.
Speaker: Philipp Kempski, Princeton University , UK - Chaired by: Alex Schekochihin, Editor, JPP
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Date/Time: Thursday 22nd June 2023, 4PM BST/11AM EDT
Title: Towards a new theory of cosmic ray transport
Abstract: Detailed energy spectra of relativistic cosmic rays (CRs) measured close to Earth provide powerful constraints on the physics of their transport in the Milky Way (MW). According to these measurements, the escape time of CRs from the MW is orders of magnitude larger than the MW’s light-crossing time and has an approximately power-law dependence on CR energy. In the standard paradigm of CR transport, their escape rate is set by resonant interactions with volume-filling, small-amplitude magnetic-field fluctuations that permeate the galactic volume. In this talk, I will argue that these traditional propagation models remain theoretically uncertain and are generally not in good agreement with observations. This suggests that the microphysical theory of CR propagation needs to be revisited. I will discuss possible new models of CR transport that may bridge the gap between theory and observations. In particular, I will argue that CR propagation may depend on the geometry of small-scale magnetic field reversals and scattering in intermittent regions of resonant field-line curvature.
Speaker: Xavier Garbet, CEA and NTU, Singapore - Chaired by: Peter Catto, Associate Editor, JPP
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Date/Time: Thursday 15th June 2023, 4PM BST/11AM EDT
Title: Shear flow generation in magnetised plasmas
Abstract: Mean shear flows are essential to control confinement in magnetised plasmas for fusion. In absence of external torque, and atomic physics processes, two types of processes are known to drive or damp flows: collisional drag and turbulent flow generation. While the theory of flow generation is well documented in axisymmetric magnetic configurations like tokamaks, it is much less understood when 3D effects are accounted for. In tokamaks, 3D effects may come from magnetic ripple due to the finite number of coils, or magnetic perturbations produced with external coils.
In this talk, the physics at play, collisional or turbulent, will first be clarified in a unified framework. Analytical theory is compared to a set of comprehensive simulations based on drift-kinetic or gyrokinetic approaches able to treat both collisions and turbulence on an equal footing, thus allowing an assessment of competing processes. This methodology has recently been applied to understand the improvement of confinement with plasma current. When 3D effects are accounted for, it appears that collisional drag prevails over turbulence beyond a threshold in ripple amplitude that is expected to be reached in ITER. The talk will end with some considerations on the edge-core interplay regarding flow generation.
Speaker: Clarisse Bourdelle, CEA, France - Chaired by: Nuno Loureiro, Associate Editor, JPP
Date/Time: Thursday 8th June 2023, 4PM BST/11AM EDT
Title: Separatrix parameters and core performances across WEST L-mode database
Abstract: WEST database analysis shows a correlation of the recycled neutral source around the separatrix with core performances. This observation questions the causality chain between particle source and turbulent transport up to the core in L-mode, high recycling plasmas, an unavoidable phase of all scenarios. The best core performances correlate with the lowest values of the density at the separatrix n_sep, similarly to AUG and JET in H-mode [G. Verdoolaege et al 2021 Nucl. Fusion 61 076006]. Reflectometry in the midplane provides n_sep, while T_sep is inferred by the ‘two-point model’ using Langmuir Probe data on divertor targets. Lower separatrix resistivity does not correlate with better core performances, unlike H-mode observations [T. Eich et al 2020 Nucl. Fusion 60 056016]. As expected in presence of an efficient neutral source due to recycling fluxes, n_sep correlates with the D recycled particle flux at the divertor measured by visible spectroscopy. Coherently, at a given controlled central line integrated density n ̅, lower n_sep correlates with larger density gradient around the separatrix as well as larger global density peaking, n ̅/〈n〉 measured by interferometry. The later correlates as well with lower collisionality in the core, similarly to JET and AUG H-modes [C. Angioni et al 2007 Nucl. Fusion 47 1326]. The correlations reported allow phrasing the subsequent causality question: What is the interplay chain between low neutral recycling at the divertor plates, low density at the separatrix, high density peaking at the separatrix, high global density peaking, higher central temperature, better core energy confinement quality? The causality chain understanding is essential to prepare ITER operation and design DEMO scenarios where the ratio of the divertor leg to the ionization length will be larger and where the pumping efficiency with respect to the vacuum vessel volume will be weaker with respect to actual operating tokamaks.
Speaker: Alessandro Zocco, IPP Greifswald, Germany - Chaired by: Per Helander, Associate Editor, JPP
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Date/Time: Thursday 1st June 2023, 4PM GMT/11AM EDT
Title: Plasma instabilities and turbulence through a stellarator lens
Abstract: Having non-planar magnetic axis and rotating highly-shaped poloidal cross sections, stellarators are the most complex devices in the family of toroidal chambers used for magnetic confinement fusion. Their complexity, if not dealt with care, can lead to a string of undesirable transport properties such as unsustainable collisional heat losses, hollow density profiles, impurities accumulation, and lack of confinement of trapped and energetic particles.
The discovery of stellarator quasi-symmetries, however, opened the way to the solution of the many problems stellarators faced for decades. Today, the stellarator is a viable reactor concept whose potential is constantly reinvigorated by the results of the Wendelstein 7-X (W7-X) experiment, based at Greifswald, Germany.
In this talk, we will examine some questions that a modern experiment like W7-X is compelling us to reconsider, and propose the answers to them. Are heat losses turbulent? Which turbulence? How does complex geometry change turbulence? What suppresses or exacerbates this turbulence? How does it saturate? Do electromagnetic fluctuations affect the turbulence saturation process? Do impurities accumulate? Do we observe current-driven magnetic reconnection in a stellarator, even if it was designed to be virtually "current free"?
We will close our discussion by summarising the key steps needed to be taken to address the problem of turbulence and stability in high-performance stellarator reactors.
Speaker: Davide Manzini, École Polytechnique, France - Chaired by: Francesco Califano, Associate Editor, JPP
Date/Time: Thursday 25th May 2023, 4PM GMT/11AM EDT
Title: Subion-scale turbulence driven by Magnetic Reconnection
Abstract: The interplay between plasma turbulence and magnetic reconnection remains an unsettled question in astrophysical and laboratory plasmas. Here we report the first observational evidence that magnetic reconnection drives subion-scale turbulence in magnetospheric plasmas by transferring energy to small scales. We employ a spatial coarse-grained model of Hall magnetohydrodynamics, enabling us to measure the nonlinear energy transfer rate across scale ℓ at position x. Its application to Magnetospheric Multiscale mission data shows that magnetic reconnection drives intense energy transfer to subion-scales. This observational evidence is remarkably supported by the results from Hybrid Vlasov-Maxwell simulations of turbulence to which the coarse-grained model is also applied. These results open new pathways to investigate the interplay between turbulence, reconnection and energy dissipation in collisionless magnetized plasmas and can potentially explain spacecraft observations in various planetary magnetosheaths that showed the ubiquity of turbulent fluctuations at sub-ion scales even when no energy cascade emerges from the larger (MHD) scales.
Speaker: Paul Cassak, West Virginia University, USA - Chaired by: Thierry Passot, Associate Editor, JPP
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Date/Time: Thursday 18th May 2023, 4PM BST/11AM EDT
Title: A Kinetic Theory Generalization of the First Law of Thermodynamics Capturing Non-Equilibrium Effects
Abstract: Plasmas in many settings are out of local thermodynamic equilibrium (LTE) because the time scales for collisions to achieve LTE are longer than dynamical time scales in the plasma. Examples abound in naturally occurring and laboratory plasmas - the solar corona, interplanetary space, magnetospheres of Earth and other planets, interstellar space, compact astrophysical objects, magnetically confined fusion plasmas, high energy density plasmas, and low temperature plasmas. How energy is converted in plasmas that are not in LTE is a forefront research area across the plasma sciences, and is only well-understood for small departures from LTE. However, many plasmas can be far from LTE, and therefore a non-perturbative approach is needed. The standard approach to studying energy conversion in plasmas that can be far from LTE is to investigate changes in thermal energy and the work being done that changes the density, which are both captured by the first law of thermodynamics. In this talk, we argue that this approach omits energy conversion associated with any other higher order moments of the phase space density f, and that energy conversion associated with these moments can be important for systems far from LTE. We perform a first-principles, non-perturbative, analytic theory of the energy associated with all higher order moments of f. It is based on the concept of entropy in kinetic theory and is derived from the Boltzmann equation. We use the result to calculate energy conversion in particle-in-cell simulations of collisionless two-dimensional magnetic reconnection, which reveal that energy conversion associated with higher order moments of f can be locally significant. The theoretical results may be useful in a wide array of plasma settings.
Speaker: Alsu Sladkomedova, Tokamak Energy Ltd, UK - Chaired by: Alex Schekochihin, Editor, JPP
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Date/Time: Thursday 11th May 2023, 4PM BST/11AM EDT
Title: Intermittency of density fluctuations and zonal flow generation in MAST edge plasmas
Abstract: Edge-plasma fluctuations in a tokamak represent a complex system exhibiting intermittent behaviour due to the presence of coherent structures. These structures are long-lived plasma filaments with large-amplitude positive (blobs) and negative (holes) density fluctuations. It is crucial to understand edge fluctuations because of the link between the pedestal pressure and core confinement. Coherent structures may also impact on the scrape-off-layer width, which is important for predicting heat loads on plasma-facing components.
In this presentation, we analyse the edge ion-scale density fluctuations using the Beam Emission Spectroscopy diagnostic on the spherical tokamak MAST. We provide evidence of high-amplitude coherent structures, which are blobs and density holes, existing amongst ambient turbulent fluctuations. Our measurements of radial velocity and skewness of the density fluctuations indicate that density holes propagate radially inwards, with the skewness profile peaking at 7−10 cm inside the separatrix.
Our research reveals that the bursts in the density-fluctuation power, originating from turbulence and radially moving density holes, are followed by an increase in the power of a Geodesic Acoustic Mode. We utilise bispectral techniques to illustrate non-linear interactions between zonal flows and density oscillations.
Speaker: Sir Ian Chapman (CEO, UKAEA), UK Atomic Energy Authority, UK - Chaired by: Hartmut Zohm, Associate Editor, JPP
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Date/Time: Thursday 4th May 2023, 4PM BST/11AM EDT
Title: UKAEA’s fusion energy strategy
Abstract: This talk will begin by explaining the UK government strategy to delivering fusion and setting this in the context of other major fusion programmes internationally. We will explore the new facilities and programmes being pursued at UKAEA, including an overview of the progress made on the prototype powerplant STEP – the Spherical Tokamak for Energy Production. Finally, the talk will explore the relationships between the public and private sector that will be important in the delivery of fusion power.
Speaker: Christoph Pfrommer, Leibnitz Institute for Astrophysics, Germany - Chaired by: Antoine Bret, Associate Editor, JPP
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Date/Time: Thursday 27th April 2023, 4PM BST/11AM EDT
Title: Cosmic ray plasma physics and magnetic dynamos in galaxy formation
Abstract: Understanding the physics of galaxy formation is an outstanding problem in modern astrophysics. Recent cosmological simulations have demonstrated that feedback by star formation, supernovae and active galactic nuclei appears to be critical in obtaining realistic disk galaxies and to slow down star formation to the small observed rates. However the particular physical processes underlying these feedback processes still remain elusive. In particular, many of these simulations neglected magnetic fields and relativistic particle populations (so-called cosmic rays). Those are known to provide a pressure support comparable to the thermal gas in our Galaxy and couple dynamically and thermally to the gas, which seriously questions their neglect.
Early attempts to include magnetic fields and cosmic rays put the plasma kinetics of wave-particle interactions into the limelight for explaining the power of galactic winds and for regulating star formation in and cosmic accretion onto galaxies. After introducing the underlying physical concepts, I will present our recent efforts to model kinetic plasma physics in macroscopic hydrodynamical models of galaxy formation. In particular, I will explain how cosmic rays interact with and propagate through the magnetized plasma in the interstellar and circumgalactic media. I will elucidate the role of plasma instabilities in regulating the momentum transfer from cosmic rays to the background plasma and how we can observationally test these theoretical considerations using new high-sensitivity MeerKAT observations. I will then demonstrate that cosmic rays play a decisive role in the formation and evolution of spiral galaxies by providing feedback that regulates star formation and drives gas out in galactic winds. Comparing cosmic ray spectra of electrons and protons to observational data and studying the correlation of the far-infrared emission with the gamma-ray and radio emission from galaxies enables us to test the cosmic ray feedback and dynamo models for the growth of galactic magnetic fields. This argues that a complete understanding of galaxy formation necessarily includes plasma astrophysics.
Speaker: Frederico Fiuza, SLAC, Stanford, USA - Chaired by: Luís O. Silva, Associate Editor, JPP
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Date/Time: Thursday 20th April 2023, 4PM BST/11AM EDT
Title: Energy partition in collisionless shocks: a microphysical perspective
Abstract: Astrophysical shock waves are among the most powerful particle accelerators in the Universe. Generated by violent interactions of supersonic plasma flows with the interstellar or intergalactic medium, shocks are inferred to heat the plasma, amplify magnetic fields, and accelerate electrons and protons to highly relativistic speeds. However, the exact mechanisms that control energy partition in shocks remain a mystery. This is particularly challenging for high Mach number shocks, such as those associated with supernova remnants, where spacecraft data in the relevant regime is scarce and the shock structure cannot be directly resolved from observations. I will review recent progress in using the combination of fully kinetic simulations and laser-driven laboratory experiments to study energy partition in high-Mach number collisionless shocks. In particular, I will present results on magnetic field amplification, plasma heating and particle acceleration, and discuss how experimental measurements are helping benchmark models of the shock microphysics.
Speaker: Gabe Plunk, IPP Greifswald, Germany - Chaired by: Per Helander, Associate Editor, JPP
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Date/Time: Thursday 13th April 2023, 4PM BST/11AM EDT
Title: Advancing the concept of the quasi-isodynamic stellarator
Abstract: Quasi-isodynamic (QI) stellarators, like the Wendelstein 7-X experiment built in Greifswald, Germany, are an elegant solution to the problem of magnetic confinement for fusion energy. In addition to the usual benefits promised by stellarators, like inherent stability and the promise of steady state operation, the QI stellarator offers other advantages like low plasma currents, which makes the designed magnetic field robust to the presence of plasma, and intrinsic stability to certain micro-instabilities that drive turbulence, which would otherwise degrade confinement. In this talk, we will present recent theoretical work that sheds light on the design space of QI stellarators, including the key geometric parameters that define separate previously unknown classes, and unexplored areas of parameter space. Solutions with unexpected properties, like low plasma elongation, field period numbers, aspect ratio, and broken stellarator symmetry, suggest new optimization lines to explore in the search for the next generation of stellarator experiments and reactor concepts.
Speaker: Rogerio Jorge, Instituto Superior Técnico, Portugal - Chaired by: Bill Dorland, Editor, JPP
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Date/Time: Thursday 6th April 2023, 4PM BST/11AM EDT
Title: Advances in Stellarator Design in Nuclear Fusion Research: The Direct Optimisation Architecture
Abstract: In pursuit of magnetic confinement nuclear fusion, magnetic fields of premier quality are imperative. These fields must sustain high-heat plasmas for an adequate amount of time while also enabling control of plasma density, fast particle management, and turbulence regulation. The standard design of stellarator machines, as seen in experiments such as HSX and W7-X, optimises magnetic fields and coils separately, resulting in limited engineering tolerances and often neglect turbulent transport in the optimisation process. Furthermore, such process is highly dependent on the initial conditions used and often needs multiple restarts with relaxed requirements, making the process inefficient and potentially compromising the optimal balance between alpha particles, neoclassical transport, and turbulence. Recent breakthroughs in the optimisation of stellarator devices, such as direct near-axis designs, integrated plasma-coil optimisation algorithms, precise quasisymmetric and quasi-isodynamic fields, and direct turbulence optimisation, are revolutionising the way in which these machines are designed. The main outcomes of these advancements, as well as the prospects for a more efficient fusion device, will be discussed in this presentation.
Speaker: Annie Kritcher, Lawrence Livermore National Laboratory, USA - Chaired by: Cary Forest, Associate Editor, JPP
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Date/Time: Thursday 30th March 2023, 4PM BST/11AM EDT
Title: Design of the first fusion laboratory experiment to achieve target gain > 1
Abstract: The inertial fusion community have been working towards ignition for decades, since the idea of inertial confinement fusion (ICF) was first proposed by Nuckolls, et al., in 1972. On August 8, 2021 and Dec 5th 2022, the Lawson criterion for ignition was met and more fusion energy was created than laser energy incident on the target at the National Ignition Facility (NIF) in Northern California. The first experiment produced a fusion yield of 1.35 MJ from 1.9 MJ of laser energy and appears to have crossed the tipping-point of thermodynamic instability according to several ignition metrics. Building on this result, improvements were made to increase the fusion energy output to >3MJ from 2.05 MJ of laser energy on target, resulting in target gain exceeding unity for the first time in the laboratory. This result is important in that it proves that there is nothing fundamentally limiting controlled fusion energy gain in the laboratory.
Speaker: Sergio Servidio, University of Calabria, Italy - Chaired by: Alex Schekochihin, Editor, JPP
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Date/Time: Thursday 23rd March 2023, 3PM GMT/11AM EDT
Title: The role of structures and kinetic effects in astrophysical plasma turbulence
Abstract: Astrophysical plasmas exhibit complex behavior similar to the erratic motion of turbulent viscous fluids. Despite the undeniable differences between collisionless plasmas and classical collisional fluids, there are a few (but remarkable) similarities: turbulent plasmas, like ordinary fluids, are characterized by the presence of a sea of coherent, long-living structures at which dissipation might occur. Local magnetic reconnection events, flux tubes, velocity shears, small-scale compressive layers, and so on are examples of such structures.
In this talk, we will start our journey in the heliosphere by reviewing some recent (and less recent) results and focusing on the role that the above fauna of coherent patterns plays in turbulence. Since these intermittent spikes are highly correlated with local non-Maxwellian (kinetic) patterns, they could be the "hot spots" for energy conversion mechanisms in poorly-collisional systems. The end result of this interaction between coherent fields and particles is a "phase space" turbulent cascade, which has been invoked by theory, supported by simulations, and confirmed by observations. We investigate the locality of the phase space dynamics both in homogeneous turbulence as well as in the unique interaction between astrophysical shocks and turbulence, by presenting a new coarse-graining transport model.
Finally, taking advantage of this experience with the heliospheric wind, we will end our trip nearby highly turbulent, supermassive black holes, demonstrating the need to model plasma accretion onto compact objects by microscopic, kinetic plasma models.
Speaker: Giuseppe Arrò, Katholieke Universiteit Leuven, Belgium - Chaired by: Francesco Califano, Associate Editor, JPP
Date/Time: Thursday 16th March 2023, 3PM GMT/11AM EDT
Title: Spectral properties and energy transfer at kinetic scales in collisionless plasma turbulence
Abstract: Turbulence in collisionless magnetized plasmas is a complex process involving the transfer of energy from large magnetohydrodynamic (MHD) scales to small ion and electron kinetic scales. This multi-scale nature of plasma turbulence is reflected in the properties of the turbulent spectra that have different shapes in different ranges of scales. Satellite observations show that at MHD scales the magnetic field spectrum follows a power-law that breaks at ion scales, where a steeper power-law develops. A second break and steepening is observed also towards electron scales but the shape of the spectrum in this range is still under debate and it is not clear if it follows a new power-law or if an exponential falloff develops.
By means of a fully kinetic simulation of freely decaying plasma turbulence, we study the spectral properties and the energy transfer characterizing the turbulent cascade from large to kinetic scales. We find that both the magnetic field and electron velocity spectra have an exponential decay at scales of the order of the electron gyroradius. On the other hand, the ion velocity spectrum shows a steep power law behaviour at sub-ion scales, without reaching far into electron scales.
The dynamics responsible for the development of such spectral features is investigated by analysing the scale-filtered energy balance. Our study reveals the presence of an indirect electron-driven mechanism that channels the electromagnetic (e.m.) energy from large to sub-ion scales more efficiently than the direct nonlinear scale-to-scale transfer of e.m. energy. This mechanism consists of three steps: in the first step the e.m. energy is converted into electron fluid flow energy at large scales; in the second step the electron fluid flow energy is nonlinearly transferred toward sub-ion scales; in the final step the electron fluid flow energy is converted back into e.m. energy at sub-ion scales.
Speaker: Ian Abel, University of Maryland, USA - Chaired by: Cary Forest, Associate Editor, JPP
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Date/Time: Thursday 9th March 2023, 4PM GMT/11AM EST
Title: The Equilibrium and Axisymmetric Stability of a Rapidly-Rotating Mirror
Abstract: Magnetic mirror configurations, once a cornerstone of the fusion programme, have enjoyed a recent renaissance. The US Department of Energy, through its ARPA-E BETHE programme, has recently funded two mirror configurations to test their prospects of achieving high-performance plasmas -- the Wisconsin High-field Axisymmetric Mirror (WHAM) and the Centrifugal Mirror Fusion Experiment (CMFX). CMFX is an example of a centrifugally-confined mirror plasma, using plasma rotation to overcome the key issues experienced by previous axisymmetric mirrors. Initial explorations of this concept were performed on the Maryland Centrifugal Experiment (MCX). Preliminary performance of the CMFX machine has been extremely promising, and prompts a more detailed analysis of the equilibrium and stability of such machines.
In this talk we will briefly summarize existing results for magnetic mirrors and then present a class of axisymmetric magnetic equilibria for these devices, derived in the rapidly-rotating large-Mach-number limit. The prototypical equilibrium is comprised of a narrow layer of plasma, reminiscent of an accretion disk, coupled to an exterior vacuum field.
In the plasma, the equilibrium is characterised by the balance between centrifugal forces and magnetic tension. We provide detailed expressions for the structure of the equilibrium in the plasma layer. These equilibria exhibit many commonalities with equilibria used for studying astrophysical disks. By exploring the coupling to the exterior vacuum solution we can study the parameters for which these equilibria exist. This may provide a hard limit on the rotation speed achievable in future centrifugal mirror experiments. Finally, we examine the stability of these equilibria and explore how more elongated mirror configurations may naturally collapse into the thin disks presented here.
Speaker: Emily Belli, General Atomics, USA - Chaired by: Alex Schekochihin, Editor, JPP
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Date/Time: Thursday 2nd March 2023, 4PM GMT/11AM EST
Title: Multiscale Nature of Turbulence in the Tokamak Pedestal: Exploiting Extreme-Scale Computing with Gyrokinetic Simulations
Abstract: Plasma confinement in tokamaks is limited by slow particle and energy losses due to turbulence, driven by unstable drift waves triggered by plasma inhomogeneities. Understanding the mechanisms that drive turbulence in burning plasmas is necessary to design next-generation fusion reactors like ITER with optimal confinement properties and fusion performance. While ion-scale turbulence in the tokamak core has been modeled extensively, gyrokinetic simulations of turbulent transport in the edge pedestal of high-confinement mode (H-mode) regimes are more difficult due to the large magnetic shear, strong flux-surface shaping, and proximity to ballooning stability limits. In addition, in the H-mode pedestal, large gradients in density and temperature can drive multiple drift-wave instabilities across a broad range of scales, namely ion and electron scales that differ in size by nearly two orders of magnitude. Simulating this so-called "multiscale turbulence", which captures the complex interaction between the slow, large scale motion of the hydrogenic ions and the fast, small-scale motion of the electrons, requires leadership-scale computing resources and advanced numerical algorithms. In this talk, we will present a first analysis of the spectral transition of multiscale turbulence in pedestal-like regimes. The ensemble of multiscale simulations required over 250k node-hours on the 200-petaflop OLCF Summit supercomputer. The simulations show that the experiments can lie in an interesting bifurcation region between ion-scale and multiscale-dominated turbulence regimes. The mechanisms by which electron-scale transport can be reduced by nonlinear mixing with ion-scale fluctuations and ion-scale driven zonal flows will be reviewed. These simulations were enabled using recents advances in numerical algorithms for gyrokinetic simulations that use spectral/pseudospectral algorithms to target scalability on next-generation, exascale HPC systems that use multicore and GPU-accelerated hardware. Preparations for scaling-up to Frontier, the first supercomputer to achieve exaflop performance, will also be discussed.
Speaker: Chris Hamilton, IAS, Princeton, USA - Chaired by: Dmitri Uzdensky, Associate Editor, JPP
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Date/Time: Thursday 23rd February 2023, 4PM GMT/11AM EST
Title: On the Kinetics of Spinning Stellar Systems or, The Galactic Tokamak
Abstract: Many galaxies, including our own galaxy the Milky Way, have a ‘bar’ structure at their center— an elongated collection of millions of stars, that spins as if it were a solid body. Galaxies are also embedded in massive dark matter haloes. When the rate at which the bar rotates resonates with a dark matter particle’s orbital frequency, the dark matter can suck angular momentum out of the bar, causing it to slow down. Previous theories of the bar-halo interaction calculated this ‘dynamical friction’ on the bar in a manner directly analogous to Landau’s calculation of the collisionless damping of an electric wave (and subsequently to O’Neil’s nonlinear generalisation thereof).
This is no coincidence — bar-halo interactions are just one of a plethora of gravitational dynamics problems that have direct plasma-kinetic analogues.
In this talk I will introduce the astrophysical context of galactic bars and their host dark matter haloes, and describe some of the aforementioned classic studies of the bar-halo interaction. However, those studies routinely ignored the fact that dark matter particles also experience random ‘diffusive’ kicks from other passing dark matter clumps, gas clouds, and so on. I will describe recent work done in collaboration with Princeton plasma theorists on quantifying the impact of diffusion on bar-halo friction, a problem which turns out to be mathematically identical to that of understanding particle energization in tokamaks. More broadly, I will argue that galactic dynamics has, over the last several decades, largely failed to learn from its more well-developed cousin and therefore has a lot of catching up to do. But I will also argue that in return, we stellar dynamicists can provide you plasma theorists with fresh contexts in which to ply your trade, and an opportunity to work on something both fantastically beautiful and totally useless.
Speaker: Marta Fajardo, Instituto Superior Técnico, Portugal - Chaired by: Luís O. Silva, Associate Editor, JPP
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Date/Time: Thursday 16th February 2023, 4PM GMT/11AM EST
Title: Solid density plasmas with ultrafast x-rays
Abstract: Solid density plasmas are present in Inertial Confinement Fusion, astrophysical objects and in material processing with lasers. With moderate temperatures (ranging from 0.1 to 100 eV) and roughly solid densities (0.1 to 10 times nsolid), the Warm Dense Matter state sits in the transition between solid, liquid gas and plasma. Warm dense plasmas have comparable thermal and Fermi energies, and sufficiently high densities for the Coulomb potential to exceed ion kinetic energy. Challenging to describe accurately with reduced theoretical frameworks, this last frontier of plasma physics is now being explored thanks to recent developments in numerical and experimental capabilities. Our group has been pursuing the diagnostic of warm, solid density plasmas for the past decade. Such states can be isolated by ultrafast isochoric heating, via X-ray laser heating of bulk solid targets, or IR heating of skin-depth thin foils. Probes capable of penetrating solid densities and with femtosecond resolution are required to capture their properties. Our team has led the use of High Harmonic Generation in the EUV to probe these elusive states of matter. In parallel with bulk heating experiments at XFELs, we have built a platform for creating and probing Warm Dense Matter by femtosecond heating of free-standing nano-foils in Lisbon. Here, we give an overview of the insights gained from XFEL and IR laser experiments on solid density plasmas, and propose novel opportunities for WDM studies from small-scale to large-scale facilities.
- All-optical Time-and-space resolved measurements of electron-ion coupling in solid density titanium plasma. GO Williams, S. Antunes, M Fajardo – to be published
- Collisional ionization and recombination in degenerate plasmas beyond the free-electron-gas approximation. GO Williams, M Fajardo - Physical Review E, 2020
- Impact of free electron degeneracy on collisional rates in plasmas. GO Williams et al- Physical Review Research, 2019
Speaker: Mario Riquelme, University of Chile, Chile - Chaired by: Antoine Bret, Associate Editor, JPP
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Date/Time: Thursday 9th February 2023, 4PM GMT/11AM EST
Title: PIC simulations of the magnetorotational instability (MRI) in stratified, collisionless accretion disks
Abstract: Plasma accretion onto astrophysical compact objects, such as black holes and neutron stars, is often considered to occur in the collisionless regime. This implies that kinetic plasma processes may play a crucial role in the physics of accretion in these systems, giving rise to important phenomena like non-thermal particle acceleration, temperature anisotropies and different temperatures between ions and electrons. Kinetic simulations, beyond MHD modeling, are thus needed in order to acquire a fully self-consistent picture of the processes that participate in disk accretion and to predict their observational signatures. In this talk, we will first present our initial results of 2D and 3D fully kinetic, particle-in-cell (PIC) plasma simulations of the collisionless magnetorotational instability (MRI), an MHD-scale instability that is crucial to produce outward transport of angular momentum in accretion disks. Our simulations are local and stratified, which means that we use the shearing box approximation and self-consistently include the vertical stratification of the disk. We find that the MRI saturation and the transport of angular momentum in our stratified simulations are more efficient than in the case where disk stratification is not included. Particle acceleration in our runs is efficient and mainly driven by magnetic reconnection, and is also more significant when the simulations include stratification. In the final part of the presentation, we will also focus on the non-linear evolution of (microscopic) temperature anisotropy instabilities, which play a crucial role in the MRI evolution by providing an effective collisionality to the plasma. We will show that these instabilities can also contribute to non-thermal particle acceleration in collisionless accretion disks. Our simulation results mainly concentrate on the sub-relativistic plasma regime, relevant for accretion onto black hole systems like Sgr A* and M87.
Speaker: Maarit Korpi-Lagg, Aalto University, Finland - Chaired by: Tünde Fülöp, Associate Editor, JPP
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Date/Time: Thursday 2nd February 2023, 4PM GMT/11AM EST
Title: On the origin of magnetic fluctuations in low magnetic Prandtl number plasmas
Abstract:Magnetic fields on small scales are ubiquitous in the universe. For example, the fluctuating magnetic fields in star-forming regions of galaxies are more than twice the strength of the magnetic fields coherent over large scales. On the solar surface, magnetic fields are mostly concentrated in medium and small-scale structures, while the proportion comprising the mean field strength is even lower than in galaxies. The generation mechanisms of the fluctuating magnetic fields are not fully understood. One possibility is the so-called small-scale dynamo (SSD), the other is tangling of the large-scale field structures through turbulence acting on them. In the interstellar medium of galaxies, the resistivity η is much lower than the viscosity ν, such that magnetic instabilities are easier to excite relative to the turbulence. SSD in such high magnetic Prandtl number (Pm= ν / η) conditions has therefore been predicted to be easily excited. In the Sun and cool stars, Pm is much lower, namely in the range of 10-6 - 10-3. Both theoretically and especially numerically, SSD is more difficult to excite at such very low magnetic Prandtl numbers. Indeed, some recent numerical studies had indicated that the threshold for SSD excitation should systematically increase with decreasing Pm, concluding that SSD would be impossible in the Sun and cool stars. Continuing increases in CPU resources, for now, have permitted us to perform even higher-resolution simulations employing the lowest Pm values to date to mimic solar conditions. Contrary to earlier findings, the SSD turns out not only to be possible for Pm down to 0.0031, but even to become increasingly easy to excite for Pm below ≃ 0.05. We relate this behaviour to the known hydrodynamic phenomenon, referred to as the bottleneck effect. Extrapolating our results to solar values of Pm indicates that an SSD would be possible under such conditions. We have recently developed a GPU-accelerated version of our solver, capable of bringing us to the solar parameter regime in terms of Pm in the near future, rendering the extrapolations into real findings in the solar regime.
Speaker: Luis O. Silva, Instituto Superior Técnico, Universidade de Lisboa, Portugal - Chaired by: Antoine Bret, Associate Editor, JPP
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Date/Time: Thursday 26th January 2023, 4PM GMT/11AM EST
Title: Electron scale kinetic instabilities in magnetized plasmas via radiation reaction and laser ionization
Abstract: Anisotropic electron distribution functions in a strong magnetic field can be unstable to the electron cyclotron/synchrotron maser instability (ECMI), leading to the generation of coherent radiation. We explore laboratory and astrophysical scenarios where this instability can be present and/or generated due to collective processes and/or by carefully tuning the experimental conditions. We show that radiation reaction, in the classical and quantum regimes, depicts a general property that can lead to ring-like electron populations that are unstable to the ECME. A theoretical model is developed and compared with QED PIC simulations [1]. Laboratory scenarios where ring-like distributions can be generated via radiation reaction are also explored and demonstrated.
We generalize previous works on anisotropic distributions driven by laser-ionized gases to demonstrate that similar distribution functions and the ensuing instability are naturally driven via a careful combination of laser parameters (intensity and polarization), plasma density, and external magnetic fields [2]. Fully self-consistent simulations demonstrate the key signatures of these processes, with an excellent agreement on the instability's features and the emitted radiation properties.
Our results open two closely connected new directions, showing that radiation reaction naturally leads to strongly anisotropic ring-like distribution functions, prone to collective plasma instabilities and that these distributions and conditions can be mimicked in the laboratory allowing for the detailed exploration of maser radiation with the state-of-the-art laser technology.
[1] P. Bilbao, L. O.Silva, arXiv:2212.12271
[2] T. Silva, P. Bilbao, L. O. Silva, in preparation (2023)
Speaker: Paolo Ricci, EPFL, Switzerland - Chaired by: Bill Dorland, Editor, JPP
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Date/Time: Thursday 19th January 2023, 4PM GMT/11AM EST
Title: New insights on plasma turbulence in the boundary region of tokamaks
Abstract: One of the greatest uncertainties in the success of the fusion program is related to the turbulent dynamics of the fusion fuel in the boundary region. The plasma behavior in the boundary governs the overall confinement properties of the device, regulates the impurity dynamics and the level of fusion ashes, and determines the heat load to the vessel walls – a showstopper for the whole fusion program if material requirements cannot be met. With the goal of improving our understanding of the boundary dynamics, the GBS code was developed during the past years. By solving the drift-reduced Braginskii equations self-consistently with the kinetic neutral atom dynamics, GBS simulates the plasma turbulence in the boundary of tokamaks as it results from external sources, recycling, turbulent transport and flows, and plasma losses at the vessel. GBS simulations have allowed us to advance the basic understanding of boundary turbulence. This incudes progress in estimating the SOL width, a crucial quantity to determine the heat flux on the vessel walls, and the identification of a turbulent regime characterized by a catastrophically large turbulent transport, which has been associated with the crossing of the density limit. We will present an overview of our simulation and theoretical results, as well as their comparison with experimental measurements from several tokamaks worldwide. Predictions for ITER and other future reactors will be discussed.
Speaker: Minna Palmroth, University of Helsinki, Finland - Chaired by: Francesco Califano, Associate Editor, JPP
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Date/Time: Thursday 12th January 2023, 4PM GMT/11AM EST
Title: 6-dimensional hybrid-Vlasov modelling of the near-Earth space: First Vlasiator results
Abstract: Numerical simulations are key in modern space physics, as they can be used as 1) context to data, 2) predict future behaviour of the system, 3) understand the system using unforeseen boundary conditions, and increasingly also in 4) discovering new phenomena that are hard to be observed using point-wise satellite measurements. Especially, the discovery of new phenomena pertains to global systems, where phenomena of interest may be initiated far away from the point of observations. The most typical method of simulating the global solar wind - magnetosphere - ionosphere system is based on magnetohydrodynamics (MHD), which is however not representing the actual plasma behaviour in locations where kinetic physics becomes important. Such regions are e.g., the foreshock - magnetosheath interaction, reconnection, and the inner magnetosphere.
Vlasiator is the world’s first and so far the only global simulation based on the hybrid-Vlasov approach that simulates the ion distributions accurately without noise. Earlier Vlasiator results have shown without a doubt that ion-kinetic effects cannot be neglected from the large scales, as small-scale phenomena affect large scales and vice versa. This scale coupling leads to phenomena that are not predicted using local simulations without proper boundary conditions, or with spacecraft measurements lacking the global context.
Here, we present the world’s first global 6-dimensional ion-kinetic global magnetospheric simulation run, accurate both locally and globally. The simulation box extends from the dayside to the nightside, and includes global dynamics and both dayside and nightside reconnection regions. We show the newest runs showing a magnetotail eruption, and discusses the physics leading to this phenomenon that has puzzled space scientists for decades.
Speaker: Andrei Beloborodov , Columbia University, USA - Chaired by: Dmitri Uzdensky, Associate Editor, JPP
Date/Time: Thursday 15th December 2022, 4PM GMT/11AM EST
Title: Magnetospheric waves and bursts from magnetars
Abstract: Magnetars are bursting neutron stars with ultrastrong magnetic fields. Their magnetospheric plasma is extreme in a few ways. Magnetic energy strongly dominates over the plasma rest mass, so magnetohydrodynamic (MHD) waves propagate with nearly speed of light. Dense (X-ray) radiation around the star strongly affects the plasma behavior -- photons create the plasma itself, and radiative processes generate a drag on particle motions. A canonical problem of magnetar physics is how the magnetospheric plasma responds to a kHz perturbation from a star quake. Observationally, such events are associated with powerful X-ray bursts, the main form of magnetar activity. Furthermore, this activity is linked to fast radio bursts of enormous strength. Magnetospheric perturbations responsible for these phenomena can be shear Alfvén waves or compressive (fast magnetosonic) waves. Both can be modeled from first principles, using relativistic MHD and kinetic descriptions. This work investigates the plasma response to the waves, how the waves energize particles and launch relativistic explosions, and how the accompanying radiative processes can produce powerful X-ray and radio emission. Similar physics may occur in binary neutron stars, generating quasi-periodic bursting activity before the binary merges.
Speaker: David Kirtley, Helion Energy, USA - Chaired by: Troy Carter, Associate Editor, JPP
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Date/Time: Thursday 8th December 2022, 4PM GMT/11AM EST
Title: Fundamental Scaling of Adiabatic Compression of Field Reversed Configuration Thermonuclear Fusion Plasmas
Abstract: Field Reversed Configuration (FRC) plasmas are plasma devices that have demonstrated that through magnetic compression can be heated to thermonuclear fusion conditions in the parameter space of an energy-producing generator. Of particular interest, FRCs are high-beta, in that the plasma particle kinetic energy is in balance with an externally applied magnetic field at all stages of operation. Dr. Kirtley will present the fundamental theory of FRC formation, scaling, and stability. A brief summary of experimental FRC programs dating to 1990 will be detailed. As will be presented, a cylindrical approximation for the energy and particle distribution within an FRC can, within 10%, match the fusion performance results of both full Magnetohydrodynamic (MHD) simulations as well as all robust, modern theoretical spatial and energy distribution models. Further, by using the cylindrical model, detailed fusion reaction, radiation, and energy transport equations are now numerically-tractable and can be modelled over a wide parameter space. A detailed numerical model will be presented with the key theoretical performance of the compression of high-beta fusion plasmas in both Deuterium-Tritium (D-T) and Deuterium-Helion (D-He-3) fuels. As will be shown, a high-beta D-He-3 plasma outperforms the traditional, low-beta D-T fuel and can theoretically yield a net-positive fusion generator.
Speaker: Fabio Bacchini, Katholieke Universiteit, Leuven, Belgium - Chaired by: Dmitri Uzdensky, Associate Editor, JPP
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Date/Time: Thursday 1st December 2022, 4PM GMT/11AM EST
Title: Fully kinetic simulations of plasma accretion in a three-dimensional shearing box
Abstract: One of the main uncertainties in the physics of plasma accretion onto compact objects (black holes and neutron stars) is represented by the dynamics of microscopic processes at kinetic scales. Although often expected to be collisionless, accretion flows around black holes are customarily studied with magnetohydrodynamic (MHD) models; these models however do not include out-of-equilibrium physics and cannot describe nonthermal particle acceleration and radiation, particle scattering and diffusion, and the transport of angular momentum through astrophysical accretion disks. To study these processes self-consistently, fully kinetic simulations are necessary; but the prohibitive costs typically associated to such numerical experiments have so far inhibited significant progress in this direction.
In this talk, we will present the first mesoscale (i.e. attaining global MHD-like behavior) simulations of plasma accretion carried out with a fully kinetic approach in three dimensions. Our work is based on the shearing-box paradigm, which allows the first-principles study of a localized sector of an accretion disk at affordable costs. Our 3D kinetic simulations are large enough to reach convergence (with respect to physical parameters and box size), allowing us to analyze the detail of particle acceleration and angular-momentum transport in the turbulent plasma dynamics developing in a typical collisionless accretion flow.
There was no talk on November 24th due to Thanksgiving.
Speaker: Tammy Ma, Lawrence Livermore National Laboratory, USA - Chaired by: Edward Thomas, Associate Editor, JPP
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Date/Time: Thursday 17th November 2022, 4PM GMT/11AM EST
Title: Accelerating the rate of discovery: Toward high-repetition-rate laser experiments for High-Energy Density (HED) Science and Inertial Fusion Energy (IFE)
Abstract: As high-intensity short-pulse lasers that can operate at high-repetition-rate (HRR) (>10 Hz) come online around the world, the high-energy-density (HED) science they enable will experience a radical paradigm shift. The >10^3 increase in shot rate over today’s shot-per-hour drivers translates into dramatically faster data acquisition and more experiments, and thus the potential to significantly accelerate the advancement of HED science. However, to fully realize the potential benefits of HRR facilities requires a fundamental shift in how they are operated, and in fact, how the experiments done on them are designed and executed. Current energetic driver facilities depend on the ability to manually tune the lasers, the targets, the diagnostics settings, and more, between single shots or sets of shots through a manual feedback loop of data collection, data analysis, and optimization largely driven by experience and intuition. At 10 Hz, this paradigm is no longer sustainable as more complex data is collected more quickly than is possible to analyze manually. Simultaneously, on-the-fly optimization of experiments will become ever more crucial as higher repetition rates will lead to more deliberate inter-shot variations and the improved operational range to allow exploration over larger regions of phase space. Consequently, it is likely that the next generation of laser facilities will be limited not by their hardware but by our ability to use it effectively. We will present the vision and ongoing work to realize a HRR framework for rapidly delivered optimal experiments coupled to cognitive simulation to provide new insights in HED science, and lay the groundwork for Inertial Fusion Energy (IFE) rep rate operations.
Speaker: László Bardóczi, General Atomics, USA - Chaired by: Troy Carter, Associate Editor, JPP
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Date/Time: Thursday 10th November 2022, 4PM GMT/11AM EST
Title: The root cause of 2,1 tearing instabilities in DIII-D H-mode plasmas
Abstract: Tearing modes open magnetic islands on rational surfaces of tokamaks, which ruin the confinement and can lead to plasma termination. The Neoclassical Tearing Mode is the leading single root cause of disruptions of high performance tokamak experiments. In order to develop and project stable operating solutions to future machines, a first principles based and experimentally validated model is necessary. I will present the analysis of an ensemble of over 16,000 DIII-D H-mode discharges exploring the root cause of 2,1 tearing onsets and its parameter dependency. The onset time distribution is exponential and the instability onset rate is constant in intermediate and high edge safety factor plasma scenarios, in accordance with Poisson-point processes as for example the radioactive decay. This supports that the instability has a threshold for growth and it is triggered by random events, in line with the Rutherford-theory of neoclassical tearing modes (NTMs). In scenarios characterized by low edge safety factor, such as the ITER baseline scenario, the onset rate increases over the course of the βN flattop, indicating that this group of plasmas evolve toward more unstable conditions. The onset statistics are well replicated by Poisson point process models, in contrast to models of classical stability index evolution. At a low edge safety factor, the increasing rate parameter is correlated with the drop of differential rotation between the q=1 and q=2 surfaces to near zero. Explanation is offered by the reduction of stabilizing polarization currents acting on sawtooth driven 2,1 seed islands when the differential rotation is approaching zero. The rate parameter and onset relative frequency analyses show strong correlation with βN , in agreement with NTMs. The magnetic perturbation amplitude of these islands grows linearly in time, as expected from NTMs, and it is unaffected by the current profile relaxation, suggesting that classical effects are weak. Recent experiments testing the effect of the current profile shape at q=2 on the 2,1 tearing instability found no causality between the island onset and the shape of the current profile. Overall, the agreement of various aspects of the data with the Rutherford theory supports that the 2,1 islands are NTMs triggered by random events in DIII-D at all edge safety factor values. This database analysis also suggests that the plasma stability can be improved by reducing the elongation, triangularity, density, impurity density, increasing the internal inductance and by running the plasmas in upper single null configuration.
Speaker: Daniela Grasso, CNR-ISC / Politecnico di Torino, Italy - Chaired by: Francesco Califano, Associate Editor, JPP
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Date/Time: Thursday 3rd November 2022, 3PM GMT/11AM EDT
Title: Evolution of current end vorticity sheets in collisionless plasma turbulence
Abstract: The evolution of current and vorticity sheets in collisionless plasmas, where magnetic reconnection within turbulence may take place driven by the electron inertia, is studied. We start from a linear analysis of magnetic vs. fluid instability, that might affect these layers, to understand the complex situation that generates in a turbulent plasma. Here, due to the presence of strong velocity shears, the typical plasmoids formation, observed to influence the energy cascade in the resistive magnetohydrodynamic context, has to coexist with the Kelvin-Helmholtz instability. We find that the current density layers may undergo the plasmoid or the Kelvin-Helmholtz instability depending on the local values of the magnetic and velocity fields. The competition among these instabilities affects not only the evolution of the current sheets, that may generate plasmoid chains or Kelvin-Helmholtz-driven vortices, but also the energy cascade, that is different for the magnetic and kinetic spectra.
References:
1) Biskamp, D. 2000, Magnetic Reconnection in Plasmas (Cambridge University Press, Cambridge)
2) Dong, C., Wang, L., Y.-M. Huang, Comisso, L., & Bhattacharjee, A. 2018, Phys. Rev. Lett., 121, 16510.
Speaker: Michele Romanelli, Tokamak Energy Ltd, UK - Chaired by: Bill Dorland, Editor, JPP
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Date/Time: Thursday 27th October 2022, 4PM BST/11AM EDT
Title: ST40: Advancing the Physics Basis of Spherical Tokamak Reactors
Abstract: ST40 is a high field Spherical Tokamak, built and operated by Tokamak Energy, with Rgeo≈0.4-0.5m, A≈1.6-1.9, κ<1.7, Ip≈0.4-0.8MA and BT≈0.8-2.2T. Two co-injected hydrogen neutral beams deliver 0.8MW at 50kV and 0.7MW at 24kV. With deuterium the corresponding figures are 1.0MW/55kV and 0.8MW/24kV, respectively. The vessel walls are conditioned with helium GDC and boronization. Merging-compression start-up is used. The plasmas is mainly limited on the high field side, although some diverted plasmas were obtained. The primary goal for the 2021-2 operations was the attainment of the business objective, Ti(0) > 100MK (8.6keV). A central deuterium ion temperature of 9.6keV was obtained together with 𝑛i(0).𝑇𝑖(0).𝜏E ≈ 6 ± 2 × 1018m−3•keV•s. Such high ion temperatures have only previously been achieved in significantly larger devices and never reached in a ST. However, in order to access the high Ti regime, the electron temperature must be high enough that the collisional power transfer from ions to electrons is significantly smaller than the input power density. In the best ST40 shots, Te(0) > 3keV was observed, which is in contrast to the Te(0) <2.4keV in neutral beam heated STs of twice the major radius but with BT <0.8T. The results from ST40 are consistent with the NSTX observation of an approximately linear BT dependence of energy confinement time. A limited TF scan and comparisons between high Ti data at 1.6 and 1.92T, on axis, demonstrate the beneficial effect of high BT. The dominant loss channel is via electrons. The observation of high temperature lends support to the high field ST route to economical, compact fusion reactors.
There will be no talk on 20th October due to the APS DPD meeting.
Speaker: Mikhail Medvedev , University of Kansas, USA - Chaired by: Antoine Bret, Associate Editor, JPP
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Date/Time: Thursday 13th October 2022, 4PM BST/11AM EDT
Title: Collective modes in QED plasma
Abstract: Ultra-magnetized plasmas where the magnetic field strength exceeds the Schwinger (critical) field become of great scientific interest, thanks to the astrophysical observations of magnetar emission and current advances in laser-plasma experiments. These advances demand better understanding of how quantum electrodynamics (QED) effects that are present in ultra-strong-field environments affect plasma dynamics. Interestingly, magnetars -- neutron stars with magnetic fields of ~1e15 Gauss or greater -- do exist and QED effects on their magnetospheric plasma cannot be ignored. In particular, Maxwell's equations become nonlinear in the strong-QED regime. This effect has not been systematically considered in theoretical studies. Here we discuss how "textbook" linear plasma modes are modified in an arbitrarily strong magnetic field. These results can be important for understanding of a magnetospheric pair plasma of a magnetar and for future laser-plasma experiments.
Speaker: Laurent Gremillet, CEA-DAM-DIF, France - Chaired by: Antoine Bret, Associate Editor, JPP
Date/Time: Thursday 6th October 2022, 4PM BST/11AM EDT
Title: Advances in the understanding of ultrarelativistic beam-plasma instabilities
Abstract: Relativistic beam-plasma instabilities are thought to arise in many astrophysical systems. One of their major effects is to dissipate into heat, suprathermal particles and radiation the kinetic energy of fast outflows from powerful objects. This occurs via self-induced, kinetic-scale electromagnetic fields that scatter and decelerate particles to the point of forming collisionless shocks waves or triggering bright synchrotron-type emissions. In initially unmagnetized, relativistic beam-plasma systems, two main instability classes are known to prevail: the essentially electrostatic, oblique two-stream modes (OTSI) and the essentially magnetic, transverse current filamentation (CFI) modes. These can also operate, usually detrimentally, in laboratory schemes involving the interpenetration of relativistic electron streams and plasmas, as in plasma-wakefield accelerators or ultraintense laser-solid interactions.
My talk will cover some recent theoretical and particle-in-cell simulation results on relativistic beam-plasma instabilities. Most of this work is connected with the E 305 project underway at the SLAC/FACET-II accelerator, which aims to probe the instabilities excited by a 10 GeV electron beam through gas or solid materials. First, I will show that the OTSI dynamics in the case of a finite-size beam impinging onto a collisionless, semi-infinite plasma deviates from the standard picture of a uniform, infinite system. I will present a model for the spatiotemporal effects caused by a sharp beam front and discuss the competition between the OTSI and the self-focusing of a finite-width beam, an issue of prime relevance for experiments. Then, I will consider the case of solid-density target and demonstrate that the hierarchy between the OTSI and CFI depends on the target collisionality and the beam density. The simulation results will be interpreted in light of a general kinetic linear theory of the OTSI and CFI modes in the presence of collisional background electrons. Finally, I will report our latest analysis of the saturation mechanisms of the CFI in asymmetric plasma flows, under conditions representative of relativistic collisionless shocks.
Speaker: Benoit Cerutti, University of Grenoble, France - Chaired by: Dmitri Uzdensky, Associate Editor, JPP
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Date/Time: Thursday 29th September 2022, 4PM BST/11AM EDT
Title: Relativistic magnetospheres: current sheets, reconnection, particle acceleration
Abstract: Spinning neutrons stars and black holes are the central engines of some of the most extreme astrophysical phenomena such as gamma-ray bursts, pulsars, X-ray binaries, binary mergers or active galactic nuclei. The activity of these compact objects is often associated with the creation and the launching of a relativistic magnetized plasma within their magnetospheres. Similarly to their non-relativistic analog in the Solar System, these magnetospheres are highly dynamical. Large-scale current sheets form and reconnect, leading to efficient particle acceleration, pair production and non-thermal radiation. However, the interplay between general relativity, quantum electrodynamics and plasma physics makes this problem not easily tractable. In this talk, I will review current efforts to model pulsar and black hole magnetospheres from first principles by means of global particle-in-cell simulations. Results will be discussed in the context of gamma-ray pulsars, and recent horizon-scale observations of the weakly accreting supermassive black holes M87* and SgrA*.
Speaker: Markus Roth, Focused Energy Inc., Germany - Chaired by: Hartmut Zohm, Associate Editor, JPP
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Date/Time: Thursday 22nd September 2022, 4PM BST/11AM EDT
Title: A private path to laser-fusion energy, Focused Energy, a new startup in the fusion community
Abstract: As clean and safe energy is needed more than ever new developments have led to the rise of startup companies around the globe taking advantage of the science developed of the years and combining the results of the past with the technology of the 21st century to make fusion energy a reality.
Focused Energy is a US/German startup supported by the TU Darmstadt and deeply embedded in the international science community. We are focusing on the concept of direct-drive laser-based inertial confinement fusion and fast ignition.
In our approach, a small pellet containing a milligram of DT is directly irradiated by intense laser light and compressed to roughly 1000 times solid density. At the moment of maximum density, a burst of energetic, laser-driven ion beams is focused into a small part of the compressed fuel to rapidly rise the temperature above ignition temperature and start a bootstrap fusion reaction, which results in a supersonic burn wave consuming the fuel.
More than two decades of research have led to this path, which has recently been quoted the most promising approach in inertial fusion energy by international leaders in the field.
We have started assembling the best scientist in the field and will move on a fast track to get fusion demonstrated in a decade time frame. Thus, we build a first facility, T-STAR and UT in Austin, while we are working on laser and target development in Darmstadt.
Focused Energy finally plans to develop a demonstration facility within this decade to demonstrate ignition, burn and gain sufficient for attractive energy production based on the unique combination of high-energy and high-power lasers.
Speaker: Yuan Shi, Lawrence Livermore National Laboratory, USA - Chaired by: Dmitri Uzdensky, Associate Editor, JPP
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Date/Time: Thursday 15th September 2022, 4PM BST/11AM EDT
Title: Quantum computing for plasma physics: An overview of recent progress
Abstract: Quantum computing (QC) promises to bring game changing capabilities. However, the potential for quantum speedup of plasma problems remains unclear. This colloquium surveys recent progress on applying QC to plasma related problems. To connect plasma physics with quantum hardware, both top-down and bottom-up approaches have yielded interesting results. The top-down approach starts from future ideal computers and reconceptualizes plasma problems to fit into the QC framework. The bottom-up approach starts from near-term noisy devices and builds up toy problems towards more realistic applications. The research field is still in its early stage, and wide gaps remain to be bridged before QC will become useful for plasma physics.
Speaker: Maxim Lyutikov, Purdue University, USA - Chaired by: Dmitri Uzdensky, Associate Editor, JPP
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Date/Time: Thursday 8th September 2022, 4PM BST/11AM EDT
Title: Plasma physics of Fast Radio Bursts
Abstract: Fast Radio Bursts are millisecond long events of coherent radio emission coming from half way across the Universe. The inferred plasma and radiation conditions are extreme: radio waves carry macroscopic amount of energy, the laser non-linearity parameter $a_0$ can be as large as millions, the guiding magnetic field may reach critical quantum values. I will highlight plasma challenges of producing high brightness coherent emission in astrophysical surrounding, new regimes of plasma-laser interaction, and discuss how the concept of Free Electron Lasers may help us to understand these mysterious events.
Speaker: Hong Qin, Princeton Plasma Physics Laboratory, USA - Chaired by: Bill Dorland, Editor, JPP
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Date/Time: Thursday 1st September 2022, 4PM BST/11AM EDT
Title: Plasma wave topology and topological plasma waves
Abstract: The hairy ball theorem is well-known to plasma physicists because it implies that the surface of the magnetic bottle for confining fusion plasmas should not look like a 2D sphere. Chern’s celebrated method of 1944 to prove the hairy ball (Gauss-Bonnet-Poincare) theorem can be used to prove that the cyclotron wave must vanish somewhere on a 2D sphere enclosing the Weyl point of Langmuir-cyclotron resonance in the parameter space. A physical consequence of this fact is that there must exist a topological surface excitation called Topological Langmuir-Cyclotron Wave (TLCW) in magnetized plasmas, which can propagate along complex phase transition interfaces in a unidirectional manner and without scattering. Due to this topologically protected robustness, the TLCW could be explored as an effective mechanism to drive current and accelerate particles in plasmas. The topological methods that we recently proposed for plasma waves bear similarities to those used in condensed matter physics in the past four decades. But there are significant differences. In condensed matters, periodic lattices lead to nontrivial topology in momentum space, but we show that in continuous media such as plasmas, nontrivial topology only exists in phase space because of the contractibility of momentum space. Using the algebraic topological concepts and tools that we recently developed, such as the boundary isomorphism theorem, and an Atiyah-Patodi-Singer type of index theorem formulated by Faure, the existence of TLCW as a spectral flow across the band gap is rigorously proven. We also show that the TLCW can be faithfully represented by a tilted Dirac cone. The entire spectrum of a generic tilted Dirac cone in phase space, including its spectral flow, is found analytically.
[This research was supported by the U.S. Department of Energy (DE-AC02-09CH11466).]
References: 1) Topological Langmuir-cyclotron wave. 2) The dispersion and propagation of topological Langmuir-cyclotron waves in cold magnetized plasmas. 3) Topological phases and bulk-edge correspondence of magnetized cold plasmas.
Speaker: Jack Halliday, Imperial College London, UK - Chaired by: Bill Dorland, Editor, JPP
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Date/Time: Thursday 25th August 2022, 4PM BST/11AM EDT
Title: Investigating radiatively driven, magnetized plasmas with a university scale pulsed-power generator
Abstract: We describe results from a novel experimental platform that is able to access physics relevant to topics including indirect-drive magnetized inertial confinement fusion, laser energy deposition, various topics in atomic physics, and laboratory astrophysics (for example, the penetration of B-fields into high energy density plasmas). This platform uses the x rays from a wire array Z-pinch to irradiate a silicon target, producing an outflow of ablated plasma. The ablated plasma expands into ambient, dynamically significant B-fields (∼5 T), which are supported by the current flowing through the Z-pinch. The outflows have a well-defined (quasi-1D) morphology, enabling the study of fundamental processes typically only available in more complex, integrated schemes. Experiments were fielded on the MAGPIE pulsed-power generator (1.4 MA, 240 ns rise time). On this machine, a wire array Z-pinch produces an x-ray pulse carrying a total energy of ∼15 kJ over ∼30 ns. This equates to an average brightness temperature of around 10 eV on-target.
Speaker: Scott Baalrud, University of Michigan, USA - Chaired by: Edward Thomas, Associate Editor, JPP
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Date/Time: Thursday 18th August 2022, 4PM BST/11AM EDT
Title: Exploring Novel Properties of Strongly Magnetized Plasmas
Abstract: An often-underappreciated aspect of plasma theory is that it assumes weak magnetization, applying only when the electron gyroradius is much larger than the Debye length. This allows one to ignore the magnetic field at the scale of Coulomb interactions. Here, we use first-principles molecular dynamics simulations and new theoretical methods to explore the properties of strongly magnetized plasmas. A few novel behaviors have been uncovered. One is that the friction force on a test ion moving through a strongly magnetized plasma shifts to obtain components that act perpendicular to its velocity. These components cause qualitative changes to the average trajectory of the ion, such as changing its gyroradius and gyrofrequency in non-intuitive ways. They also translate to qualitative changes in macroscopic material properties of the plasma, such as the electrical conductivity, viscosity, and energy relaxation rates. Although strongly magnetized plasmas are not the norm, they do arise in several contexts, including non-neutral plasmas, antimatter traps, high magnetic field approaches to fusion energy, and in dense astrophysical objects such as magnetars. These results suggest that unexpected behaviors arise in these systems, and it motivates potential applications that make use of these novel properties.
Speaker: Anne White, MIT, USA - Chaired by: Peter Catto, Associate Editor, JPP
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Date/Time: Thursday 11th August 2022, 4PM BST/11AM EDT
Title: Turbulence and transport research beyond the burning plasma era
Abstract: The prospect of near-term fusion electricity opens new doors for university-based plasma physics research. Even after the grand societal challenge of putting fusion on the grid is achieved, research addressing grand intellectual challenges in plasma transport will remain vibrant. University groups will engage with sponsors and collaborators including not only governments and national labs around the world, but also private companies and utilities. In this talk I present side-by-side examples of recent research results on turbulence and transport measurements, as well as predictive simulation and modeling, carried out by researchers at MIT in support of both the nascent fusion industry and the established fission industry. I will share my perspective, as an academic department head, on the future of fusion research in universities as we move through and beyond the era of burning plasmas.
Speaker: Jon Hillesheim, UK Atomic Energy Authority, UK - Chaired by: Bill Dorland, Editor, JPP
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Date/Time: Thursday 4th August 2022, 4PM BST/11AM EDT
Title: Highlights of T and DT results from JET-ILW experiments
Abstract: JET has recently completed the first DT campaign in a tokamak since 1997, and for the first time ever with ITER-like W/Be divertor and first wall materials. Scientific results have been achieved in all goals of the campaign including demonstration of high (average Pfus>10 MW ) sustained fusion power for 5 s, demonstration of an integrated radiative scenario, demonstration of clear alpha particle effects, exploration of isotope and mixed plasma species effects on energy and particle transport, addressing plasma-wall interaction in DT plasmas, and demonstration of RF schemes relevant to ITER DT operation. Highlights from the DT campaign, and from the preparatory isotope campaigns, will be presented.
Speaker: Ami Dubois, Naval Research Laboratory, USA - Chaired by: Edward Thomas, Editor, JPP
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Date/Time: Thursday 28th July 2022, 4PM BST/11AM EDT
Title: Compressed Current Sheets in the Magnetotail: Importance of the Ambipolar Electric Field
Abstract: Micro-scale features are now being resolved by NASA’s Magnetospheric Multi-Scale (MMS) mission, which means for the first time, we are able to investigate thin and non-ideal current sheets (i.e. current sheets that cannot be explained by the Harris equilibrium model) in detail and assess their role in magnetic reconnection. We use MMS satellite data to analyze kinetic-scale structures and dynamics associated with compressed current sheets. Our analysis shows that a transverse ambipolar electric field is localized to the region of lower hybrid fluctuations and the pressure gradient in this region is comparatively small, leading to the interpretation that compression of the current sheet and the resulting velocity shear is the underlying fluctuation driving mechanism. Our kinetic equilibrium model shows that as a large scale Harris current sheet is compressed, an ambipolar electric field forms and produces velocity shear near the magnetic null, indicating that velocity shear-driven waves can arise in the center of compressed current sheets. The presence and location of shear-driven waves at the center of current sheets is notable for a couple of reasons. First, because laboratory experiments and PIC simulations have shown that shear-driven lower hybrid fluctuations are capable of producing significant anomalous cross-field transport and resistivity, which can trigger magnetic reconnection. Second, using MMS wave data we can calculate the anomalous resistivity directly and show that the resistivity is significant, particularly at the magnetic null. Finally, we show that the electron distribution function is non-gyrotropic, which theoretical arguments suggest is an indicator of the possibility for magnetic reconnection to occur. Our kinetic equilibrium shows that such non-gyrotropic distribution functions can be generated by a quasi-static electric field and does not necessarily arise from wave induced effects.
This work is supported by the US Naval Research Laboratory Base Program.
Speaker: Martin Lemoine, Institut d'Astrophysique de Paris (CNRS - Sorbonne Université), France - Chaired by: Antoine Bret, Associate Editor, JPP
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Date/Time: Thursday 21st July 2022, 4PM BST/11AM EDT
Title: Particle acceleration in turbulent plasmas: colourizing the Fermi picture
Abstract: Particle acceleration in magnetohydrodynamic (MHD) turbulence represents a fundamental dissipation process in a wide variety of astrophysical sources. This seminar discusses a generalization of the original Fermi scenario of point-like scatterings off discrete, moving structures to a turbulent MHD flow. The energization rate of a particle is first connected to the spatio-temporal gradients of the velocity of magnetic field lines; this notably implies that the intermittent nature of those gradients shapes the acceleration physics. This non-resonant Fermi process is then shown to account for the bulk of particle energization observed in numerical simulations (particle-in-cell and MHD) of relativistic, strong turbulence. Finally, I introduce a transport equation to describe the evolution of the distribution function in momentum space, paying proper account to the intermittent statistics of the accelerating structures. This transport model can reproduce the time evolution of particle energy distributions that are collected by particle tracking in the forced MHD turbulence of the Johns Hopkins University database.
Speaker: Rachael McDermott, IPP, Garching, Germany - Chaired by: Bill Dorland, Editor, JPP
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Date/Time: Thursday 14th July 2022, 4PM BST/11AM EDT
Title: Validation of low-Z impurity transport theory using boron perturbation experiments in ASDEX Upgrade
Abstract: Impurities are unavoidable in fusion plasmas and potentially problematic as they result in fuel dilution and radiative energy losses. Therefore, it is important to have accurate predictions of impurity behavior in future fusion devices, which requires a validated theoretical description of impurity transport. With this goal, an experimental technique was developed at ASDEX Upgrade (AUG) to separately identify the diffusive and convective components of the boron particle flux [1-2]. Using this technique, a database of B transport coefficients covering a wide range of plasma parameters has been assembled and can now be used to validate theoretical predictions of low-Z impurity transport [2]. This database shows that the normalized ion temperature gradient (R/LTi) is the strongest organizing parameter for both the B diffusion and convection and strong R/LTi (>6) is a necessary ingredient to obtain hollow B density profiles in AUG. This database also shows that large changes in the applied neutral beam injection (NBI) have a relatively small impact on impurity transport compared to similar changes in electron cyclotron resonance heating (ECRH). Even low levels of ECRH power dramatically increase both the diffusive and convective fluxes and lead to peaking of the impurity density profile. Comparisons to a combination of neoclassical and quasi-linear gyrokinetic simulations show good agreement in the measured and predicted diffusion coefficients. The outward convection measured in NBI dominated plasmas, however, is not well captured by the simulations, despite the inclusion of fast ions [3]. In contrast, the convection is reasonably well reproduced for plasmas with flat or peaked boron density profiles. This dataset provides an excellent experimental validation of the non-monotonic, predicted, convective-particle-flux created by the combination of pure-pinch, thermodiffusion, and roto-diffusion. In addition, this dataset demonstrates a non-monotonic dependence of the experimental particle diffusivity to ion heat conductivity (D/χi) in qualitative agreement with theoretical predictions.
[1] C. Bruhn et al Plasma. Phys. Control. Fusion 60 (2018) 085011
[2] Corrigendum Bruhn C. et al. Plasma Phys. Control Fusion 62 (2020) 049501
[2] R. M. McDermott et al. 2022 Nucl. Fusion 62 (2022) 026006
[3] P. Manas et al Nucl. Fusion (60) 2020 056005
Speaker: Alice Harding, Los Alamos National Laboratory, USA - Chaired by: Dmitri Uzdensky, Associate Editor, JPP
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Date/Time: Thursday 7th July 2022, 4PM BST/11AM EDT
Title: Plasma Physics of Pulsar Magnetospheres
Abstract: Pulsars turn out to be much more complex than a seemingly simple rotating dipole field. The relativistic plasma required for current closure in their magnetospheres requires two signs of charge, so production of electron-positron pairs in required. A great deal of progress has been made over the last 15-20 years in understanding the structure of fields and currents in the pulsar magnetosphere. However, the source of the plasma and how the microphysics of its production is self-consistently coupled with the global magnetosphere is still not resolved. While we have determined that the main site of particle acceleration and high-energy radiation is in the current sheet outside the light cylinder, the details of the mechanisms involved are also not resolved. I will review the theoretical progress to date, from force-free MHD global models to particle-in-cell simulations. I will also review the recent ideas for generating the pulsar emission from radio to Very-High-Energy wavelengths that is ultimately needed to connect with observations.
Speaker: Derek Schaeffer, Princeton University, USA - Chaired by: Troy Carter, Associate Editor, JPP
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Date/Time: Thursday 30th June 2022, 4PM BST/11AM EDT
Title: Collisionless Shockwaves in Magnetized High-Energy-Density Laboratory Plasmas
Abstract: As a fundamental process for converting kinetic to thermal energy, collisionless shocks are ubiquitous throughout the heliosphere and astrophysical systems, from Earth's magnetosphere to supernova remnants. While these shocks have been studied for decades by spacecraft, telescopes, and numerical simulations, there remain key open questions in the fundamental physics of collisionless shocks, such as: How do shocks accelerate particles to extremely high energies? or How is energy partitioned between particles across a shock?
In this talk, I will discuss results from high-energy-density experiments and simulations on the formation of supercritical collisionless shocks created through the interaction of a supersonic laser-driven magnetic piston and magnetized ambient plasma. Through proton and refractive imaging, we observe for the first time a magnetized collisionless shock, comparable to some of the strongest shocks in the heliosphere. By probing particle velocity distributions with Thomson scattering, we directly measure the coupling interactions between the piston and ambient plasmas that are critical steps in the formation of magnetized collisionless shocks. Particle-in-cell simulations constrained by experimental data further detail the shock formation process and predict key signatures that are observed in experiments. I will also discuss how the development of this experimental platform can complement, and in some cases overcome, the limitations of similar measurements undertaken by spacecraft missions and can allow novel investigations of energy partitioning and particle acceleration in shocks.
Speaker: Piero Martin, University of Padua, Italy - Chaired by: Hartmut Zohm, Associate Editor, JPP
Date/Time: Thursday 23rd June 2022, 4PM BST/11AM EDT
Title: Status of the Divertor Tokamak Test facility
Abstract: This talk illustrate the physics basis which supports the main engineering choices in the Divertor Test Tokamak facility (DTT) under construction in Frascati, Italy.
DTT is a superconducting tokamak with 6 T on-axis maximum toroidal magnetic field, carrying plasma current up to 5.5 MA in pulses with total length up to 100 s.
The D-shaped device has a major radius R=2.19 m, minor radius a=0.70 m, with an average triangularity 0.3. The auxiliary heating power coupled to the plasma at maximum performance is 45 MW, which allows matching the PSEP/R values with those of ITER and DEMO, where PSEP is the power flowing through the last closed magnetic surface.
The primary mission of DTT is the study of the plasma exhaust and of tokamak divertor performance in conditions relevant to ITER and DEMO and in regimes where plasma core and edge behaviors are integrated.
In addition to that DTT will provide a facility for high performance tokamak physics and to address core confinement and stability issues in a variety of plasma configurations, including negative triangularity scenarios and the management of transient events like disruptions and ELMs.
Speaker: Archie Bott, Princeton University, USA - Chaired by: Luís Silva, Associate Editor, JPP
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Date/Time: Thursday 16th June 2022, 4PM BST/11AM EDT
Title: Turbulence and thermodynamics in expanding, collisionless, magnetised plasma
Abstract: The magnetised plasma composing many different astrophysical systems of interest – from the solar wind to the intracluster medium of galaxy clusters – is often weakly collisional or collisionless, with the Larmor radii of the constituent particles being many orders of magnitude below their Coulomb mean free paths. This feature results in a complex interplay between a plasma's macrophysical evolution (e.g., due to expansion, compression, or large-scale shear) and its microphysical response (e.g., triggering of kinetic instabilities). In this talk, we will elucidate this phenomenon aided by the results of several hybrid-kinetic expanding-box simulations. We will show how the nonlinear dynamics of strong Alfvénic turbulence in a collisionless plasma efficiently adapts to changes in fundamental wave physics that are induced by the effect of macroscopic expansion on microscopic particle motions. This adaptation holds irrespective of a qualitative transformation to the plasma’s thermodynamics caused by pressure-anisotropy-driven kinetic instabilities. We will also demonstrate that different rates of expansion can lead to two qualitatively distinct thermodynamic states: in one state, Alfvén waves are supported; in the other, they are suppressed. These states will be characterised in detail, including the firehose-induced effective collisionality. Our results may help to disentangle the signatures of kinetic instabilities and strong Alfvénic turbulence in key observables in the near-Earth solar wind, such as magnetic power spectra and ion velocity distribution functions.
Speaker: Bill Dorland, University of Maryland, USA - Chaired by: Alex Schekochihin, Editor, JPP
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Date/Time: Thursday 9th June 2022, 4PM BST/11AM EDT
Title: Prospects for real-time, first-principles transport simulations and stellarator optimization including turbulence
Abstract: The open-source Trinity code solves for the time-dependent radial profiles of density, temperature, etc, using turbulent fluxes obtained from any radially local gyrokinetic turbulence code, neoclassical fluxes obtained from any drift kinetic solver, external sources, and edge boundary conditions supplied by the user. While originally developed for tokamak applications, the multiscale approach of Trinity is easily generalized for stellarator applications, as long as the equilibrium is assumed to consist of nested flux surfaces without islands. We present results using the original Trinity code together as well as a new Python version that will enable broader, easier uptake by the community. In 2018, we embedded Trinity into an optimization framework and demonstrated the ability to optimize tokamak shaping to maximize fusion power using first-principles estimates for turbulence-induced fluxes. Here, we will present our approach to embedding these gyrokinetic tools into the SIMSOPT framework. GX is an open-source, radially-local, GPU-native, gyrokinetic turbulence code that uses pseudo-spectral methods and native CUDA libraries to calculate turbulence-induced fluxes and critical gradients. At high resolution, GX is simply yet another GK code, but it can be run successfully at low resolution, in lieu of uncontrolled approximations and reduced models. With these tools, we demonstrate the ability to solve for the time-dependent evolution of core fusion reactor profiles in approximately real time, without resorting to reduced models. We also demonstrate the ability to find a shape, size, etc, that maximizes fusion performance by minimizing turbulence-induced losses “inside the optimization loop” for families of tokamak and stellarator reactor concepts, using equilibrium information calculated by VMEC and/or a near-axis expansion approximation, and we present machine-learned, sub-grid techniques that could further accelerate these calculations. I will show linear and nonlinear benchmarks against standard codes from the community, for both tokamak and stellarator configurations. Finally, we introduce the concept of specific computational intensity and use it to demonstrate how one can decide when to retire a given reduced model and rely instead on a higher-fidelity approach. There are many, many reduced models available for modeling fusion plasmas and without some kind of easy-to-use, objective method to distinguish the appropriateness of one approach from another, modelers and designers are often left to work out how to proceed more or less randomly. This leads to severe combinatoric complexity in design and interpretation efforts, which we hope to help bring under control.
Speaker: Elizabeth Wolfrum, IPP Garching, Germany - Chaired by: Hartmut Zohm, Associate Editor, JPP
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Date/Time: Thursday 2nd June 2022, 4PM BST/11AM EDT
Title: The road to pedestal tailoring at ASDEX Upgrade
Abstract: In the narrow edge region of a tokamak transport can be reduced by suppression of turbulence. The core plasma confinement is then elevated and consequently, the region with reduced turbulence is called ‘pedestal’. This work gives an overview of recent investigations at ASDEX Upgrade that show our current understanding of the transport mechanisms in the pedestal and how transport and stability in this narrow region can be influenced.
For electron heat transport a constant temperature gradient length hints towards a local small-scale turbulent transport mechanism. The ion heat transport is close to neoclassical values, however in some cases this only holds in the central part of the pedestal with deviations at the pedestal top and foot. The shape and position of the edge density profile are key to both stability and transport and remains the parameter which can be most varied in the pedestal.
In our search for a scenario without large edge localised modes, ballooning modes can be driven unstable at the pedestal foot. Careful balance of the drive and stabilising terms allows the pedestal to be tailored such that the global peeling-ballooning stability limit is not breached. Another globally stable regime is achieved with strong nitrogen seeding, leading to the formation of an X-point radiator. These two ELM-free regimes are important research topics for the extrapolation to larger devices.
Speaker: Shinya Maeyama, Nagoya University, Japan - Chaired by: Bill Dorland, Editor, JPP
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Date/Time: Thursday 26th May 2022, 4PM BST/11AM EDT
Title: Cross-scale interactions between ion and electron-scale turbulence in magnetized plasmas
Abstract: Recent gyrokinetic simulations have revealed the existence of cross-scale interactions between disparete turbulence at ion and electron gyroradius scales. I would like to start my talk by reviewing recent studies of multi-scale gyrokinetic simulations and discussing problems to be solved in future. For addressing one of these issues, we examine the extrapolation of cross-scale interactions toward high electron temperature burning plasmas, and demonstrate a possibility of reduction of turbulent transport by cross-scale interactions. In the latter part of this talk, I also would like to discuss the methodology for extracting and modeling cross-scale interactions between disparate-scale turbulence. To this end, we have developed a statistical analysis technique based on Mori-Zwanzig projection operator method, which decomposes time evolution of variable of interests into correlated/uncorrelated terms with regard to the explanatory variables. We discuss validity/applicability of the method to multi-scale turbulence problem based on the results of application example to simple plasma turbulence problem.
Speaker: Mark Sherlock, Lawrence Livermore National Laboratory, USA - Chaired by: Bill Dorland, Editor, JPP
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Date/Time: Thursday 19th May 2022, 4PM BST/11AM EDT
Title: Overview of plasma transport processes relevant to Inertial Confinement Fusion at the National Ignition Facility
Abstract: A predictive simulation capability is a long term goal of the Inertial Confinement Fusion research program at the Nation Ignition Facility laser. Achieving this goal requires us to understand a number of plasma transport processes in detail in order to assess their overall impact on achieving a sufficiently efficient and symmetric energy transfer from the laser “drive” to the fusion fuel. This talk will give an overview of the processes currently being explored and the associated computational and theoretical techniques. Topics include: modeling electron thermal transport in the kinetic regime with Vlasov-Fokker-Planck codes; generation and transport of magnetic field by lasers; generation of ion turbulence by strong heat flow; ion kinetic effects inside the fuel capsule; transport instabilities involving magnetic field including the thermomagnetic, collisional Weibel, electrothermal and magnetothermal instabilities; the effect of laser speckles on transport; and the theory of laser absorption in non-thermal plasmas.
Speaker: Erika Ye, MIT, USA - Chaired by: Bill Dorland, Editor, JPP
Date/Time: Thursday 12th May 2022, 4PM BST/11AM EDT
Title: Quantum-inspired methods for solving the Vlasov-Poisson equation
Abstract: Kinetic simulations of collisionless (or near-collisionless) plasmas using the Vlasov equation are often infeasible due to high resolution requirements and the exponential scaling of computational cost with respect to dimension. Recently, it has been proposed that matrix product state (MPS) methods, a quantum-inspired but classical algorithm, can be used to approximately solve partial differential equations with exponential speed up, provided that the solution can be compressed and efficiently represented as an MPS within some tolerable error threshold. In this work, we explore the practicality of MPS methods for solving the Vlasov-Poisson equations in 1D1V, and find that important features of linear and nonlinear dynamics, such as damping rates and saturation energies, can still be captured while compressing the solution by at least a factor of 8. Furthermore, by comparing the performance of different mappings of the distribution functions onto the MPS, we generate some intuition of the MPS representation and its behavior, which will be useful for extending these methods to higher dimensional problems.
Speaker: Brian Reville, Max Planck Institute for Nuclear Physics, Germany - Chaired by: Bill Dorland, Editor, JPP
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Date/Time: Thursday 5th May 2022, 4PM BST/11AM EDT
Title: Extremely shocking limits - the maximum energy particles at astrophysical shocks
Abstract: Many astrophysical shocks are observed to be coincident with sources of bright non-thermal radiation (and vice-versa). The emission is produced by relativistic particles that are thought to be accelerated by the Fermi shock acceleration process, where particles gain energy by repeatedly sampling the converging flows on opposite sides of the shock transition. The maximum energy a particle can achieve via this process is sensitive to the nature of the plasma instabilities that occur in the region around the shock, which are in general driven by the energetic particles themselves. The current predictions on the maximum energy have profound implications for cosmic-ray origin theories that assume supernova remnants are the dominant contributor. Recent gamma-ray observations are starting to constrain these models, and indicate that alternative scenarios should be seriously explored. The possible role of laboratory plasma experiments in supporting new theories will be touched upon.
Speaker: Michael Barnes, University of Oxford, UK - Chaired by: Bill Dorland, Editor, JPP
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Date/Time: Thursday 28th April 2022, 4PM BST/11AM EDT
Title: Bistable turbulence in tokamak plasmas
Abstract: The prevailing paradigm for magnetised plasma turbulence associates a unique, stationary turbulent state with a given set of equilibrium plasma parameters. In this talk we present data from gyrokinetic simulations of tokamak plasmas showing that bistable turbulence is possible in the presence of imposed flow shear. In particular, we show that at least two distinct, finite amplitude turbulent states can be obtained for the same input plasma parameters, with the initial condition determining the steady-state solution. We will argue, with the aid of example data, that the occurrence of bistable turbulence is regulated by a competition between externally-imposed, equilibrium flow shear and `zonal’ flow shear that is generated by the turbulence itself. This competition produces an unexpected behaviour in some cases: the combination of zonal and equilibrium flow shears, each of which individually suppresses turbulence, results in an enhancement of turbulence amplitudes.
Speaker: Vladimir Zhdankin , Flatiron Institute, USA - Chaired by: Troy Carter, Associate Editor, JPP
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Date/Time: Thursday 21st April 2022, 4PM BST/11AM EDT
Title: Generalized entropy production in nonthermal collisionless plasmas
Abstract: Collisionless plasmas develop nonthermal particle distributions after being energized, and thus enter a state of low Boltzmann-Gibbs entropy. While the Vlasov equation predicts that Boltzmann-Gibbs entropy is formally conserved (along with an infinite set of other Casimir invariants), anomalous entropy production may be enabled by phase mixing, nonlinear entropy cascades, etc. The characterization of entropy production for various irreversible processes is a fundamental problem at the frontier of plasma physics. In this talk, I will describe how entropy production (in a generalized sense) can be represented by the evolution of an infinite set of dimensional quantities derived from the Vlasov equation, the so-called "Casimir momenta", which characterize violations of the Vlasov equation (and therefore irreversibility) at different energy scales. I will then suggest that Casimir momenta may be used to construct models of nonthermal particle acceleration in plasma processes of sufficient complexity, such as turbulence or magnetic reconnection, providing a possible route to understanding the prevalence and properties of power-law distributions. These ideas may open a new perspective into particle energization in collisionless plasmas.
Speaker: Anatoly Spitkovsky, Princeton University, USA - Chaired by: Dmitri Uzdensky, Associate Editor, JPP
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Date/Time: Thursday 14th April 2022, 4PM BST/11AM EDT
Title: Particle acceleration in collisionless shocks: connecting micro and macro scales
Abstract: Sudden deceleration of supersonic flows results in shock waves, which in the conditions of low density plasmas are mediated by collisionless processes. Such colliisionless shocks in astrophysical environments are thought to be responsible for the generation of nonthermal particles that span many decades in energy. These particles produce synchrotron radiation from astrophysical sources, such as supernova remnants and relativistic jets, or are observed directly as energetic cosmic rays. The main acceleration mechanism for these particles is known as "diffusive shock acceleration" and involves particle scattering and diffusion around a shock wave. In the nonlinear stage, shock acceleration couples together the internal structure of the shock with magnetic turbulence generated by accelerated particles, and presents a fascinating self-propagating nonlinear system with multiscale feedbacks. With the development of ab-initio numerical simulations of collisionless shocks, many details of the shock acceleration mechanism can now be studied directly. In this talk I will review the progress in kinetic (PIC) simulations of shock structure and particle acceleration in various regimes, and focus on processes that lead to electron acceleration in non-relativistic shocks, including field amplification, electron heating, and nonlinear regulation of shock injection. The lessons learned from microscopic PIC simulations suggest pathways to larger simulations that use augmented MHD techniques to study shock acceleration on the scales of astrophysical objects. I will discuss such MHD-PIC approaches and applications of current results to morphologies and spectra of nonthermal emission from supernova remnants and galaxy clusters.
Speaker: Tom Blackburn, University of Gothenburg, Sweden - Chaired by: Troy Carter, Associate Editor, JPP
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Date/Time: Thursday 7th April 2022, 4PM BST/11AM EDT
Title: Laser-matter interactions at ultra-high intensity: how do we simulate them and what can experiments tell us?
Abstract:As the intensity frontier pushes past 1023 W/cm-2, experiments with high-intensity lasers interacting with matter, whether plasma or relativistic particle beams, enter a new regime. Here the dynamics arise from the interplay between relativistic plasma physics and strong-field, nonperturbative, quantum electrodynamics (QED). Understanding these processes is essential for developing our knowledge of extreme astrophysical environments, such as pulsars, magnetars and black-hole magnetospheres. In this talk I will present an overview of the progress that has been made in investigating the strong-field regime, from the simulation models we use, to the experiments that are possible with today's high-power lasers.
Speaker: Vladimir Yankov, Ergophos LLC, USA
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Date/Time: Thursday 31st March 2022, 4PM BST/11AM EDT
Title: Improvement of confinement in tokamaks by a weakening the poloidal magnetic field at the boundary, invariants, and attractors
Abstract: Density profiles of tokamaks are enigmatically peaked and can be described as a turbulent attractor defined by a conservation law, namely, the plasma is frozen in the poloidal magnetic field. The profiles aka Turbulent EquiPartition are accurately described by a simple formula nv=const where v is the specific poloidal volume. The formula predicts that density and temperature at the border will decrease if the v is increased. This can be done in many ways and was observed experimentally before any theory emerged. The first way observed was current rampdown and the latest way was negative triangularity. Since almost all results in tokamaks were obtained experimentally, the theory will be presented briefly as well as several new ways to improve confinement. The theory will include the origin of many plasma and tokamak invariants from the Poincare invariant.
Speaker: Axel Brandenburg, NORDITA, Sweden
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Date/Time: Thursday 24th March 2022, 3PM GMT/11AM EDT
Title: Primordial magnetic fields
Abstract: Until the late 1980s, primordial magnetic fields were frequently invoked as an alternative to dynamo theory for explaining the galactic magnetic field. Nowadays, dynamo theory is generally accepted, but primordial magnetic fields might still exist, and could permeate the entire Universe - even in the voids of galaxy clusters. The field strength today could be in the range of 1e-16 to 1e-9 Gauss at a reference scale of 1 Mpc. The lower limit is motivated by the non-observation of secondary GeV photons from TeV blazars, although plasma instabilities may provide an alternative explanation for the non-observation.
In my talk, I will discuss various scenarios for primordial magnetic field generation. A relatively straightforward idea is to invoke the chiral magnetic effect, which could lead to exponential growth of the magnetic field over a limited period of time. A completely different idea is to amplify quantum fluctuations during inflation, for example through a hypothetical coupling of electromagnetic fields to an axion field. In all these scenarios, the magnetic field might be strong enough to drive also relic gravitational waves (GWs), which could be detectable with space interferometers, pulsar timing arrays, or high precision astrometry. Particularly exciting are the prospects of detecting circular polarization of GWs, which would indicate helicity of the underlying magnetic field. Other potential detection techniques of helical primordial magnetic fields involve non-mirrorsymmetric features in the cosmic background radiation and the pattern of arrival directions of energetic photons in the sky. In all these cases, the magnetic fields are turbulent, and I present numerical simulations for various generation scenarios and the subsequent decay of magnetic fields. Finally, I discuss the prospects of learning about their energy spectra through GW measurements.
Speaker: Ian Hutchinson, MIT, USA
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Date/Time: Thursday 10th March 2022, 4PM GMT/11AM EST
Title: Electron holes in collisionless plasmas: how long do these common nonlinear structures last?
Abstract: Electron phase-space holes are now widely observed in space plasmas. They consist of a solitary positive potential peak with depleted electron population on trapped orbits that sustains the potential; and so they are intrinsically kinetic: governed by the Vlasov equation. Important new details about their speed and structure are now emerging from multi-satellite measurements. This talk will introduce the principles, observations, and simulations of electron holes; explain the ways that they behave like composite objects possessing lumped momentum, negative mass, and kinematic properties; and show how these concepts determine how and when they break up by instabilities. Instability probably determines the lifetime of a hole when collisions are negligible.
Speaker: Rachel Bielajew, MIT, USA
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Date/Time: Thursday 3rd March 2022, 4PM GMT/11AM EST
Title: Exploring edge turbulence in the low and improved confinement regimes at the ASDEX Upgrade tokamak
Abstract: Future tokamak fusion reactors will need to operate in a regime of high energy confinement while particle confinement remains low enough that impurities can be exhausted. The low confinement operating regime L-mode has no edge transport barrier and lacks high energy confinement. The high confinement operating regime H-mode has been a target for high confinement operation, however its steep pedestal gradients lead to the edge instability Edge Localized Modes (ELMs). ELMs exhaust impurities and allow for steady-state high confinement operation, but they also release substantial energy which can damage material surfaces. The “improved” confinement regime I-mode is a promising operational scenario for future fusion reactors because it features an edge energy transport barrier without a particle transport barrier and it is naturally ELM-free. The mechanism that leads to this separation of transport channels in I-mode is an open question. The nature of the edge and pedestal turbulence in I-mode plasmas, and its role in determining transport, is still under investigation. In this work we explore edge fluctuations in the L-mode and I-mode edge at the ASDEX Upgrade tokamak through detailed study with turbulence diagnostics. In conjunction, linear gyrokinetic studies probe the nature of the turbulence from the outer core to the pedestal top. We find that the pedestal Weakly Coherent Mode (WCM) remains similar in nature in L-mode and I-mode and that ion-scale fluctuations in the outer core and pedestal top also undergo little change between L-mode and I-mode. The electron scale is a potential candidate for the suppression of heat flux in I-mode, separated from possible particle flux mechanisms. Cross-scale coupling is seen to be important in the I-mode outer core and pedestal.
Speaker: Paola Mantica, ISTP-CNR, Milan
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Date/Time: Thursday 24th February 2022, 4PM GMT/11AM EST
Title: Tokamak turbulence stabilization by electromagnetic effects and fast ions
Abstract: A decade ago the experimental discovery on JET that ion temperature profile stiffness (due to the strength of turbulence reaction to changes in ion temperature normalized gradient) is reduced by increased Neutral Beam and/or Ion Cylotron Resonant Heating power triggered an intense work to understand and reproduce/expand these results. The JET results were explained by means of gyrokinetic simulations as due to non-linear electromagnetic stabilization of ion turbulence associated with pressure gradients (including thermal and suprathermal components). In both JET and ASDEX-Upgrade evidence has been found that these mechanisms are at the basis of improved ion confinement and ion temperature peaking in high power Hybrid scenarios. On DIII-D a similar stabilizing effect was found. An additional mechanism linked to a purely fast ion driven resonant linear electrostatic stabilization has been found in JET high ICRH power discharges and very recently used in ASDEX-Upgrade to design pulses with improved ion temperature peaking. Significant progress has been achieved in the theoretical understanding of these stabilizing effects and work is still in progress to better understand the extrapolability to ITER conditions, especially in presence of highly energetic α particles. This talk will present an overview of the experimental and theoretical work on this topic and will discuss its impact on our predictive capabilities of tokamak scenarios.
Speaker: Lorenzo Sironi, Columbia University, USA
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Date/Time: Thursday 17th February 2022, 4PM GMT/11AM EST
Title: Fast and furious: reconnection and turbulence in magnetically-dominated astrophysical plasmas
Abstract: In the most powerful astrophysical sources, reconnection and turbulence operate in the “relativistic” regime, where the magnetic field energy exceeds even the rest mass energy of the plasma. Here, reconnection and turbulence can lead to fast dissipation rates and efficient particle acceleration, thus being prime candidates for powering the observed fast and bright flares of high-energy non-thermal emission. With fully-kinetic particle-in-cell (PIC) simulations and analytical theory, we investigate the physics of relativistic reconnection and turbulence, and demonstrate that they can be the “engines” behind: (1) high-energy flares in blazar jets; and (2) the hard-state spectra of black hole X-ray binaries and Active Galactic Nuclei.
Speaker: Joaquim Loizu, EPFL, Switzerland
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Date/Time: Thursday 10th February 2022, 4PM GMT/11AM EST
Title: New horizons for stellarator optimization via fast 3D MHD equilibrium and stability calculations with islands and chaos.
Abstract: Fast calculations of 3D MHD equilibria are crucial for stellarator optimization. In fact, the evaluation of macroscopic plasma stability, neoclassical and turbulent transport, as well as the design of coils and heat exhaust solutions, all require the knowledge of the magnetic equilibrium. While stellarator coils can be designed such that the vacuum magnetic field lines approximately lie on nested toroidal foliations, the presence of finite plasma pressure and currents makes the field generally non-integrable, thereby consisting of an intricate combination of magnetic surfaces, magnetic islands, and magnetic field-line chaos. The accurate and fast computation of 3D MHD equilibria is a fascinating challenge, involving the solution of an intrinsically nonlinear problem that is subject to undesirable pathologies and for which only certain classes of numerically tractable solutions are guaranteed to exist. Exploiting the latter, the Stepped-Pressure Equilibrium Code (SPEC) was developed in recent years and is becoming an efficient numerical tool that can supply the required MHD input to optimization codes. In this talk, I will review some exciting questions that have been addressed with SPEC. These are, for example, the understanding of stellarator equilibrium 𝛽-limits including the effect of bootstrap current, the possibility of performing free-boundary optimization to preserve good magnetic surfaces at finite 𝛽, the unified calculation of linear ideal and resistive MHD stability, and the possibility of rapidly finding nonlinear saturation of MHD instabilities.
Speaker: Sophia Henneberg, IPP Greifswald, Germany
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Date/Time: Thursday 3rd February 2022, 4PM GMT/11AM EST
Title: Advanced Approaches in Stellarator Optimization
Abstract: Stellarators possess three-dimensional magnetic fields to generate external rotational transform -- rotational transform solely generated by the coils’ magnetic field. This reduces or even eliminates the need for generating toroidal plasma currents, which can lead to instabilities such as disruptions. However, the three-dimensionality can in general involve some drawbacks, e.g., more complicated coils are typically needed compared to the axisymmetric case. Nonetheless, with careful exploitation of the large design space via optimization, the apparent disadvantages can be diminished.
In stellarator optimization studies, the boundary of the plasma is usually described by Fourier series that are not unique: several sets of Fourier coefficients describe approximately the same boundary shape. A simple method for eliminating this arbitrariness is proposed and shown to work well in practice. Additionally, we investigate the mathematical structure of the various inter-related calculations that underpin the integrated stellarator optimization problem to better understand how the equilibrium calculation, the coil calculation, and the optimization calculation communicate with each other.
Speaker: Troy Carter, UCLA, USA
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Date/Time: Thursday 27th January 2022, 4PM GMT/11AM EST
Title: Turbulence and transport in the Large Plasma Device: shear suppression, nonlinear instability and electromagnetic turbulence
Abstract: The Basic Plasma Science Facility (BaPSF) at UCLA is a US national collaborative research facility for studies of fundamental processes in magnetized plasmas, supported by US DOE and NSF. The centerpiece of the facility is the Large Plasma Device (LAPD), a 20m long, magnetized linear plasma device. LAPD is used to study a number of fundamental processes, including: collisionless shocks, dispersion and damping of kinetic and inertial Alfvén waves, compressional Alfvén waves for ion-cyclotron range of frequencies heating, flux ropes and magnetic reconnection, three-wave interactions and parametric instabilities of Alfvén waves, turbulence and transport and interactions of energetic ions and electrons with plasma waves. An overview of the facility will be given, followed by a more detailed discussion of studies of pressure-gradient-driven turbulence and turbulent transport: suppression of turbulent transport by externally controlled flow and flow shear; simulations of LAPD turbulence that demonstrate the existence and dominance of a nonlinear/non-modal instability; and measurements of electromagnetic drift-wave turbulence, dominated by parallel magnetic field fluctuations, in moderate (~20%) beta plasmas.
Speaker: Eva Kostadinova, Auburn University, USA
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Date/Time: Thursday 20th January 2022, 4PM GMT/11AM EST
Title: Anomalous Electron Diffusion in Magnetic Islands and Stochastic Magnetic Fields
Abstract: Magnetic islands and regions of stochastic magnetic fields originate from the dynamical processes of magnetic reconnection and turbulence in plasma. These structures are ubiquitous in both laboratory settings (e.g., tokamaks and stellarators) and space environment (e.g., solar wind plasma and Earth’s magnetosphere). An interesting feature of magnetic islands and stochastic regions in plasmas is their connection to plasma particle acceleration, often resulting in anomalous diffusion. An important question is what universal principles relate the properties of energetic particles as a function of the underlying magnetic field topology in both lab and space. The answer to this question requires the development of universal transport models.
This talk will introduce a Fractional Laplacian Spectral (FLS) approach to anomalous diffusion in plasmas with magnetic islands and stochastic magnetic fields. The FLS is a novel technique which computes the probability for particle transport as a function of nonlocal interactions and stochasticity in the examined field. The inputs for the model are informed from DIII-D experiments where energetic electrons (exhibiting anomalous diffusion) were observed in the presence of resonant magnetic perturbation (RMP) of the magnetic field and from simulations of the corresponding B-field topology. The perturbation on the B-field results in two characteristic structures: magnetic islands (leading to nonlocal transport) and stochastic regions (leading to chaotic transport). We show how the interplay between typical island scale and the magnitude of stochasticity determine the resulting electron diffusion.
Work supported by NSF-1903450, DE-FC02-04ER54698, DE-FG02-95ER54309, DE-SC0021284, and DE-FG02-05ER54809.
Speaker: John Goree, The University of Iowa, USA
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Date/Time: Thursday 13th January 2022, 4PM GMT/11AM EST
Title: Dusty plasma experiments: strong coupling, shocks, and testing theories of statistical physics
Abstract: Dusty plasmas contain small solid particles, which gain large electric charges. Typically they are micron size polymer spheres. Unlike the electron and ion components, the dust particle component tends to behave like a strongly coupled plasma, with Coulomb collisions dominating to such a degree that particles arrange themselves like atoms in a liquid or a solid. Due to their large size, the dust particles can be imaged individually in video recordings. This video imaging allows experimenters to track individual particles, which is an enormously powerful diagnostic that is unavailable in traditional plasma physics experiments, where electrons and ions cannot be imaged individually. In this talk I present, as two example research topics: shocks and tests of theories of statistical physics. While the shock topic is a traditional one for plasma physics, the topic of testing statistical physics takes the discipline of plasma physics in a new direction, by exploiting particle tracking.
Speaker: Karl Krushelnick, University of Michigan Ann Arbor, USA
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Date/Time: Thursday 16th December 2021, 4PM GMT/11AM EST
Title: Relativistic plasma physics and high field phenomena using intense lasers
Abstract: The past two decades have witnessed the development of revolutionary light sources having the unprecedented ability to probe new physical regimes and control matter with atomic scale precision. The ongoing development of multi-Petawatt lasers around the world will allow exploration of fundamental yet unanswered questions regarding non-linear Quantum Electrodynamics in relativistic plasmas, including non-perturbative quantum radiation reaction and electron-positron pair production mechanisms. Further experiments enabled by such lasers will include pump-probe experiments using femtosecond x-rays as a probe of material dynamics on ultra-short timescales, the production of GeV ion beams, the generation of instabilities in electron-positron jets, the exploration of vacuum polarization effects, relativistic shocks and the production of “exotic” particles such as pions and muons. I will review recent advances in this field and also describe the new NSF funded ZEUS facility under construction at the Center for Ultrafast Optical Science (CUOS) at the University of Michigan. ZEUS will be a dual-beamline 3 PetaWatt laser system that will provide unique capabilities for research. This will be a new high power laser user facility for US scientists as well as for the wider international research community, and will have an open and transparent external review panel for facility access and 30 weeks per year dedicated to external user experiments. After completion in 2023, the ZEUS laser system will be the highest-power laser system in the US.
Speaker: Steve Tobias, University of Leeds, UK
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Date/Time: Thursday 9th December 2021, 4PM GMT/11AM EST
Title: Quasilinear Approximations and Statistical Simulation in turbulent MHD
Abstract: In many situations of interest turbulent dynamics interacts with mean flows, mean magnetic fields or rotation - this usually makes the dynamics both inhomogeneous and anisotropic. In those circumstances progress can often be made by employing quasilinear approximations and developing equivalent statistical theories. In this talk I will examine the history and utility of the quasilinear approximation in fluids and plasmas and describe a generalisation that provides a more accurate model for the dynamics. I will demonstrate the utility using models of the joint MHD instability in the solar tachocline and the driving of zonal flows. I will conclude by speculating on how these techniques can be extended to provide self-consistent, efficient sub-grid models of turbulent processes.
Speaker: Elijah Kolmes, Princeton University, USA
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Date/Time: Thursday 2nd December 2021, 4PM GMT/11AM EST
Title: Maximal Energy Release and the Rules of Rearrangement
Abstract: Throughout plasma physics, we are often interested in processes through which kinetic energy is transferred out of a distribution of particles. Examples of these processes include wave-particle interactions (for instance, the amplification of a wave) as well as the growth of turbulent internal modes. Some particle distributions are more prone to these energy transfers than others. Of interest is the maximal possible energy that can be liberated from a distribution function by wave-particle interactions with constraints on the nature of the interaction. These constraints might be that the waves can only rearrange the 6D phase space, or that they must conserve adiabatic invariants, or that instead they can only act to diffuse particles. This talk will trace the development of these ideas, starting in the 1960s. Among the developments that we will cover is the recent and surprising result that, with enough fine-tuning, the energy recoverable from diffusive processes can reach the energy recoverable from entropy-conserving processes.
Speaker: Victor Malka, Weizmann Institute of Science, Israel
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Date/Time: Thursday 25th November 2021, 4PM GMT/11AM EST
Title: Laser plasma accelerators: First results from the HIGGINS 2x100 TW laser
Abstract: Laser Plasma Accelerators (LPA) are changing the scientific and societal landscape. Opening new hopes for high energy physics, offering alternative to synchrotron light sources with the recent demonstration with LPA’s based Free Electron Radiation, and delivering particle and radiation beams for medical and security applications, they are among the most innovative tools of modern sciences. The laser plasma accelerators are a perfect illustration of what cross-domain fertilization with a zest of imagination can produce. In this talk I’ll explain the main involved concepts, and why these wonderful machines rely on our ability to control finely the electrons motion with intense laser pulses. I’ll show how the electrons collective manipulation permits to produce giant electric fields of value in the TV/m exceeding by 3 orders of magnitude or more the ones used in current machines. These collective motions when controlled permits also to modify and to shape the longitudinal and radial components of the plasma fields for either accelerating efficiently electrons or for producing energetic photons by wiggling electron during their acceleration. This control is crucial for electrons injection that is essential for delivering stable ultra-short and ultra-bright energetic particle or radiation beams. To illustrate the beauty of laser plasma accelerators I will show some concepts we recently demonstrated that allow these controls for beams improvements. Finally, I will show the commissioning of the HIGGINS dual laser system of the Weizmann Institute of Science, together with a set of first experimental results showing new insights of the relativistic plasma fields and a new approach for producing plasma refractive optics for relativistic beam manipulation.
Speaker: Jonathan Squire, University of Otago, New Zealand
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Date/Time: Thursday 18th November 2021, 4PM GMT/11AM EST
Title: The helicity barrier: how low-frequency turbulence triggers high-frequency heating of the solar wind
Abstract: Turbulence, as the agent responsible for converting mechanical into thermal energy, controls the thermodynamic properties of many astrophysical plasmas. For low collisionality plasmas, a wide array of channels are available — ions might be more heated than electrons (or vice versa), or the heating may be anisotropic with respect to the magnetic field. In the solar corona and fast solar wind, which are heated strongly above the Sun’s surface by the liberation of energy in magnetic fields, observations show that heating is perpendicular, affecting heavy ions more than protons, and protons more than electrons. For low-frequency Alfvénic-turbulence models, which are a leading candidate to explain the fast-wind’s properties, this detailed heating partition has been a puzzle: theories predict a variety of outcomes, with electron heating dominating in the highly anisotropic, low-beta limit that seems most relevant to coronal conditions. Another theory, involving resonance with high-frequency ion-cyclotron waves (ICWs), naturally explains details of the heating rates, but it has proved difficult to explain the source of ICWs. In this talk, I will explain how a bizarre effect in plasma turbulence — termed the “helicity barrier” — may resolve this conundrum. This barrier halts the flux of turbulent energy to the smallest scales when the stirring at large scales is dominated by waves propagating in one direction (imbalanced). This inhibits electron heating, causing the energy in magnetic fields to build up in time until it generates ICWs, thus preferentially heating ions instead. In our 6D hybrid-kinetic simulations, the resulting turbulence bears detailed resemblance to a wide array of in-situ measurements from the solar wind, capturing the steep “transition range,” observed magnetic-helicity signatures, and key features of the ion distribution function. Based on the predicted dependence of the ion-to-electron heating ratio on imbalance, we suggest that, combined with expansion, the effect could play an important role in regulating global features of the solar wind such as its bimodal speed distribution.
NOTE: There will be no speaker 2021.11.11 - enjoy APS-DPP!
Speaker: Daniel Groselj, Colombia University, USA
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Date/Time: Thursday 4th November 2021, 3PM GMT/11AM EDT
Title: Microphysics of Relativistic Collisionless Electron-Ion-Positron Shocks
Abstract: Weakly magnetized, relativistic collisionless shocks have been studied extensively over the past couple of decades using electron-ion and pair plasma compositions, whereas the broader landscape of electron-ion-positron mixtures has been left unexplored. The more general case is of astrophysical relevance for the early afterglow phase of gamma-ray bursts (GRBs), where the prompt radiation loads the external medium ahead of the shock with electron-positron pairs. In this talk, I will address the microphysics of external, pair-loaded GRB shocks using a set of first-principles kinetic simulations. I will show that even a small number of electron-positron pairs per ion significantly changes the shock structure. In particular, I will demonstrate that a pair-loaded shock is mediated by the Larmor gyration of ions in the compressed mean magnetic field even when this field is extremely weak, and therefore, pair-loaded shocks accelerate ions only in the limit of vanishing external magnetization. Electrons, on the other hand, can form distinctively non-thermal distributions even when the ions are essentially thermal. Although the shock structure significantly changes with respect to the plasma composition, the energy fraction carried by the pairs downstream of the shock is nearly independent of the pair-loading factor. Finally, I will comment on the implications of the results for the early afterglow emission of GRBs.
Speaker: Ilya Dodin, Princeton Plasma Physics Laboratory, USA
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Date/Time: Thursday 28th October 2021, 4PM BST/11AM EDT
Title: Applications of modern geometrical optics to modeling radiofrequency waves and plasma turbulence
Abstract: Modern geometrical optics is a powerful framework that allows modeling wave processes more efficiently than just via solving wave equations like any other PDEs. I will overview some of the recent applications of this theory to reduced linear modeling of radiofrequency waves, including mode conversion, cutoffs, and caustics, which is usually assumed to require the full-wave approach. I will also show how modern wave theory helps fundamentally improve quasilinear theory and understand inhomogeneous turbulence. The presentation is mainly targeted at curious theorists, but the applications addressed are also of practical importance for fusion science and beyond.
Speaker: Alessandro Geraldini, EPFL, Switzerland
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Date/Time: Thursday 21st October 2021, 4PM BST/11AM EDT
Title: The magnetised plasma sheath and its role in the boundary of magnetic fusion devices
Abstract: Sheaths form wherever a plasma is in contact with a solid target. They are characterised by a spatial variation of the electrostatic potential and density over very small length scales normal to the surface. Within a few Debye lengths of the target, the electric field is so strong that the plasma is non-neutral. Debye sheaths exist in order to repel electrons, which are lighter and more mobile than ions. When a magnetic field is present in the plasma at an oblique angle with the target, the sheath develops a two-scale structure, with a part of the electrostatic potential variation occurring in a larger quasineutral magnetic presheath (or Chodura sheath) a few ion gyro radii from the target. When simulating turbulence in the open field line region (Scrape-Off Layer) of a fusion device, it is numerically prohibitive to resolve the small timescales and length scales of the magnetised sheath, which comprises the magnetic presheath and Debye sheath. Instead, an iterative method can be used to directly obtain numerical solutions for the electrostatic potential in the magnetised sheath in steady state. This is demonstrated using a fully kinetic model exploiting the grazing magnetic field angle (typical of fusion devices) to approximate the ion and electron trajectories. Numerical solutions include the ion distribution function reaching the target, important for sputtering predictions. Analytical calculations show that the kinetic Chodura condition must be satisfied by the ion distribution function reaching the magnetised sheath from the rest of the plasma. The implications of this for boundary conditions to gyrokinetic codes of the open field line region are discussed.
Speaker: Scott Hsu, ARPA-E, USA
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Date/Time: Thursday 14th October 2021, 4PM BST/11AM EDT
Title: Overview of ARPA-E’s Fusion R&D Programs
Abstract: Since 2015 with the launch of the ALPHA program [1], the Advanced Research Projects Agency-Energy (ARPA-E) of the U.S. Department of Energy has supported over 50 R&D projects relating to fusion energy. ARPA-E’s fusion R&D portfolio is focused on high-risk, high-reward translational/applied R&D to enable timely commercially viable fusion energy, while incentivizing collaborations between privately and publicly funded fusion teams. This colloquium will be organized into two parts: (1) brief overview of the mission/approach of ARPA-E as an R&D funding agency, and how fusion-energy R&D is motivated/pursued within the overall context of the agency, and (2) overview of ARPA-E’s active fusion programs (BETHE, GAMOW, and Fusion Diagnostics), including its technology-to-market (T2M) approach, and technical research highlights from selected fusion projects.
[1] C. L. Nehl et al., J. Fusion Energy 38, 506 (2019); https://doi.org/10.1007/s10894-019-00226-4.
Speaker: Hubertus Thomas, DLR — German Aerospace Centre, Germany
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Date/Time: Thursday 7th October 2021, 4PM BST/11AM EDT
Title: Research on Complex/Dusty Plasmas in the Lab and in Space
Abstract: Complex/dusty plasmas are plasmas containing small solid particles, which get charged by the collection of plasma electrons and ions. Due to their high charge in laboratory plasmas they start to interact strongly and can form liquid and solid structures, the latter is called plasma crystal. This can be seen as a classical condensed matter system where the main component – the solid particles – can be visualized and tracked dynamically. This allows investigations of fundamental processes in liquids and solids and their transitions. Solid particles of sizes of around a micrometer in diameter start to react strongly on gravity and levitating forces are mandatory. The sheath electric field of a rf-discharge allows the trapping of microparticles in the sheath and can be used to form 2-dimensional (horizontal) or compressed 3-dimensional (with a small extend in the vertical direction) complex plasma systems. To study large 3-dimensional complex plasmas in the bulk of a discharge microgravity experiments are necessary. PK-4 is the third plasma crystal facility on the International Space Station ISS continuing the successful research under microgravity conditions started in 2001 already. In this presentation I will give an overview on complex plasma research and will show recent results like active matter and electrorheological plasmas from ground based and ISS-based laboratories.
This work was supported in part by DLR (BMWi), ESA, Roscosmos and NASA/NSF.
Speaker: Maria Gatu Johnson, MIT, USA
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Date/Time: Thursday 30th September 2021, 4PM BST/11AM EDT
Title: Exploring Stellar Nucleosynthesis and Basic Nuclear Science using High Energy Density plasmas at OMEGA and the NIF
Abstract: Thermonuclear reaction rates and nuclear processes have been explored traditionally by means of accelerator experiments, which are difficult to execute at conditions relevant to Stellar Nucleosynthesis. High-Energy-Density (HED) plasmas closely mimic astrophysical environments and are an excellent complement to accelerator experiments. This talk will focus on HED experiments to study the T+T reaction at the OMEGA laser facility, and the mirror 3He+3He reaction at OMEGA and at the National Ignition Facility (NIF). We present neutron spectra from the T(t,2n)α(TT) reaction measured in HED experiments at ion temperatures from 4 to 18 keV, corresponding to center-of-mass energies (Ec.m.) from 16 to 50 keV. A clear difference in the shape of the TT-neutron spectrum is observed between the two Ec.m., providing the first conclusive evidence of a variant TT-neutron spectrum in this Ec.m. range. Preliminary data from a recent discovery science experiment at the NIF exploring the solar 3He+3He reaction at Ec.m. from 60-120 keV will also be discussed. In addition, the talk will cover the potential of this new field of research, ongoing efforts, and future directions for studying nuclear astrophysics-relevant nuclear processes at OMEGA and the NIF. This work was supported in part by the U.S. DOE, the MIT/NNSA CoE, LLE and LLNL.
Speaker: Saskia Mordijck, College of William & Mary, USA
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Date/Time: Thursday 23rd September 2021, 4PM BST/11AM EDT
Title: The role of turbulence in determining the density profile in magnetic confinement devices
Abstract: The fusion gain in a tokamak is directly linked to the density of the plasma. However, due to the high temperatures necessary for fusion, it is impossible to fuel the core of the plasma directly and directly influence the core density. Without any direct fueling in the core of a tokamak, the plasma density is fully controlled by transport perpendicular to the confining magnetic field surfaces. In this talk, I will show how cross-field transport of electrons is dominated by turbulence in the plasma core by comparing experiments with existing models. These models capture how various types of turbulence influence transport and thus the density profile. While the density profile in the core is fully determined by turbulent transport, at the plasma edge, the picture is more complicated. At the edge of the tokamak, turbulent transport effects intermingle directly with fueling through ionization of the surrounding gas. To better understand the impacts of turbulence on the particle flux, we perform a series of experiments on LAPD varying the neutral density and electron density gradient. While some trends follow linear predictions of resistive drift wave turbulence, other phenomena cannot be explained using linear predictions.
Speaker: Dmitri Uzdensky, University of Colorado Boulder, USA
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Date/Time: Thursday 16th September 2021, 4PM BST/11AM EDT
Title: Extreme Plasma Astrophysics: a Shining New Frontier
Abstract: While traditional plasma physics deals with plasmas made up of a fixed number of electrons and ions that are nonrelativistic and nonradiative, there exist in the Universe important plasma environments with physical conditions so extreme that additional “exotic physics” (from a plasma physicist’s point of view) processes come into play: special and general relativity, strong coupling between plasma particles and photons, and, in most extreme cases, QED effects like pair production and annihilation. These processes alter the plasma dynamics near compact relativistic astrophysical objects — neutron stars and black holes — arguably, the most enigmatic and fascinating objects in the Universe. Understanding how collective plasma processes (waves, instabilities, shocks, magnetic reconnection, turbulence, etc.) operate under these exotic conditions calls for the development of a new, richer physical framework, which forms the scope of Extreme Plasma Astrophysics. I will review the rapid progress that is being made now in exploring this exciting new frontier, stimulated by the exploding astrophysical motivation and enabled by concerted, vigorous theoretical efforts and recent computational breakthroughs such as the advent of novel relativistic kinetic plasma codes incorporating radiation reaction and pair creation. I will illustrate this progress with recent studies of radiative relativistic turbulence and magnetic reconnection with pair creation in the context of accreting black-hole coronae and neutron-star magnetospheres. I will end by outlining the future directions of the burgeoning field of Extreme Plasma Astrophysics.
Speaker: Allen Boozer, Columbia University, USA
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Date/Time: Thursday 9th September 2021, 4PM BST/11AM EDT
Title: Fast Magnetic Reconnection
Abstract: When a magnetic field undergoes a near-ideal evolution that involves all three spatial coordinates, mathematics and Maxwell's equations give a characteristic time scale for the initiation of reconnection. This time is given by the ideal evolution multiplied by a factor that depends only logarithmically on the strength of the non-ideal effects. The critical mathematical concept is chaos, which means the streamlines of the ideal flow of the magnetic field lines can separate exponentially in time. The mathematics of vector representations in three dimensions together with Faraday's law define the ideal flow velocity of magnetic field lines as well as an electromotive-like constant on each line which gives the non-ideality. Maxwell's equations imply chaotic flows are energetically impossible in a two-dimensional evolution, which makes conventional two-dimensional reconnection theory an extremely specialized subject. The magnitude of the current density in a three-dimensional reconnection depends only logarithmically on the strength of the non-ideal effects instead of being inversely proportional as in two dimensions. The current density lies in numerous thin but wide ribbons along the magnetic field lines. The concepts that underlie three-dimensional reconnection theory are unfamiliar to the plasma physics community. The talk will both explain these concepts and give simple examples of their application. To ensure those who would like have time to assess unfamiliar concepts, the slides that will be used are attached here.
Speaker: Lorin Swint Matthews, Baylor University, USA
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Date/Time: Thursday 2nd September 2021, 4PM BST/11AM EDT
Title: Modeling Complex Interactions in a Complex Plasma
Abstract: A complex, or dusty, plasma consists of sub-micron to micron-sized grains immersed in a plasma environment. Micron-sized dust grains have been successfully employed as non-perturbative probes to measure variations in plasma conditions on small spatial scales, such as those found in plasma sheaths. Within a sheath, ions are accelerated from the bulk plasma towards the charged boundary. Ions flowing past a dust grain form a positively charged spatial region downstream of the grain, called the ion wake. The ion wake in turn modifies the interaction potential between charged grains and can contribute to the stability of the dust structures which are formed in a given plasma environment. Thus, although dust grains can be used as non-invasive probes on “small scales”, on even “smaller scales” the perturbations to the plasma flow are necessary to establish a stable dust configuration. Here we present a multi-scale N-body model of the dust-plasma interactions. Results are compared with ground-based lab experiments as well as microgravity experiments onboard the International Space Station to determine quantities such as the charge on individual grains, the electric field within the region, and the nature of the ion wakefield.
Speaker: David Hosking, University of Oxford, UK
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Date/Time: Thursday 26th August 2021, 4PM BST/11AM EDT
Title: Reconnection-controlled decay of magnetohydrodynamic turbulence and the role of invariants
Abstract: In this talk, I will describe a new theoretical picture of magnetically dominated, decaying turbulence in the absence of a mean magnetic field. I shall argue that the energy-decay rate of such a system is governed by the reconnection of magnetic structures, and not necessarily by ideal dynamics, as has previously been assumed. I will explain how a prediction for the decay law of magnetic energy can be obtained by assuming reconnection-mediated dynamics that respects the conservation of certain integral invariants, which represent topological constraints satisfied by the reconnecting magnetic field. As is well known, the magnetic helicity is such an invariant for initially helical field configurations, but does not constrain non-helical decay, where the volume-averaged magnetic-helicity density vanishes. For such a decay, I shall propose a new integral invariant, analogous to the Loitsyansky and Saffman invariants of hydrodynamic turbulence, that expresses the conservation of the random [scaling as volume^(1/2)] magnetic helicity contained in any sufficiently large volume. The existence of this `Saffman helicity invariant’ leads to a natural explanation of the inverse-transfer phenomenon reported by previous numerical studies. Finally, I shall describe an application of these results to the decay of primordial magnetic fields in the early Universe.
Speaker: Julia Stawarz, Imperial College London, UK
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Date/Time: Thursday 19th August 2021, 4PM BST/11AM EDT
Title: Magnetospheric Multiscale Observations of Collisionless Plasma Turbulence in Earth’s Magnetosheath: Turbulent Electric Fields & Turbulence-Driven Magnetic Reconnection
Abstract: Plasmas throughout the Universe undergo complex, highly nonlinear turbulent dynamics, which transfer energy from large to small-scale fluctuations and in the process generate a multitude of small-scale structures, such as current sheets. However, many space plasmas are nearly collisionless, making the question of how the turbulent fluctuations are dissipated a particularly challenging question. NASA’s Magnetospheric Mutiscale (MMS) mission is a formation of four Earth-orbiting satellites providing the high-resolution plasma measurements and inter-spacecraft separations necessary to examine plasma dynamics at scales approaching those of the electrons. In this presentation, I will discuss two recent studies that make use of the unique measurements from MMS in Earth’s magnetosheath to examine the small sub-proton scale dynamics of turbulent plasmas in greater detail than previously possible. In the first study, the behaviour of the turbulent electric field is examined by directly measuring the contributions from the terms in generalised Ohm’s law from fluid to electron-scales. In the second study, MMS observations are used to systematically identify magnetic reconnection events at the thin current sheets that are generated by the turbulent fluctuations. The large-scale properties of the turbulent fluctuations, in particular the correlation length, are found to influence the nature of the reconnection dynamics potentially leading to, so called electron-only reconnection, in which there is not enough space for ion to fully couple to the newly reconnected magnetic fields. Both of these studies provide insight into the nonlinear couplings, and potentially the dissipative dynamics, in collisionless plasmas.
Speaker: Omar Hurricane, Lawrence Livermore National Laboratory, USA
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Date/Time: Thursday 12th August 2021, 4PM BST/11AM EDT
Title: Some basic principles of inertial confinement fusion and some recent “burning plasma” results*
Abstract: Inertial confinement fusion (ICF) has existed as a field of study since the 1970s, but the field was born out of the Cold War. In the decades since the 1970s, pioneering research developing the principles and technologies of ICF culminated in the creation of several major facilities that exist today. While the technology of ICF facilities themselves is fascinating, this talk concentrates upon a handful of basic physics principles of “indirect-drive” (x-ray driven) targets fielded on the National Ignition Facility (NIF) in Northern California and upon some key results from the last decade of research, including some recent experiments that appear to have broached the burning plasma regime [1,2,3].
- [1] A.B. Zylstra, O.A. Hurricane, D.A. Callahan, et al., in preparation (2021)
- [2] J.S. Ross, J.E. Ralph, A.B. Zylstra, et al., in preparation (2021)
- [3] A.L. Kritcher, C.V. Young, H.F. Robey, et al., in preparation (2021)
*Work performed under the auspices of the U. S. Department of Energy by LLNL under contract DE-AC52-07NA27344
Speaker: Samuel Cohen, Princeton Plasma Physics Laboratory, USA
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Date/Time: Thursday 5th August 2021, 4PM BST/11AM EDT
Title: Progress and plans for Princeton Field-Reversed-Configuration Research
Abstract: The Princeton Field Reversed Configuration-2 (PFRC-2) is a research device for studying innovative physics methods to enable small clean fusion reactors. PFRC novel physics regimes are characterized by J┴B and kinetic conditions. Based on the limited availability of one fuel component, 3He, such reactors would be limited to use in niche applications, such as for spacecraft propulsion or emergency terrestrial power generation. First experiments, motivated by single-particle simulations of plasma heating by rotating magnetic fields of odd-parity symmetry (RMFo), produced electron temperatures in excess of 100 eV. The present research program addresses three topics: ion heating by RMFo; confinement; and stability. To achieve bulk ion energies in excess of 100 eV, ARPA-E-supported upgrades are being made to machine hardware, modeling capabilities, and diagnostics. Two new diagnostics have been installed, two-photon laser-induced fluorescence (TALIF, PU-MAE) and Thomson scattering (TS, ORNL). The TALIF diagnostic has measured the H° density in quasi-state-state and puffed gas discharges, allowing evaluation of particle confinement time and energy loss by CX. TS is now being put into operation. Additional planned increased capabilities include reflectometry (UC-Davis), DFSS for internal fields (ORNL), an ion energy analyzer (PPPL), and a PIC simulation code (U Rochester). Benefits of and the requirements for scrape-off-layer modification are described.
Speaker: Emily Lichko, University of Arizona, USA
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Date/Time: Thursday 29th July 2021, 4PM BST/11AM EDT
Title: Effects of distribution structure on predictions of plasma behavior in marginally unstable plasma
Abstract: Due to low collisionality in space and astrophysical plasmas, distributions of ions and electrons observed by spacecraft exist in a state far from thermodynamic equilibrium.The non-Maxwellian features in these distribution functions can trigger microinstabilities, which likely play a role in some of the largest open questions in solar physics, including coronal heating, heating of the bulk solar wind, and accounting for high-frequency waves observed alongside the Alfvenic turbulent cascade. While there is a tremendous amount of information in the structure of these distribution functions, they are typically only represented by a fit of one or two Maxwellian or bi-Maxwellian distributions. In this work, we examine how the fidelity of the model to the observed distribution function affects our predictions for the stability of the plasma, and how much of the information in the distribution function is needed to accurately predict the behavior of the plasma. To do this, we use marginally stable one-dimensional, electrostatic simulations of the electron two-stream instability. For these simulations, there is significantly better agreement between the behavior of the plasma and the predictions of linear theory when a higher-fidelity representation of the distribution function is used. Preliminary work on the extension of these electrostatic results to the electromagnetic regime and the comparison of the predictions of linear wave activity with mesurements of waves from Parker Solar Probe will be presented as well.
Speaker: Muni Zhou, MIT, USA
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Date/Time: Thursday 22nd July 2021, 4PM BST/11AM EDT
Title: From the Weibel instability to flux-tube coalescence – a pathway to seeding plasma dynamos
Abstract: The amplification of seed magnetic fields by the turbulent dynamo is believed to be essential in forming large-scale cosmic magnetic fields with dynamical strengths. The Weibel instability is a crucial mechanism that can produce seed fields from unmagnetized plasmas, but only at the plasma kinetic scale. It remains unclear whether such microscopic seed fields, under the joint action of their own nonlinear evolution and the background turbulence, can contribute to the formation of macroscopic magnetic fields. In the first part of this talk, I will demonstrate how an initially unmagnetized plasma may magnetize itself through the Weibel instability arising under the action of large-scale motions as simple as a shear-flow. The generation and nonlinear evolution of the Weibel seed fields are studied within a fully kinetic framework. For the second part, I will focus on our study of a system composed of an ensemble of magnetic flux tubes, the dynamics of which can represent the long-term evolution of filamentary Weibel seed fields. The formation of large-scale magnetic fields from initial small-scale fields and the associated inverse energy transfer have been identified as a result of the coalescence of magnetic structures through magnetic reconnection. An analytical model for the time evolution of quantities such as the magnetic energy and the energy-containing scale is constructed within the magnetohydrodynamic description, and is confirmed by our direct numerical simulations. In the end, we apply our study to estimate the length scale and strength of magnetic fields generated through this pathway to seed plasma dynamos.
Speaker: William Daughton, Los Alamos National Laboratory, USA
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Date/Time: Thursday 15th July, 4PM BST/11AM EDT
Title: Challenges and Opportunities for Understanding Magnetic Reconnection in Large Systems
Abstract: Some of the most difficult challenges in reconnection physics arise from the large separation between the macroscopic scales where the magnetic fields become stressed and the dissipation scales where the frozen-in condition is broken. Within intermediate scale problems, such as the Earth’s magnetosphere, agreement between kinetic simulations and spacecraft observations suggest we are making good progress towards understanding the reconnection physics. Despite this success, important questions remain regarding the influence of 3D instabilities and the cross-scale coupling with the larger dynamics. While this physics is difficult to study with spacecraft observations, new progress may be possible with large-scale kinetic simulations and reproducible laboratory experiments. In much larger systems, such as the solar corona, our understanding of magnetic reconnection remains completely untested. While the plasmoid instability offers an appealing scenario for bridging MHD and kinetic-scales, it appears unlikely this hypothesis can be rigorously tested with either in situ or remote observations. However, upcoming laboratory experiments are poised to provide the first experimental studies of this regime. In order to design and interpret these experiments, fully kinetic VPIC simulations are allowing researchers to consider realistic geometry, drive coils, and Coulomb collisions to rigorously model the transition between MHD and kinetic regimes. This talk will give an overview of these challenges, with an emphasis on recent progress and new opportunities.
Speaker: Benedikt Geiger, University of Wisconsin, Madison, USA
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Date/Time: Thursday 8th July 2021, 4PM BST/11AM EDT
Title: Results and future directions of the HSX stellarator experiment
Abstract: The Helically Symmetric eXperiment (HSX) at UW Madison, Wisconsin is the world’s first neoclassically optimized stellarator. It started operation in 2001 and has since then significantly contributed to the understanding of neoclassical and turbulent transport in 3D magnetic field geometries. To further extend the operational space of HSX, the device is currently undergoing a major upgrade consisting of a new 500 kW electron cyclotron resonance heating system, an increase of the magnetic field strength and new diagnostic hardware. In this talk, I will give a detailed introduction to the HSX experiment, together with its past key physics findings. Moreover, the current status of the upgrade will be presented, and the expected performance and addressable physics will be detailed. Of particular interest are hereby studies of turbulent heat and particle transport as the magnetic field structure of HSX provides access to unique studies of non-linear turbulence saturation mechanisms. Turbulence saturation mechanisms, such as three wave coupling, are not yet fully understood and the validation of turbulence codes and theories is urgently required for robust design of the next generation of optimized stellarator experiments.
Speaker: Olga Alexandrova, LESIA, Observatoire de Paris, France
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Date/Time: Thursday 1st July 2021, 4PM BST/11AM EDT
Title: Solar Wind Turbulence: in-situ observations from magneto-fluid to kinetic plasma scales
Abstract: Solar wind turbulence was mostly studied at MHD scales: there, magnetic fluctuations follow the Kolmogorov spectrum. The fluctuations are mostly incompressible and they have non-Gaussian statistics (intermittency), due to the presence of coherent structures in the form of current sheets, as it is widely accepted. Kinetic range of scales is less known and the subject of debates. We study the transition from Kolmogorov inertial range to small kinetic scales with a number of space missions. It becomes evident that if at ion scales (100-1000 km) turbulent spectra are variable, at smaller scales they follow a general shape. Thanks to Cluster/STAFF, the most sensitive instrument to measure magnetic fluctuations by today, we could resolve electron scales (1 km, at 1 AU) and smaller (up to 300 m) and show that the end of the electromagnetic turbulent cascade happens at electron Larmor radius scale, i.e., we could establish the dissipation scale in collisionless plasma. Furthermore, we show that intermittency is not only related to current sheets, but also to cylindrical magnetic vortices, which are present within the inertial range as well as in the kinetic range. This result is in conflict with the classical picture of turbulence at kinetic scales, consisting of a mixture of kinetic Alfven waves. The dissipation of these waves via Landau damping may explain the turbulent dissipation. How does this picture change if turbulence is not only a mixture of waves but also filled with coherent structures such as magnetic vortices? These vortices seem to be an important ingredient in other instances, such as astrophysical shocks: for example, they are observed downstream of Earth's and Saturn's bow-shocks. With the new data of Parker Solar Probe and Solar Orbiter we hope to study these vortices closer to the Sun to better understand their origin, stability and interaction with charged particles.
Speaker: Tim Horbury, Imperial College London, UK
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Date/Time: Thursday 24th June 2021, 4PM BST/11AM EDT
Title: A new era in studying the solar wind
Abstract: The solar wind is an accessible astrophysical plasma, in which we can make measurements with remarkable precision. While it has long been studied, in the last few years several spacecraft have launched into the inner heliosphere which, together, will produce a step change in our ability to characterise the solar wind plasma itself, its evolution on small and large scales, and how changes in conditions in the Sun’s surface and atmosphere determine these properties. I will describe this constellation - Parker Solar Probe, BepiColombo and Solar Orbiter – and the contribution that each can make. Concentrating on Solar Orbiter, launched in February 2020, I will describe its capabilities and some early results including fine scale solar dynamics, waves generated by cometary ions, switchbacks, and the interaction of Venus’ atmosphere with the solar wind. I will describe prospects for the next few years, where together these missions will help to solve some key questions in plasma physics, including the processes heating and accelerating the Sun’s corona to form the solar wind.
Speaker: Christopher Reynolds, University of Cambridge, UK
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Date/Time: Thursday 17th June 2021, 4PM BST/11AM EDT
Title: Thermal conduction in the intracluster medium of galaxy clusters
Abstract: The baryonic component of galaxy clusters is dominated by the intracluster medium (ICM), a hot and tenuous plasma atmosphere in an approximate state of hydrostatic equilibrium within the gravitational potential of the dark matter halo. The ICM is an important actor in many astrophysical processes within the cluster - the ram pressure of the ICM can strip cold gas out of orbiting galaxies, and radiative cooling can lead to significant galaxy building in the ICM core in a manner that is well-known to be regulated by feedback from the central supermassive black hole. However, all of these phenomena are influenced by transport processes within the weakly-collisional and high-beta ICM which are still poorly understood. In this talk I focus on the physics and astrophysical role of thermal conduction in the ICM. I summarize recent developments in understanding the role of whistler modes in the regulation of thermal heat transport and proceed to discuss some astrophysical implications of this new transport model. I end by discussing the future observational landscape of these ICM plasma studies.
Speaker: Josefine Proll, Eindhoven University of Technology, Holland
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Date/Time: Thursday 10th June 2021, 4PM BST/11AM EDT
Title: Reduced turbulence in optimised maximum-J stellarators
Abstract: Turbulence is one of the main obstacles to a working fusion reactor. Especially in stellarators, the large space of available magnetic field shapes allows for optimisation towards low levels of turbulence. A useful nonlinear measure of turbulence is that of available energy. Here I will show that the available energy calculated for trapped-electron-mode turbulence in different magnetic configurations can predict trends in the (simulated) heat flux of trapped-electron mode (TEM) turbulence in these configurations and could thus serve as a valuable proxy in future optimisation routines. Both, the available energy and the nonlinear simulations, support a previous linear prediction: that the class of optimised maximum-J stellarators, amongst them Wendelstein 7-X, particularly benefits from reduced turbulence. Previously, we had analytically shown that in these devices, the electron-driven TEM is absent. Here I will show that the stabilising property of the electrons also extends to ion-temperature gradient (ITG) modes and can thus explain the levels of low turbulence in the record-shots of Wendelstein 7-X at finite density gradient. Finally, I will present evidence that in the absence of TEMs, the universal instability can emerge and actually dominate the turbulence in optimised stellarators.
Speaker: Edward Thomas, Auburn University, USA
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Date/Time: Thursday 3rd June 2021, 4PM BST/11AM EDT
Title: Expanding frontiers for dusty plasmas: magnetic fields to microgravity
Abstract: The presence of charged, solid, particulate matter in plasmas, i.e., “dust”, is ubiquitous. From stellar nurseries to planetary rings and from fusion experiments to plasma processing reactors, “dusty” plasmas are found in a wide variety of naturally occurring and human-made plasma systems. Therefore, understanding the physics of dusty plasmas can provide insights into a broad range of astrophysical and technological problems. This presentation will focus on how the small charge-to-mass (q/m) ratio of the charged microparticles gives rise to many of the unique spatio-temporal properties of dusty plasmas. Moreover, this small charge-to-mass ratio strongly influences how magnetic field and microgravity studies of dusty plasmas are performed, leading to new investigations of previously unexplored regimes of plasma parameters. This presentation will discuss results from our studies of dusty plasmas in high magnetic fields (B ≥ 1 T) using the Magnetized Dusty Plasma Experiment (MDPX) device at Auburn University and in microgravity experiments using the Plasmakristall-4 (PK-4) laboratory on the International Space Station. At the end, the presentation will discuss the prospects for the future of dusty plasma research.
Speaker: Plamen Ivanov, University of Oxford, UK
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Date/Time: Thursday 27th May 2021, 4PM BST/11AM EDT
Title: Zonally dominated dynamics and the transition to strong turbulence in cold-ion Z-pinch plasma
Abstract: Following the discovery of the Dimits shift (Dimits et al. 2000), the role of zonal flows (ZFs) for the transition to turbulence in tokamak plasmas has been an area of intense research. We attempt to shed some light on this problem by studying the transition to turbulence in a simplified cold-ion fluid model for ion-scale turbulence in Z-pinch magnetic geometry. Our equations are obtained in a highly collisional, cold-ion, asymptotic limit of the ion gyrokinetic equation and capture the two well-known ion-temperature-gradient (ITG) instabilities driven by either magnetic curvature or parallel compression. We find that this model has a well-defined Dimits (low-transport, ZF-dominated) state characterised by a staircase-like arrangement of ZFs and zonal temperature that suppresses turbulence. Viscous decay of the ZFs leads to occasional turbulent bursts that reconstitute the staircase by providing a negative zonal turbulent viscosity. In 2D, at sufficiently large equilibrium temperature gradients, the zonal turbulent viscosity switches sign, hence the turbulent bursts no longer reinforce the zonal staircase and the Dimits state is destroyed. In 3D, the Dimits state is much more resilient and can always be sustained provided sufficient parallel extent of the system. This is because the large-scale curvature-driven perturbations go unstable to small-scale "parasitic" 3D slab-ITG modes that give rise to a negative zonal turbulent viscosity and provide an effective thermal diffusion for the large-scale modes. If we restrict the parallel extent of the system, the Dimits state is destroyed, and a strongly turbulent, high-transport state is established. In this state, energy is injected into large-scale perturbations by the curvature-ITG instability, then transferred into the parasitic small-scale modes, and finally dissipated by the finite collisionality. Moreover, we find that sufficient parallel resolution is critical for the 3D Dimits state and failure to resolve the small parallel scales of the parasitic modes results in a non-physical transition to strong turbulence. This analysis is based on analytical calculations and numerical simulations of the cold-ion fluid model.
Speaker: Noah Hurst, University of Wisconsin, USA
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Date/Time: Thursday 20th May 2021, 4PM BST/11AM EDT
Title: Vortex dynamics in non-neutral electron plasmas subject to externally imposed ExB flows
Abstract: A series of experiments is described in which magnetized non-neutral electron plasmas are subjected to strong applied electric fields in the plane perpendicular to the magnetic field. The resulting ExB drift dynamics are isomorphic to those of a two-dimensional ideal fluid described by the Euler equations. In this correspondence, the electron density is analogous to the fluid vorticity, and so the plasmas mimic the behavior of fluid vortices. The transverse electric fields act as externally imposed ExB strain flows which can deform and destroy the vortices. Details of the experimental procedure are given, as well as an overview of the experiments that have been carried out so far using this technique. Recent work is then discussed in greater detail, including studies of adiabatic behavior of elliptical electron vortices subject to slowly growing strain flows, and studies of spatial Landau damping of vortex oscillations due to a fluid-wave resonance near the vortex edge. The results are compared with a low-dimensional theoretical model of elliptical vortices, and with particle-in-cell simulations. Finally, the relationship of these results to other similar systems in geophysics, astrophysics, and plasma physics is discussed.
Speaker: Seth Davidovits, Lawrence Livermore National Laboratory (LLNL), USA
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Date/Time: Thursday 13th May 2021, 4PM BST/11AM EDT
Title: Turbulence in high-energy-density experiments: inference and generation
Abstract: High-energy-density (HED) experiments pursuing fusion or X-ray generation can become turbulent. Facilities for HED experiments are also utilized for generating plasma turbulence for study, often with astrophysical applications in mind. The first part of this talk discusses the inference of turbulent flow in experiments without spatial (diagnostic) resolution of the flows; a need for such inference often arises in fusion or X-ray generation experiments, where the plasma is rapidly compressed to small size. Here I highlight examples from Z-pinch experiments optimized for X-ray production, and also briefly discuss recent work showing that turbulence in such two-dimensional compressions may exhibit stronger growth rates with decreasing volume than three-dimensional compressions. The second part of the talk discusses the turbulence generation principles underlying a new experimental design being developed for future laboratory studies of astrophysically-relevant turbulence.
Speaker: Eugene Churazov, Max Planck Institute for Astrophysics, Garching, Germany and Space Research Institute, Moscow, Russia
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Date/Time: Thursday 6th May 2021, 4PM BST/11AM EDT
Title: X-ray view of the Coma galaxy cluster with SRG/eROSITA
Abstract: Coma (Abell 1656) is a massive nearby galaxy cluster famous for being the first object where the presence of Dark Matter was noted by Fritz Zwicky back in 1933. In radio band, it became the first cluster where a “radio halo” and a “radio relic” were detected. In X-rays, which are emitted by hot plasma filling the cluster gravitational well, it is one of the three brightest clusters in the sky. Coma is also a spectacular case of cluster merger with a smaller galaxy group. All this makes Coma a testbed for studies of the phenomena ranging from collisionless dynamics of merging clusters to hydrodynamics, particle acceleration, and weakly collisional intracluster plasma on small scales. In X-rays, the only “trouble” is the large angular size (a few degrees) of the Coma cluster, which is difficult to map with telescopes having a small field of view. This difficulty was recently overcome with the SRG/eROSITA observations yielding a spectacular X-ray map of the entire cluster. Preliminary results of the analysis of these data will be discussed.
Speaker: Marija Vranic, Istituto Superior Tecnico, Lisbon, Portugal
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Date/Time: Thursday 29th April 2021, 4PM BST/11AM EDT
Title: Direct laser acceleration (DLA) of leptons in plasma channels in radiation-dominated regime
Abstract: DLA occurs in partially void plasma channels as a consequence of the simultaneous interaction of particles with the laser field and the plasma background. The particles perform betatron oscillations in the large-scale electric and magnetic field generated by displacing plasma electrons. In addition, they oscillate in the rapidly alternating laser field. By gaining momentum in the direction of laser propagation, the particles perceive a lower laser frequency, and the two types of oscillations can become resonant. The DLA electrons to ~500 MeV were obatianed in experiments using near-critical plasma densities and ps optical lasers. The principal advantage of DLA is that it generates relativistic electron beams with > 100 nC of charge. Using the next generation of lasers (~10 PW power), one could expect energies > 10 GeV, maintaining the high-charge content. In this regime, the interaction becomes dominated by the radiation losses, which counter-intuitively become favourable for acceleration. With a few modifications, DLA can be used for positron acceleration as well. I will address the underlying physics, the analytical model of the acceleration and the scaling laws predicting the asymptotic energy of the accelerated particles. The presented results are supported by particle-in-cell simulations.
Speaker: Felix I. Parra, University of Oxford, UK
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Date/Time: Thursday 22nd April 2021, 4PM BST/11AM EDT
Title: Collisional transport in large aspect ratio stellarators
Abstract: Collisional transport at the small collision frequencies characteristic of fusion reactors can be enormous in stellarators. In order to reduce this transport and the associated energy loss, the position and strength of the external magnets that produce the magnetic field in stellarators must be optimized. In this talk, I will revisit collisional transport in the particularly interesting limit of large aspect ratio stellarators. I will derive a new formulation to calculate collisional transport in the small collision frequency regime relevant to stellarator reactors. This new formulation has been implemented in KNOSOS (KiNetic Orbit-averaging SOlver for Stellarators), a very efficient, fast code that calculates collisional transport in a variety of regimes and can hence be used in stellarator optimization exercises. I will show both numerical and analytical results obtained using the new model that illustrate the nature of stellarator collisional transport at small collision frequencies.
Speaker: Alessandro di Siena, University of Texas, Austin, USA
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Date/Time: Thursday 15th April 2021, 4PM BST/11AM EDT
Title: Understanding the complex interaction between supra-thermal particles and turbulence in magnetic confinement devices
Abstract: The performance of magnetic confinement devices is strongly limited by turbulent transport inducing particle and energy losses and reducing plasma confinement. Among the different experimental actuators of turbulence, supra-thermal particles – generated via external heating schemes – are typically considered one of the most efficient in suppressing ion-temperaturegradient (ITG) driven turbulence in the core of fusion devices. In this talk, I will present some of the most recent insights into understanding the underlying physical mechanisms responsible for this turbulence regulation from first principle gyrokinetic simulations, theory and experiments. Finally, I will discuss the possible implications of this turbulence stabilization via energetic particles to existing and future tokamak and optimized stellarator devices.
Speaker: Tünde Fülöp, Chalmers University of Technology, Sweden
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Date/Time: Thursday 8th April 2021, 4PM BST/11AM EDT
Title: The runaway electron landscape of cooling plasmas
Abstract: The phenomena of runaway acceleration in plasmas has general importance in many fields of physics, for example it is a candidate mechanism for lightning initiation in thunderstorms and electron acceleration in solar flares. In fusion plasmas, understanding of runaways has a great practical importance, as the severity of runaway avalanches increases strongly with plasma current. Therefore, generation of runaways is expected to be a serious issue in ITER and other high-current reactor-scale fusion devices. We will discuss the characteristics and consequences of runaway generation, as well as possible mitigation strategies in fusion devices.
Speaker: Kristopher Klein, University of Arizona, USA
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Date/Time: Thursday 1st April 2021, 4PM BST/11AM EDT
Title: HelioSwarm: Leveraging Multi-point, Multi-scale Observations to Uncover the Nature of Turbulence in Space Plasmas
Abstract: There are many fundamental questions about the temporal and spatial structure of turbulence in space plasmas. Answering these questions is complicated by the multi-scale nature of the turbulent transfer of mass, momentum, and energy, with characteristic scales spanning many orders of magnitude. The solar wind is an ideal environment in which to measure turbulence, but multi-point observations with spacecraft separations spanning these scales are needed to simultaneously characterize structure and cross-scale couplings. Recently selected for phase A study for NASA’s Heliophysics MidEx Announcement of Opportunity, the HelioSwarm Observatory proposes to transform our understanding of the physics of turbulence in space and astrophysical plasmas by deploying nine spacecraft to measure the local plasma and magnetic field conditions at many points, with separations between the spacecraft spanning MHD and ion scales. HelioSwarm resolves the transfer and dissipation of turbulent energy in weakly-collisional magnetized plasmas with a novel configuration of spacecraft in the solar wind. These simultaneous multi-point, multi-scale measurements of space plasmas allow us to reach closure on questions of how energy is distributed in typical solar wind conditions, as well as in extreme conditions relevant to astrophysical plasmas.
Speaker: Bart Ripperda, Flatiron Institute and Princeton University, USA
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Date/Time: Thursday 25th March 2021, 3PM GMT/11AM EDT/12AM JST
Title: Magnetic reconnection and plasmoid formation in black hole accretion flows
Abstract: Plasmoids, or hot spots, forming due to magnetic reconnection in current sheets, are conjectured to power frequent X-ray and near-infrared flares from Sgr A*, the black hole in the center of our Galaxy. It is unclear how, where, and when current sheets form in black-hole accretion flows. We show extreme resolution 3D general-relativistic resistive magnetohydrodynamics and 2D general-relativistic particle-in-cell simulations to model reconnection and plasmoid formation in black hole magnetospheres. Plasmoids can form in thin current sheets In the inner 15 Schwarzschild radii from the event horizon, after which they can merge, grow to macroscopic hot spots of the order of a few Schwarzschild radii and escape the gravitational pull of the black hole. Large plasmoids are energized to relativistic temperatures via magnetic reconnection near the event horizon and they significantly heat the jet, contributing to its limb-brightening. We find that only hot plasmoids forming in magnetically dominated plasmas can potentially explain the energetics of Sgr A* flares. The flare period is determined by the reconnection rate, which we find to be consistent with studies of reconnection in isolated Harris-type current sheets.
Speaker: Alexander Ivanov, Budker Institute of Nuclear Physics, Novosibirsk, Russia
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Date/Time: Thursday 18th March 2021, 3PM GMT/11AM EDT/12AM JST
Title: Studies of plasma confinement in a gas-dynamic trap
Abstract: The gas dynamic trap (GDT) was invented by Vladimir Mirnov and Dmitrii Ryutov in Novosibirsk in the late 1970s. It is basically a version of a magnetic mirror which is characterized by a long mirror-to-mirror distance exceeding the effective mean free path of ion scattering into a loss cone, a large mirror ratio (R ~ 100) and axial symmetry. Under these conditions the plasma confined in a GDT is isotropic and Maxwellian. The plasma loss rate out of the end mirrors is governed by a set of simple gas-dynamic equations, hence the device's name. Plasma magnetohydrodynamic stability in GDT can be achieved through a favorable averaged pressure-weighted curvature of the magnetic field lines, as was initially proposed, or, alternatively through a sheared plasma rotation at periphery induced by electrically biased electrodes at the end wall. A high flux volumetric neutron source based on a GDT is proposed, which benefits from the high β achievable in magnetic mirrors. Axial symmetry also makes the GDT neutron source more maintainable and reliable, and technically simpler. This review discusses the results of a conceptual design of the GDT-based neutron source which can be used for fusion materials development and as a driver of fission–fusion hybrids. The main physics issues related to plasma confinement and heating in a GDT are addressed by the experiments at the GDT device in Novosibirsk. The review concludes by updating the experimental results obtained, a discussion about the limiting factors in the current experiments and a brief description of the design of a future experimental device for more comprehensive modeling of the GDT-based neutron source. Conceivable approaches to improvement of plasma confinement in a GDT are also considered which would allow to consider the concept application in a fusion reactor.
Speaker: Matthew Kunz, Princeton University, USA
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Date/Time: Thursday 11th March 2021, 4PM GMT/11AM EST
Title: Waves, Turbulence, and Transport in Weakly Collisional, High-Beta Plasmas
Abstract: Many space and astrophysical plasmas are so hot and dilute that they cannot be rigorously described as fluids. These include the solar wind, low-luminosity black-hole accretion flows, and the intracluster medium of galaxy clusters. All of these plasmas are magnetized and weakly collisional, with plasma beta parameters of order unity or even much larger (“high-beta”). In this regime, deviations from local thermodynamic equilibrium ("pressure anisotropies") and the kinetic instabilities they excite can dramatically change the material properties of such plasmas and thereby influence the macroscopic evolution of their host systems. This talk outlines an ongoing programme of kinetic calculations aimed at elucidating from first principles the physics of waves, turbulence, and transport under these conditions. Three key results will be featured. (1) Shear-Alfvén waves “interrupt” themselves at sufficiently large amplitudes by adiabatically driving a field-biased pressure anisotropy that both nullifies the restoring tension force and excites a sea of ion-Larmor-scale instabilities. (2) Ion-acoustic waves in a collisionless, high-beta plasma similarly excite Larmor-scale instabilities, which ultimately aid the waves' propagation by rendering the plasma more fluid-like and, therefore, incapable of Landau damping. (3) Pressure anisotropy generated either by turbulent fluctuations or by global expansion (as in the solar wind) qualitatively change the properties of magnetized turbulence, affecting plasma heating and the so-called “critical balance”. Contact with observations of the near-Earth solar wind and the intracluster medium of galaxy clusters will be made.
Speaker: Howard Wilson, University of York, UK
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Date/Time: Thursday 4th March 2021, 4PM GMT/11AM EST
Title: Plasma Modelling in Support of the STEP Fusion Reactor Programme
Abstract: STEP – the Spherical Tokamak for Electricity Production – aims to deliver a prototype power plant by 2040 that will deliver net electricity at the 100MW level. This is a challenging timescale, that will require disruptive changes to how we design and regulate. The plasma scenario presents some of the biggest challenges, and this talk will discuss some of them. A spherical tokamak plasma has some advantages over one at conventional aspect ratio, allowing access to high elongation and beta (ratio of thermal plasma pressure to magnetic field pressure), in a compact geometry. Therefore, while much of the physics in the two designs is similar, there are also key differences. For example, the compact nature means there is little space for a solenoid, so non-inductive current drive is essential; the low magnetic field and high density require novel radio frequency methods for this current drive; the high beta affects the micro-instabilities that drive plasma turbulence and influence confinement; magnetohydrodynamic instabilities must be controlled to limit disruptions while achieving high fusion power and bootstrap current fraction; novel systems are required to manage the exhaust power loads, especially for the inner divertor leg. This talk will explore progress and challenges in modelling these key physics issues in support of STEP.
Speaker: Hye-Sook Park, Lawrence Livermore National Laboratory, USA
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Date/Time: Thursday 25th February 2021, 4PM GMT/11AM EST
Title: Astrophysical collisionless shock formation and nonthermal electron acceleration in laboratory experiments
Abstract: Collisionless shocks are ubiquitous in astrophysical environments such as in supernova remnants, jets in active galactic nuclei and gamma ray bursts and are known to be responsible for cosmic ray acceleration. While the theory of diffusive, or Fermi, shock acceleration (DSA) is well-established, the plasma microphysics responsible for the generation of the shocks, the nature of their resulting magnetic turbulence residue, and the injection of particles into DSA is not yet well understood. With the advent of high-power lasers, laboratory experiments with high-Mach number, collisionless plasma flows can provide critical information to help understand the mechanisms of shock formation by plasma instabilities and self-generated magnetic fields. A series of experiments were conducted on Omega and the National Ignition Facility to observe: the filamentary Weibel instability that seeds microscale magnetic fields [1, 2]; collisionless shock formation (seen by an abrupt ~4x increase in density and ~6x increase in temperature); and electron acceleration distributions that deviated from the thermal distributions [3]. Experimental results along with theoretical interpretations aided by particle-in-cell simulations will be discussed.
- [1] H.-S. Park et al., HEDP 8, 38 (2012)
- [2] C. Huntington et al., Nature Physics 11, 173 (2015)
- [3] F. Fiuza et al., Nature Physics, 16, 916 (2020)
Speaker: Uri Shumlak, University of Washington, USA and Zap Energy, Inc.
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Date/Time: Thursday 18th February 2021, 4PM GMT/11AM EST
Title: Thermonuclear Fusion in an Equilibrium Z Pinch
Abstract: The equilibrium Z pinch is a novel approach to magnetic confinement fusion because it does not rely on external magnetic field coils. Equilibrium conditions are reached through the use of sheared plasma flows, which enhance stability and provide a path to thermonuclear fusion. Simple geometry and strong scaling of fusion gain with pinch current form the cornerstones of this compact fusion device. The sheared-flow-stabilized Z pinch has been developed through integrated computational and experimental investigations at the University of Washington in collaboration with Lawrence Livermore National Laboratory. Experimental results demonstrate plasma stabilization, sustained thermonuclear fusion, and agreement with theoretical and computational predictions. Building on these advances, Zap Energy Inc. is developing a low-cost fusion reactor core based on the equilibrium Z pinch.
Speaker: Hartmut Zohm, Max Planck Institute for Plasma Physics, Germany
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Date/Time: Thursday 11th February 2021, 4PM GMT/11AM EST
Title: Plasma Physics Challenges on the Road to a Tokamak DEMOnstration Fusion Power Plant
Abstract: In the EU Roadmap to Fusion Electricity, DEMO is the step between ITER and a commercial power plant. It is supposed to generate net electricity and have a self-sufficient fuel cycle. The pre-conceptual studies carried out for a tokamak-based DEMO show that the plasma scenario cannot be simply transferred from the ITER Q=10 scenario. We will discuss the physics issues encountered and possible solutions how to overcome them. The focus of the talk will be on the plasma physics aspects, not on reactor integration, and therefore meant to be exciting for plasma physicists from all fields.
Speaker: Artem Smirnov, TAE Technologies, Inc
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Date/Time: Thursday 4th February 2021, 4PM GMT/11AM EST
Title: Progress and Challenges in TAE's Quest Towards an FRC-Based Fusion Reactor
Abstract: TAE Technologies, Inc. (TAE) is a privately funded company pursuing a novel approach to magnetic confinement fusion, which relies on Field-Reversed Configuration (FRC) plasmas composed of mostly energetic and stable particles. This advanced FRC-based system simplifies the reactor design and could offer a path forward to clean, safe, and economical aneutronic p-B11 fusion. To validate the science behind the FRC-based approach to fusion, an active experimental program is underway at TAE’s state-of-the-art plasma research facility in Orange County, California. The core of the facility is the world’s largest FRC device named Norman. In Norman, tangential injection of variable energy neutral beams (15 – 40 keV hydrogen, up to 20 MW total), coupled with plasma edge biasing, active plasma control, and advanced surface conditioning, led to production of steady-state, hot FRC plasmas dominated by fast ion pressure. High-performance, advanced beam-driven FRCs were produced,1-4 characterized by (1) macroscopic stability, (2) steady-state plasma sustainment, and (3) dramatically reduced transport rates (more than an order of magnitude improvement over conventional FRCs). Collectively, these accomplishments represent a strong argument validating the FRC-based approach to fusion power. This talk will provide a comprehensive overview of the TAE experimental program.
Speaker: Anvar Shukurov, Newcastle University, UK
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Date/Time: Thursday 28th January 2021, 4PM GMT/11AM EST
Title: Random flows and rotation in galactic coronae
Abstract: After a review and summary of the observational evidence for random flows and rotation at large altitudes (1-10 kpc) above the discs of spiral galaxies, I discuss the physical parameters of the off-planar gas and the energy sources of the random motions. I argue that the effects of the turbulent viscosity on large-scale gas flows are significant and propose an exploratory model of the viscous coupling of the rotating galactic disc and the corona.
Speaker: Benjamin Chandran, University of New Hampshire, USA
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Date/Time: Thursday 21st January 2021, 4PM GMT/11AM EST
Title: Understanding the Building Blocks of the Solar Wind and How They Fit Together: Heat Flux, Radiation, and Alfven-Wave Turbulence
Abstract: A growing body of evidence suggests that the solar wind is powered to a large extent by an Alfven-wave energy flux that is generated by convective motions on the surface of the Sun. The solar wind and solar corona are also affected by the flux of heat, including conductive losses into the radiative lower solar atmosphere. Numerical simulations that account for the above physics are increasingly able to reproduce remote observations of the corona and solar wind. On the other hand, we still lack an analytic theory that provides formulas for key quantities such as the solar mass-loss rate. Analytic treatments are needed for several reasons. They deepen our understanding by distilling complex processes into their most essential elements, they show how different quantities scale with one another, and they encapsulate our understanding into a portable form that can be applied to other systems and used by anyone. In this talk, I will present a recently developed analytic theory of coronal heating and solar-wind acceleration that provides analytic formulas and intuitive explanations for the solar mass-loss rate, the solar-wind speed far from the Sun, the coronal temperature, the heat flux from the corona into the lower solar atmosphere, and the plasma density at the base of the corona.
Speaker: Elizabeth Tolman, Institute for Advanced Study, USA
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Date/Time: Thursday 14th January 2021, 4PM GMT/11AM EST
Title: Drift kinetic theory of alpha particle transport by tokamak perturbations
Abstract: Upcoming deuterium-tritium tokamak experiments are expected to have large energetic alpha particle populations. These experiments can be used to study the interaction between these alpha particles and perturbations to the tokamak’s electric and magnetic fields. In this talk, I will first describe why this behavior is important and interesting. Then, I will discuss a new drift kinetic theory to calculate the alpha heat flux resulting from a wide range of perturbation frequencies and periodicities. This theory suggests that the alpha heat flux caused by toroidal field ripple, one type of perturbation, is small. Applied to the toroidal Alfvén eigenmode (TAE), another type of perturbation, the theory suggests a significant alpha heat flux that scales with the square of the TAE amplitude. The TAE amplitude calculated from one saturation condition suggests that TAEs in SPARC, one upcoming deuterium-tritium experiment, will not cause significant alpha transport via the mechanisms in this theory. However, saturation above the level suggested by the simple condition, but within numerical and experimental experience, could cause significant transport.
Speaker: Anna Grassi, Sorbonne University, Paris, France
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Date/Time: Thursday 7th January 2021, 4PM GMT/11AM EST
Title: Exploring the physics of turbulent collisionless shocks in conditions of laboratory experiments
Abstract: Collisionless shocks are ubiquitous in astrophysical plasmas and play an important role in magnetic field generation/amplification and particle acceleration. While diffusive shock acceleration (DSA) is well established, the details of particle injection into DSA remain a long-standing puzzle, particularly for electrons. High-energy-density (HED) plasma experiments and kinetic plasma simulations offer a promising route to identify the dominant processes at play. Very recently experiments performed at the National Ignition Facility have observed for the first time the formation of high-Mach number collisionless shocks mediated by electromagnetic instabilities and nonthermal electron acceleration. I will discuss the physics behind shock formation and particle acceleration in these laboratory systems and how they can be connected to astrophysical models. Using large-scale, multi-dimensional particle-in-cell (PIC) simulations, we find that the inhomogeneous profiles of laser-ablated plasmas lead to shock formation that can be up to 10 times faster than previous models predicted. The shock front can also develop strong corrugations at the ion gyroradius scale, which can be controlled by changing the electron temperature of the flow. Finally, we show that electrons can be effectively accelerated to nonthermal energies and injected into DSA via a Fermi-like mechanism occurring within the finite, turbulent shock transition. These findings can help guide the development and interpretation for current experimental programs and open exciting prospects for studying the microphysics of turbulent collisionless shocks in the laboratory.
Speaker: Frank Jenko, IPP Garching, Germany
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Date/Time: Wednesday 16th December 2020, 4PM GMT/11AM EST
Title: When will we be able to predict plasma confinement in fusion devices?
Abstract: It is a key goal of magnetic confinement fusion (MCF) research to develop and build devices that allow us to create a plasma at sufficiently high pressure and energy confinement time, so that Lawson's criterion for a burning plasma can be met. There was breathtaking progress along these lines between the 1970s and 1990s, largely based on a "trial-and-error" approach. With the preparation of ITER operation and attempts to design first versions of future fusion power plants, it became clear, however, that a more targeted "predict-first" approach is needed at this point to save significant amounts of time and resources in the further development of fusion energy. Fortunately, the power of High Performance Computing keeps growing at a remarkable speed, with exascale systems around the corner. These platforms open up new possibilities to solve the complex nonlinear equations underlying many observed phenomena in MCF plasmas, and to move from an interpretative to a truely predictive approach. In this context, computing, data analysis, and machine learning are increasingly intertwined to provide reliable predictions. So how and when will we be able to predict plasma confinement in fusion devices? This question will be at the heart of this presentation.
Speaker: Ellen Zweibel, University of Wisconsin, Madison, USA
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Date/Time: Wednesday 9th December 2020, 4PM GMT/11AM EST
Title: Some open questions in the plasma physics of cosmic rays
Abstract: It's now widely recognized that cosmic rays have considerable influence on the dynamics and energy balance of thermal gas in and around galaxies. While it's their collisional interactions that render cosmic rays most directly visible, in many respects their collisionless interactions, mediated by kinetic scale plasma waves and instabilities, are more significant. These microscale interactions are now captured by fluid models and used in large scale simulations of galaxy formation and evolution, where they are revealing surprising behavior. After briefly reviewing the basics, I will discuss some open questions in the microphysics of cosmic ray - thermal gas coupling, and how the large scales react back on the small ones.
Speaker: James Drake, University of Maryland, USA
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Date/Time: Wednesday 2nd December 2020, 4PM GMT/11AM EST
Title: Whistler-regulated MHD: Transport equations for electron thermal conduction in the high β intracluster medium of galaxy clusters
Abstract: Transport equations for electron thermal energy in the high \beta_e intracluster medium (ICM) are developed that include scattering from both classical collisions and self-generated whistler waves. The calculation employs an expansion of the kinetic electron equation along the ambient magnetic field in the limit of strong scattering and assumes whistler waves with low phase speeds V_w ~ v_{te}/\beta_e << v_{te} dominate the turbulent spectrum, with v_{te} the electron thermal speed and \beta_e >> 1 the ratio of electron thermal to magnetic pressure. We find: (1) temperature-gradient-driven whistlers dominate classical scattering when L_c > L/\beta_e, with L_c the classical electron mean-free-path and L the electron temperature scale length, and (2) in the whistler dominated regime the electron thermal flux is controlled by both advection at V_w and a comparable diffusive term. The findings suggest whistlers limit electron heat flux over large regions of the ICM, including locations unstable to isobaric condensation. This description of thermal transport can be used in macroscale ICM models. The basic physics underlying whistler wave dynamics and associated electron scattering will be highlighted.
Speaker: Ahmed Diallo, Princeton Plasma Physics Laboratory (PPPL), USA
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Date/Time: Wednesday 18th November 2020, 4PM GMT/11AM EST
Title: Characterizing and Understanding the Instabilities Localized in the Edge of Fusion Devices to Improve Pedestal Predictive Models
Abstract: The simultaneous achievement of high-performance core plasma and highly dissipative boundary plasma is key for future fusion reactors. The critical region of interaction is the edge transport barrier (also known as the H-mode pedestal), which mediates the tension between core and edge, and plays a defining role in the performance of both. Fusion performance in these reactors hinges critically on the efficacy of the edge transport barrier at suppressing energy losses. In this talk, I will describe new insights into the characterization and understanding of instabilities localized in the edge of fusion devices as well as challenges for the development of edge models for future devices. I will focus on the dynamics leading up to the most common global instabilities occurring in the edge pedestal that are the edge localized modes (ELMs). In addition, I will present recent investigations on the triggering mechanism of ELMs.
Speaker: Louise Willingale, University of Michigan, Ann Arbour, USA
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Date/Time: Wednesday 4th November 2020, 4PM GMT/11AM EST
Title: Magnetic signatures of radiation-driven double ablation fronts
Abstract: Laser-plasma interactions produce strong temperature and density gradients that generate megagauss strength magnetic fields through the Biermann-battery effect. We performed experiments using the OMEGA EP laser system and used proton radiography to measure the strength, spatial profile, and temporal dynamics of the self-generated magnetic fields. The target material was varied and the plastic (CH), aluminum, copper, or gold targets exhibited different magnetic field structures. Mid-Z targets had two distinct regions of magnetic field, one produced by gradients from electron thermal transport and the second from radiation-driven gradients. Extended magnetohydrodynamic simulations including radiation transport reproduced the key experimental features, including the magnetic field generation and double ablation front formation.
Speaker: Eliot Quataert, Princeton University, USA
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Date/Time: Wednesday 28th October 2020, 3PM GMT/11AM EST
Title: The Physics of Galaxy Cluster Plasmas
Abstract: Galaxy clusters are among the largest gravitationally bound objects in the universe. The majority of the baryonic mass in clusters resides in a hot, low density plasma that pervades the intracluster medium (rather than in stars). The heating and cooling processes in this plasma must be understood in order to make progress on a number of key problems in galaxy formation, including the formation of the most massive galaxies and black holes in the universe. An understanding of galaxy cluster thermodynamics is also important for the use of clusters as cosmological probes into the nature of dark matter and dark energy. In this talk, I will describe new insights into the physics of galaxy cluster plasmas, focusing on the subtle interplay between cosmic rays (relativistic particles) and the dilute thermal plasma. Remarkably, this interplay can drive both sound waves and internal gravity waves linearly unstable. I will describe the possible importance of these instabilities for understanding how black holes heat cluster plasmas, regulating the growth of massive galaxies.
Speaker: Stuart Bale, UC Berkley, USA
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Date/Time: Wednesday 21st October 2020, 4PM BST/11AM EDT
Title: Parker Solar Probe at the edge of the streamer belt
Abstract: The NASA Parker Solar Probe mission was launched in late 2018 and has, to date, made six perihelion passes with the latest at 20.4 solar radii. The PSP instruments have measured a rich variety of plasma physics phenomena, including Alfvenic fluctuations and turbulence, interplanetary dust, ion- and electron-scale plasma instabilities and solar radio emissions. I will review some of the key initial results, including the phenomena of Alfvenic 'switchbacks' or jets, and describe more recent measurements as PSP encounters the dense structures associated with the streamer belt.
Speaker: Jeff Parker, University of Wisconsin-Madison, USA
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Date/Time: Wednesday 14th October 2020, 4PM BST/11AM EDT
TITLE: Topological phase in plasmas
ABSTRACT: Recent discoveries have demonstrated that matter can be distinguished on the basis of topological considerations, giving rise to the concept of topological phase, which was recognized by the 2016 Nobel Prize in Physics. Introduced originally in condensed matter physics, the physics of topological phase can also be fruitfully applied to plasmas. The rich band structures and wave physics supported by plasmas may contain a plethora of topological phenomena. I will introduce topological band theory, including a discussion of Berry phase, Berry connection, Berry curvature, and Chern number. One of the clear physical manifestations of topological phase is the bulk-boundary correspondence, the existence of localized, robust modes at the interface between topologically distinct phases. Two examples illustrate these concepts. First, a simple magnetized cold plasma is topologically nontrivial, leading to an electromagnetic surface wave existing at the boundary of magnetized plasma and vacuum. Second, the Alfven continuum exhibits a topology that depends on the sign of the magnetic shear, and hence there exists a topological phase transition across a layer of zero shear, which corresponds to the reversed-shear Alfven eigenmode. More broadly, topological phase offers a tool for theoretically discovering novel interface modes. These recent developments also provide the opportunity to experimentally study in plasmas for the first time manifestations of topological waves and topological protection.
Speaker: E. Paulo Alves, SLAC/Stanford, USA
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Date/Time: Wednesday 7th October 2020, 4PM BST/11AM EDT
TITLE: Accelerating our understanding of complex nonlinear plasma phenomena using machine learning
ABSTRACT: The increasing quantity and quality of plasma data being produced by laboratory experiments, spacecraft observations and high-fidelity numerical simulations is creating new opportunities for innovation in the way we do plasma physics. In particular, deep learning techniques are offering powerful new ways of building highly predictive data-driven models for various important applications (including disruption prediction of fusion plasmas). The inherent complexity of these data-driven models, however, limits their interpretability, challenging the development of a theoretical understanding of the plasma physics underlying the data. In this talk, I will discuss how sparse regression techniques can be used to infer interpretable plasma physics models (in the form of nonlinear partial differential equations) directly from the data of fully kinetic particle-in-cell (PIC) simulations. The potential of this approach will be illustrated through the recovery of the fundamental hierarchy of plasma physics models, from the Vlasov equation to magnetohydrodynamics, based solely on data of complex plasma dynamics captured by first-principles PIC simulations. I will discuss how this data-driven methodology offers a promising new route to accelerate the development of reduced theoretical models of complex nonlinear plasma phenomena and to design computationally efficient algorithms for multi-scale plasma simulations.
Special Colloquium with three speakers (Alex Creely, Pablo Rodriguez-Fernandez and Ryan Sweeney)
Status of the SPARC Physics Basis
Date/Time: Thursday 1st October 2020, 4PM BST/11AM EDT
Speaker: Alex Creely - Commonwealth Fusion SystemsTitle: Overview of the SPARC tokamak
Abstract: The SPARC tokamak is a critical next step toward commercial fusion energy. SPARC is designed as a high-field (B0 = 12.2T), compact (R0 = 1.85m, a = 0.57m), superconducting, D-T tokamak with the goal of producing fusion gain Q > 2 from a magnetically confined fusion plasma for the first time. Currently under design, SPARC will continue the high-field path of the Alcator series of tokamaks, utilizing new magnets based on rare-earth barium copper oxide (REBCO) high temperature superconductors (HTS) to achieve high performance in a compact device. The goal of Q > 2 is achievable with conservative physics assumptions (H98,y2 = 0.7) and, with the nominal assumption of H98,y2 = 1, SPARC is projected to attain Q ≈ 11 and Pfusion ≈ 140 MW. SPARC will therefore constitute a unique platform for burning plasma physics research with high density ( ≈ 3*10^20m−3), high temperature ( ≈ 7 keV), and high power density (Pfusion/Vplasma ≈ 7 MW m−3) relevant to fusion power plants. SPARC’s place in the path to commercial fusion energy, its parameters, and the current status of SPARC design work are presented.
Speaker: Pablo Rodriguez-Fernandez - Massachusetts Institute of TechnologyTitle: Predictions of core plasma performance for the SPARC tokamak
Abstract: Empirical predictions with conservative physics indicate that SPARC baseline plasmas would reach Q=11, which is well above its mission objective of Q>2. Thanks to the development and validation of theory-based reduced models for heating and transport, and in order to build confidence that SPARC will be successful, physics-based integrated modeling has also been performed. Simulations of pedestal stability and gyro-fluid turbulence indicate that the baseline plasma discharge in SPARC would reach Q=9, in remarkable agreement with the empirical predictions. This large margin with respect to its performance goals indicates that SPARC will be capable of studying and demonstrating operation in the burning plasma regime, and alpha physics and transport relevant for fusion power plants will be investigated. This talk will describe the approach being used to perform physics-based predictions of SPARC performance, the assumptions behind the models, and main results for the baseline scenario.
Speaker: Ryan Sweeney - Massachusetts Institute of Technology
Title: MHD stability and disruptions in the SPARC tokamak
Abstract: SPARC is being designed to operate with a normalized beta of 1.0, a normalized density of 0.37 and a safety factor at the 95% poloidal flux surface of 3.4, providing a comfortable margin to their respective disruption limits. Further, a low beta poloidal of 0.19 at the safety factor q=2 surface reduces the drive for neoclassical tearing modes, which together with a frozen-in classically stable current profile might allow access to a robustly tearing-free operating space. Although the inherent stability is expected to reduce the frequency of disruptions, the disruption loading is comparable to and in some cases higher than that of ITER. The machine is being designed to withstand the predicted unmitigated axisymmetric halo current forces up to 50 MN and similarly large loads from eddy currents forced to flow poloidally in the vacuum vessel. Runaway electron (RE) simulations using GO+CODE show high flattop-to-RE current conversions in the absence of seed losses, although NIMROD modelling predicts losses of 80%; self-consistent modelling is ongoing. A passive RE mitigation coil designed to drive stochastic RE losses is being considered and COMSOL modelling predicts peak normalized fields at the plasma of order percent that rises linearly with a change in the plasma current. Massive material injection is planned to reduce the disruption loading. A data-driven approach to predict an oncoming disruption and trigger mitigation is discussed.
Speaker: Elena Amato, Osservatorio Astrofisico di Arcetri, Italy
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Date/Time: Wednesday 30th September 2020, 4PM BST/11AM EDT
TITLE: Some recent puzzles on the acceleration and transport of Galactic Cosmic Rays.
ABSTRACT: Recent developments on the subjects of acceleration and transport of Galactic Cosmic Rays will be discussed. High energy astrophysical observations, and direct cosmic ray detection data, collected by AMS-02 in particular, have revealed a number of unexpected features in the spectra of both primary and secondary cosmic rays. These findings seem to challenge the standard description of cosmic-ray propagation through the Galaxy and also impact our theories on the origin of these particles. I will discuss how at least some, if not all, of the presumed anomalies allow for an explanation based on non-linear effects associated with CR transport, and more generally what they suggest about the origin and propagation of Galactic Cosmic Rays.
Speaker: Cary Forest, Wisconsin Plasma Physics Laboratory, USA
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News: $5 Million Plasma Fusion Mirror Devices Grant
Date/Time: Wednesday 23rd September 2020, 4PM BST/11AM EDT
TITLE: The Wisconsin HTS Axisymmetric Mirror (WHAM) on a faster path to lower cost fusion energy
ABSTRACT: A public-private partnership between the UW Madison, MIT and Commonwealth Fusion Systems has been formed to build and operate a compact, high-field simple mirror WHAM (the Wisconsin HTS Axisymmetric Mirror) showing how compact end plugs can now be built for axisymmetric tandem mirrors. It builds on recent physics breakthroughs in stability and confinement, critical technological advances in superconductivity, and the availability of high power reactor relevant heating systems. Two mirror coils will be constructed using REBCO high temperature superconducting material by CFS (17 T mirrors). Hot and high density target plasmas will be created using high frequency ECH from modern gyrotrons. Fast sloshing ions will be created and energized by a novel RF heating scenario in which neutral beam injection is used to fuel ions which are then accelerated in situ to high energy by High Harmonic Fast Wave. Quasi-stationary plasmas (plasma duration >> ion slowing down and characteristic confinement times) will be created with electron temperatures of 1 keV, average ion energies of 20 keV and densities that approach the plasma pressure limit. The end product will be a realistic conceptual design of a low cost Break Even Axisymmetric Tandem (BEAT) that leads to and HTS Axisymmetric Magnetic Mirror Experimental Reactor (HAMMER). Finally, several lucrative off-ramps for this research have been identified that would lead to fusion neutron sources useful for academic and industrial purposes that may help accelerate progress towards fusion energy by stimulating additional investment.
Speaker: Eve Stenson, Max Planck Institute for Plasma Physics (IPP), Germany
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Date/Time: Wednesday 16th September 2020, 4PM BST/11AM EDT
TITLE: En route to magnetically confined matter-antimatter plasmas
ABSTRACT: The large mass asymmetry between electrons and ions is a key element of the rich complexity for which plasma physics is well known. Correspondingly, the behavior of "pair plamsas", comprising negatively and positively charged particles of equal mass, is predicted to be markedly simpler in a number of ways. Not so simple, unfortunately, are the options for creating and studying such plasmas in the laboratory. This talk will describe the approach being pursued the APEX (A Positron Electron eXperiment) collaboration, along with recent milestones and latest developments toward the goal of magnetically confined e+e- pair plasmas.
Speaker: Lina Hadid, LPP, École Polytechnique, France
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Date/Time: Wednesday 9th September 2020, 4PM BST/11AM EDT
TITLE: Turbulent energy cascade rate in the Earth's magnetosheath and the solar wind using in-situ spacecraft data
ABSTRACT: Compressible turbulence has been a subject of active research within the space physics community for the last three decades and is actually believed to be essential for understanding the physics of the solar wind (for instance the heating of the fast wind), of the interstellar medium (in cold molecular clouds) and other astrophysical and space phenomena. In this talk I will give a review of the different studies that we have done regarding the compressible and incompressible cascade rates in the interplanetary space. Firstly, using the exact law of compressible isothermal magnetohydrodynamic (MHD) turbulence [Banerjee & Galtier, PRE, 2013], we give an estimation of the compressible energy cascade rate (|εC|) in the Earth’s magnetosheath using THEMIS and CLUSTER spacecraft data and show that it is at least three orders of magnitude larger than its value in the solar wind. Moreover, we show the role of the density fluctuations in increasing the spatial anisotropy in the Earth's magnetosheath [Hadid et al., PRL, 2018]. Secondly, using the exact law of compressible Hall MHD turbulence [Andrés & Sahraoui, PRE, 2017] we give a complete estimation of |εC | at the MHD and the sub-ion scales in the Earth's magnetosheath using MMS data [Andrés et al., PRL, 2019]. Finally we show the radial evolution of the turbulent cascade rate from the Sun (~0.2 A.U.) up to Mars (~1.5 A.U.), using Parker Solar Probe and Maven data [Andrés et al. in prep.].
Speaker: Miho Janvier, IAS, France
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Date/Time: Wednesday 2nd September 2020, 4PM BST/11AM EDT
TITLE: From observations of the solar corona to MHD simulations: a 3D standard model of solar flares.
ABSTRACT: Solar flares are among the most energetic events in our solar system. Accompanied by intense UV and even X-ray emissions, they can inject energetic particles into the interplanetary medium and be accompanied by coronal mass ejections (CMEs). These clouds of magnetized plasma interact with planetary environments, hence the interest for our human societies to gain deeper knowledge in predicting flares and their subsequent evolution. Increased temporal and spatial resolutions of ground and space observatories have allowed us to refine a standard model for eruptive flares, which can explain their generic features (the presence of flare ribbons, flare loops and a twisted erupting magnetic structure). In particular, 3D MHD modelling has provided us with some predictions on the magnetic field behaviour during the eruption, such as the evolution of regions where the magnetic field energy is converted. These predictions are nowadays well documented with the help of observations with, e.g., the AIA and HMI instruments aboard the NASA mission Solar Dynamics Observatory. We will look at how these predictions can be validated with a careful study of the active region configuration with 3D MHD numerical models. This will show how approaches combining modelling techniques and observations provide a major step in extending and completing the standard model for eruptive flares in its 3D version.
Speaker: Elizabeth Paul, Princeton University, USA
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Date/Time: Wednesday 26th August 2020, 4PM BST/11AM EDT
Title: Adjoint methods for stellarator shape optimization
Abstract: The design of modern stellarators with acceptable confinement properties requires numerical optimization of the magnetic field in the non-convex, high-dimensional spaces describing their geometry. Another major challenge facing the stellarator program is the sensitive dependence of confinement properties on coil shapes, necessitating construction under tight tolerances. These challenges are addressed with the application of adjoint methods and shape sensitivity analysis. Adjoint methods enable the efficient computation of the gradient of a function that depends on the solution to a system of equations, such as linear or nonlinear PDEs. We present the first applications of adjoint methods for stellarator shape optimization. Several examples of this approach will be discussed, including the optimization of neoclassical properties based on the solution of an adjoint drift kinetic equation [1] and the optimization of equilibria based on the solution of an adjoint MHD force balance equation [2,3]. These advances enable a reduction in cost by several orders of magnitude over traditional optimization methods and provide additional insight into the local sensitivity of confinement properties.
[1] E.J. Paul, I.G. Abel, M. Landreman, W. Dorland, J. PLASMA PHYS. 85, 795850501 (2019).
[2] T. Antonsen, E.J. Paul, M. Landreman, J. PLASMA PHYS. 85, 905850207 (2019).
[3] E.J. Paul, T. Antonsen, M. Landreman, W.A. Cooper, J. PLASMA PHYS. 86, 905860103 (2020).
Speaker: Carlos Paz-Soldan, General Atomics, USA
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Date/Time: Wednesday 19th August 2020, 4PM BST/11AM EDT
Title: Measurement and Control of Relativistic Electrons in Tokamaks
Abstract: Rapid plasma shutdowns in the tokamak fusion reactor configuration have the potential to generate intense beams of relativistic (multi MeV) electrons that pose a severe risk to the integrity of plasma-facing components. Demonstrating control of these MeV populations will be among the first research challenges to overcome on the path to achieving the net fusion gain mission of the next generation of tokamak reactors currently under design and construction. Insight into the dynamics of the relativistic electrons is gained by measuring their volumetric emission via either the bremsstrahlung or synchrotron mechanisms. A unique diagnostic to measure bremsstrahlung-emitted MeV photons was deployed in the DIII-D tokamak, capable of stress-testing theoretical models. Disagreements between theory and experiment point to interactions with plasma instabilities driven by the electrons themselves as an important, yet previously ignored, mechanism governing the electron behavior. Guided by this information, a variety of instabilities of the relativistic electron beam have been observed, many for the first time. An active international effort is now underway to deploy several of these instabilities to control and mitigate the relativistic beam population. This seminar will describe the development of the novel measurements, their interaction with emerging theoretical models, and finally discuss ongoing activities to harness instabilities for beam control applications. This material is based upon work supported by the Department of Energy under Award Number(s) DE-FC02-04ER54698.
Speaker: Jan Egedal, University of Wisconsin, USA
Date/Time: Wednesday 12th August 2020, 4PM BST/11AM EDT
Title: Exploring driven collisionless reconnection in the Terrestrial Reconnection Experiment (TREX) J. Egedal, J. Olson, S. Greess, A. Millet-Ayala, and C.B. Forest
Abstract: The Terrestrial Reconnection Experiment (TREX) [1] executed in the Big Red Ball at the University of Wisconsin is optimized to study magnetic reconnection in a regime where Coulomb collisions between electrons and ions are sufficiently infrequent that kinetic effects in the electron dynamics are retained. In this colloquium I will review the experimental configuration and review early significant results but then turn to important recent discoveries. Upgrades to drive have now allows us to access are more fully collisionless regime in which electron pressure anisotropy develops and is fundamental to the structure of the electron diffusion region. The observed signatures of reconnection include narrow electron jets and current layers with widths down to the scale length of the electron skin depth, confirming previous results from fully kinetic simulations. Driven reconnection scenarios are important to a range of systems including the interaction of stellar winds with planetary magnetospheres. We have also now observed a shock interface to form between the supersonically driven plasma inflow; a region of magnetic flux pileup permits the normalized reconnection rate to self-regulate to a fixed value where the inflow speed is about 50% of the Alfvenic outflow speeds observed in the reconnection exhaust. Consequently, the normalized reconnection rate, is larger than those typically observed in undriven systems. [1] J. Olson, et al., Phys. Rev. Lett 116, 255001 (2016) [2] C.B. Forest, et al., Jour. Plasma Phys., 81, 345810501 (2015)
Speaker: Cami Collins, General Atomics USA
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Date/Time: Wednesday 5th August 2020, 4PM BST/11AM EDT
Title: Understanding & Controlling Transport of Fast Ions By Alfvén Eigenmodes in Tokamaks
Abstract: In fusion reactors, fast-ions are highly energetic particles (EPs) that play critical roles in heating, current drive, momentum input, and plasma stability. A key goal of worldwide EP physics research is to develop the experimental and theoretical knowledge to predict and optimize EP behavior in ITER and future reactors. In order to achieve and maintain a burning plasma state, tokamaks must confine a large enough number of EPs for long enough times to allow for the transfer of energy through collisions to the colder, background thermal plasma. However, large gradients in the EP pressure profiles can provide free energy to drive plasma instabilities, such as Alfvén eigenmodes (AEs), through wave-particle interactions. These instabilities can in turn transport EPs away from the core of the plasma, leading to reduced fusion performance and losses that could seriously damage reactor walls. Experiments using the extensive diagnostic suite on the DIII-D tokamak have shown that fast-ion transport suddenly increases when multiple AEs cause particle orbits to become stochastic, resulting in a “stiff” fast-ion density profile that stops increasing despite higher injected beam power, along with neutron rates that approach only 50% of expected values. In experiments to develop a steady-state tokamak operating scenario, fast-ion confinement was improved by ~25% using control techniques that alter or suppress AEs by manipulating equilibrium profiles, increase mode damping by changing background plasma parameters, and decrease mode drive by reducing the spatial fast-ion gradient. Measurements are being used to develop validated transport models that can efficiently calculate beam deposition, EP profiles, and losses over a wide parameter regime for predictive discharge modeling and burning plasma scenario optimization. This material is based upon work supported by the Department of Energy under Award Number(s) DE-FC02-04ER54698.
Speaker: Sir Steve Cowley, Princeton Plasma Physics Laboratory, USA
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Date/Time: Wednesday 29th July 2020, 4PM BST/11AM EDT
Title: Plasma equilibrium, the energy landscape and explosive ballooning instability
Abstract: Since the pioneering work of Kruskal and Kulsrud in 1958 we have understood plasma equilibria as stationary states of the energy. We have also associated instability with the plasma seeking lower energy equilibrium states. For example, the Energy Principle of Bernstein et. al. identifies linear instability with the accessibility of lower energy states (not necessarily equilibria) by linear plasma displacements. In a sense this is probing the local behavior of the energy landscape. The nonlinear (nonlocal) energy landscape is however largely unexplored. However understanding is critical if we are to understand the impact of instability and meta-stability — particularly the explosive dynamics expected with large changes in the energy of the equilibrium states. In this talk I will examine the nonlinear energy landscape for interchange/ballooning type displacements. In particular I will construct singular equilibrium states arising from displacing isolated narrow flux tubes from a smooth initial equilibrium. These are the nonlinear equilibria that arise from ballooning type instabilities. I will show tokamak cases where the original equilibrium is stable to small displacements but unstable to finite perturbations. In such cases the original state is metastable and the displaced equilibrium state is lower energy. This phenomena is expected in ELMs and some disruption dynamics in tokamaks. The flux tube equilibrium states are singular and will dissipate rapidly due to reconnection — I will estimate the rate of reconnection and the subsequent evolution.
Speaker: Sasha Philippov, CCA, Flatiron Institute, USA
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Date/Time: Wednesday 22nd July, 4PM BST/11AM EDT
Title: Extreme plasma astrophysics of black holes and neutron stars
Abstract: Neutron stars and black holes are powerful sources of broad-band non-thermal electromagnetic emission, including coherent radio and high-energy radiation. The collective behavior of plasmas that produce these emission signatures is still poorly understood. In this talk I will present global radiative kinetic plasma simulations of neutron star and black hole environments, which allow modeling emission signatures from first principles. I will describe applications of these methods to the understanding of the multi-wavelength emission mechanism of rotating magnetized neutron stars (pulsars), including the 50-year old problem of the generation of coherent radio waves. I will also highlight recent work on pair plasma discharges and magnetic flares near supermassive black holes and electromagnetic precursors to gravitational wave events.
Speaker: Carolyn Kuranz, University of Michigan, USA
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Date/Time: Wednesday 15th July 2020, 4PM BST/11AM EDT
Title: Creating Astrophysically Relevant Systems in the Laboratory in the High-Energy-Density Regime
Abstract: High-energy-density experiments can provide insight into astrophysical processes, which are often observed from great distances under uncontrolled and unknown conditions. In order for an experiment to be well-scaled to an astrophysical process, several specific conditions must be considered, including key governing equations, specific spatial scaling, and similar global dynamics. In many cases, these conditions can be met using high-energy-density experimental facilities, such as, high-energy laser or pulsed power devices. I will discuss general scaling rules and several astrophysically-relevant high-energy-density physics experiments, specifically an experiment conducted at the National Ignition Facility relevant to core-collapse supernova SN1993J, a red supergiant, where a radiative shock is near a hydrodynamically unstable interface. We found that significant energy fluxes from radiation and thermal heat conduction affect the hydrodynamics structure at the interface. In the experiments, a blast wave structure similar to those in supernovae is created in a plastic layer. The blast wave crosses a three-dimensional interface that produces unstable growth dominated by the Rayleigh-Taylor instability. We have detected the evolution of the interface structure under these conditions and will show the resulting experimental and simulation data.
Speaker: Jack Hare, Imperial College London, UK
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Date/Time: Wednesday 8th July 2020, 4PM BST/11AM EDT
Title: Magnetic Reconnection Driven by Pulsed Power
Abstract: Magnetic reconnection is a fundamental process in plasma physics, which breaks and reforms magnetic fields lines whilst rapidly converting magnetic energy into heat and motion. Reconnection is responsible for dramatic events such as Coronal Mass Ejections from the surface of the Sun, and plays an important role in setting the energy balance in the universe.
I will present laboratory experiments which study magnetic reconnection driven by the MAGPIE pulsed power generator at Imperial College London. We use exploding wire-arrays to produce supersonic, magnetised plasma flows which collide to create a long lasting, quasi-2D reconnection layer. These experiments access a regime in which the magnetic, thermal and kinetic energies within the inflowing plasma are in rough equipartition, a regime which is particularly relevant to astrophysics, and which is distinct from other reconnection experiments. The properties of the reconnection process can be strongly modified by our choice of wire array material and geometry. Using a suite of diagnostics such as laser interferometry, high speed framing, collective Thomson scattering and Faraday rotation imaging, I will present results from super-Alfvénic, radiatively cooled reconnection using aluminium wires, and sub-Alfvénic, plasmoid-unstable reconnection using carbon wires.
I will also discuss our latest results, which show evidence for 3D effects within the reconnection layer, such as enhanced Thomson scattering, which hints at instability-driven turbulence, and the development of a large-scale kink instability. Finally I will discuss the PUFFIN pulsed power generator which I will build at MIT to study fundamental plasma processes, including magnetic reconnection, over long time scales.
Speaker: Anna Tenerani, University of Texas, USA
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Date/Time: Wednesday 1st July 2020, 4PM BST/11AM EDT
Title: Alfvénic fluctuations and switchbacks in the solar wind
Abstract: Large amplitude, turbulent Alfvénic fluctuations have been commonly observed in the solar wind since the first in-situ measurements, and they are thought to provide a possible mechanism to heat the solar corona and accelerate the solar wind. An important property that remains unexplained is that, despite the large excursion of such fluctuations, the magnitude of the total magnetic field remains nearly constant. This condition corresponds to spherical polarization and it implies an intrinsic degree of phase coherence in the fluctuating fields, which is necessary in order to maintain such a nonlinear state. How is this Alfvénic turbulent state achieved and maintained in the solar wind remains a fundamental open question in space physics. The mystery only deepens with Parker Solar Probe, whose observations during the first perihelion have shown the ubiquitous and persistent presence of magnetic field lines which are strongly perturbed to the point that they produce local inversions of the radial magnetic field, known as switchbacks. The corresponding signature of switchbacks in the velocity field is that of local enhancements in the radial speed (or jets) that display the typical velocity-magnetic field correlation that characterizes Alfvén waves propagating away from the Sun. After reviewing the main properties of Alfvénic fluctuations and switchbacks in the solar wind, we will address how their stability and evolution is affected by nonlinearities and kinetic effects in different plasma beta regimes. Emphasis will be given to the onset and evolution of parametric instabilities and to the impact of kinetic effects on the saturated nonlinear state. The implications of our results for models of switchback generation will be discussed, and we will conclude by outlining remaining open issues.
Speaker: Caterina Riconda, Sorbonne University, France
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Date/Time: Wednesday 24th June 2020, 4PM BST/11AM EDT
Title: Laser-plasma interaction experiment for solar burst studies
Abstract: Recent laser-plasma interaction experiments will be presented that allowed to explore some fundamental processes of wave coupling at the origin of interplanetary radio emissions. This experiments are relevant for the electromagnetic (EM) emission at twice the plasma frequency (2ωp) observed during solar bursts and thought to result from the coalescence of two Langmuir waves (LWs). In the interplanetary medium, the first LW is excited by electron beams, while the second is generated by electrostatic decay of Langmuir waves. In the laboratory experiment, instead of an electron beam, an energetic laser propagating through a plasma excites the primary LW, with characteristics close to those at near-Earth orbit. Analogously to what happens in the interplanetary medium, electrostatic decay of Langmuir waves generates the second LW. The resulting EM radiation at 2ωp is observed at different angles. Its intensity, spectral evolution, and polarization confirm the LW coalescence scenario.
Speaker: Irina Zhuravleva, University of Chicago, USA
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Date/Time: Wednesday 17th June 2020, 4PM BST/11AM EDT
Abstract: The intracluster medium (ICM) between galaxies in galaxy clusters is in a form of hot, X-ray-emitting plasma, permeated by weak magnetic fields. Although the magnetic fields are energetically subdominant, they modify the transport properties of the plasma, and, thereby, many large-scale phenomena in the ICM: from feedback processes to cluster mergers and subsequent energy flow and heating. The large sizes and relative simplicity of galaxy clusters make them ideal laboratories for measuring fundamental properties of such plasma, which are inaccessible by other means. In this talk, I will review how high-resolution imaging and spectroscopic data from X-ray observations are used to probe turbulence, gas viscosity, and thermal conduction in the ICM. I will present recent constraints on the effective gas viscosity in the bulk intergalactic plasma based on the observed density fluctuations on the Coulomb mean free path scale. At the end of the talk, exciting possibilities to probe plasma physics with near-future XRISM observatory will be discussed.
Speaker: Roger Blandford, KIPAC, Stanford University; 2020 SHAW PRIZE WINNER
Date/Time: Friday 12 June 2020, 4PM BST/11AM EDT
Title: Some New Frontiers in Plasma Astrophysics
Abstract: The advent of “Multi-messenger Astronomy” is revealing many new types of source where the physical conditions are extreme and the plasma physics seems central to explaining what is observed but is poorly understood. I will briefly describe four scenarios that may be relevant to interpreting these cosmic sources. (I) There is now good, circumstantial evidence that fast radio bursts are associated with young neutron stars endowed with ~10-100 GT magnetic fields called magnetars. Surface flares or quakes can launch Alfven waves into the outer magnetosphere where they may steepen to form fronts with thickness of order a radio wavelength. (II) The famous black hole shadow in M87 is defined by a ring that has been generally interpreted as a torus supported by ~ 100 MeV protons.An alternative possibility is that we are observing an ergo-magnetosphere powered by black hole spin, not accretion. (III) Many cosmic sources, including relativistic jets and the Crab Nebula, exhibit remarkably rapid variability requiring efficient, volumetric release of electromagnetic energy. This may be effected by the magnetoluminescent untangling of magnetic flux tubes as opposed to shock fronts or reconnection. (IV) Ultra-high energy cosmic rays (up to ~ 100 J) may be accelerated by strong shocks that form around nearby rich clusters of galaxies. The efficacy of this process depends upon the cosmic ray scattering caused by hydromagnetic disturbances around the shock and in the intergalactic and interstellar media they must traverse to reach earth.
Speaker: Félicie Albert, Lawrence Livermore National Laboratory, USA
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Date/Time: Wednesday 3 June 2020, 4PM BST/11AM EDT
Abstract: Bright sources of x-rays, such as synchrotrons and x-ray free electron lasers (XFEL) are transformational tools for many fields of science. They are used for biology, material science, medicine, or industry. Such sources rely on conventional particle accelerators, where electrons are accelerated to gigaelectronvolts (GeV) energies. The accelerating particles are also wiggled in magnetic structures to emit x-ray radiation that is commonly used for molecular crystallography, fluorescence studies, chemical analysis, medical imaging, and many other applications. One of the drawbacks of synchrotrons and XFELs is their size and cost, because electric field gradients are limited to about a few 10s of MeV/M in conventional accelerators.
This seminar will review particle acceleration in laser-driven plasmas as an alternative to generate x-rays. A plasma is an ionized medium that can sustain electrical fields many orders of magnitude higher than that in conventional radiofrequency accelerator structures. When short, intense laser pulses are focused into a gas, it produces electron plasma waves in which electrons can be trapped and accelerated to GeV energies. This process, laser-wakefield acceleration (LWFA), is analogous to a surfer being propelled by an ocean wave. Betatron x-ray radiation, driven by electrons from laser-wakefield acceleration, has unique properties that are analogous to synchrotron radiation, with a 1000-fold shorter pulse. This source is produced when relativistic electrons oscillate during the LWFA process.
An important use of x-rays from laser plasma accelerators we will discuss is in High Energy Density (HED) science. This field uses large laser and x-ray free electron laser facilities to create in the laboratory extreme conditions of temperatures and pressures that are usually found in the interiors of stars and planets. To diagnose such extreme states of matter, the development of efficient, versatile and fast (sub-picosecond scale) x-ray probes has become essential. In these experiments, x-ray photons can pass through dense material, and absorption of the x-rays can be directly measured, via spectroscopy or imaging, to inform scientists about the temperature and density of the targets being studied.
Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344, supported by the LLNL LDRD program under tracking code 13-LW-076, 16-ERD-024, 16-ERD-041, supported by the DOE Office of Fusion Energy Sciences under SCW 1476 and SCW 1569, and by the DOE Office of Science Early Career Research Program under SCW 1575.
Speaker: Chris Chen, Queen Mary University of London, UK
Date/Time: Wednesday 27th May 2020, 4PM BST/11AM EDT
Abstract: Parker Solar Probe (PSP), launched in August 2018, is set to become the first spacecraft to fly through the solar corona, the tenuous outer atmosphere of the Sun. The spacecraft carries a comprehensive set of instruments to fully characterise the local plasma environment; it has currently reached 27 solar radii from the Sun and by 2024 will get to within 9 solar radii. One of the primary goals of the mission is to investigate the related open problems of coronal heating and solar wind acceleration. Plasma turbulence is thought to be one of the key processes underlying these phenomena. Here, I will present initial results from the first two orbits of PSP to investigate the properties of the turbulence near the Sun and the possible role it plays in the generation of the solar wind. Significant differences of the turbulence closer to the Sun include much higher energy levels, a greater level of imbalance, all Alfvenic fields taking a -3/2 spectrum, and a smaller slow mode energy fraction. Comparison to the solar wind model of Chandran et al. (2011) indicates fluxes consistent with a turbulence-driven solar wind and a generation of the inward Alfvenic fluctuations (necessary for the turbulence to occur) by reflection from the large-scale gradient in Alfven speed, meaning that turbulence driven by Alfven waves from the Sun remains a viable explanation for solar wind acceleration. If time permits, I will also discuss some more recent findings, and future plans for the mission.
Speaker: Matt Landreman, University of Maryland, USA
Date/Time: Wednesday 20th May 2020, 4PM BST/11AM EDT
Abstract: To achieve good orbit confinement, recent stellarators such as HSX and W7-X have been designed using optimization, with numerical calculation of a 3D MHD equilibrium at each objective function evaluation. Here we present two new design methods which reduce the computational cost by orders of magnitude, while also providing new insights into the space of solutions [1-3]. These benefits are made possible by an expansion in large local aspect ratio [4], which is shown to be accurate in experimentally relevant geometries. The expansion permits a precise understanding of how many unique quasisymmetric configurations are possible (close to the magnetic axis). One of the new design approaches is a direct geometric construction of stellarator shapes with either quasisymmetry or omnigenity, providing good orbit confinement. This construction also makes it possible to achieve quasisymmetry to much higher accuracy than reported before. This construction can be ~ 10^7 x faster than traditional optimization, enabling wide surveys over possible stellarator configurations, and discovery of qualitatively new stellarator configurations. In a second new design approach, coil shapes and the constructed solutions are optimized simultaneously to achieve consistency. This approach has enabled the first simultaneous plasma-and-coil design of a quasisymmetric stellarator using analytic derivatives.
[1] LANDREMAN, SENGUPTA, & PLUNK, J PLASMA PHYS 85, 905850103 (2019).
[2] PLUNK, LANDREMAN, & HELANDER, J PLASMA PHYS 85, 905850602 (2019).
[3] LANDREMAN & SENGUPTA, J PLASMA PHYS 85, 815850601 (2019).
[4] Garren & Boozer, Phys Fluids B, 3, 2805 (1991).
Speaker: Eli Viezzer, University of Seville, Spain
Date/time: Wednesday 13th May 2020, 4pm BST/11am EDT
Abstract: In magnetically confined fusion devices, enhanced particle and energy transport induced by magnetohydrodynamic (MHD) fluctuations can deteriorate the plasma confinement and endanger the integrity of the device. One of the most prominent MHD fluctuations in a tokamak plasma is the edge localized mode (ELM) which expels jets of hot plasma, similar to solar flares on the edge of the Sun. ELMs appear during a mode of tokamak operation in which energy is retained more effectively and pressure builds up at the plasma edge (pedestal region). This mode of operation is called high confinement mode and is the operational regime foreseen for ITER. To avoid erosion of the divertor target plates from the heat and particle fluxes caused by ELMs, the mitigation or even full suppression of ELMs is required for future magnetic fusion devices. The successful realization of fusion relies, therefore, in a thorough understanding of edge stability, ELM-induced transport and ELM control. The small spatial width of the pedestal (outermost 5% of the confined plasma) and the fast temporal changes associated to ELMs (duration of about 1 ms, corresponding to 1-2% of the confinement time for ASDEX Upgrade) require high-resolution measurements to enable the analysis of the pedestal transport. To date, most effort has been placed in modelling the electron channel as those measurements are routinely available with a temporal resolution down to several tens of μs. The development of advanced diagnostics on the ASDEX Upgrade tokamak has paved the way to study the dynamic behaviour of both ions and electrons during an ELM cycle. We found that the ion energy transport recovers on similar time scales as the electron particle transport, while the electron energy transport is delayed. The dominant effect comes from the depletion of energy caused by the ELM. The local sources and sinks for the electron channel in the steep gradient region are much smaller compared to the energy flux arriving from the pedestal top, indicating that the core plasma may dictate the local dynamics of the ∇Te recovery during the ELM cycle.
Speaker: Ethan Peterson, MIT, USA
Date/time: Wednesday 6th May 2020, 4pm BST/11am EDT
Abstract: Eugene Parker developed the first theory of how the solar wind interacts with the dynamo-generated magnetic field of the Sun. He showed that the wind carries the magnetic field lines away from the star, while their footpoints are frozen into the corona and twisted into an Archimedean spiral by stellar rotation. The resulting magnetic topology is now known as the Parker spiral and is the largest magnetic structure in the heliosphere. The transition between magnetic field co-rotating with a star and the field advected by the wind is thought to occur near the so-called Alfvén surface - where inertial forces in the wind can stretch and bend the magnetic field. According to the governing equations of magnetohydrodynamics, this transition in a magnetic field like the Sun's is singular in nature and therefore suspected to be highly dynamic. However, this region has yet to be observed in-situ by spacecraft or in the laboratory, but is presently the primary focus of the Parker Solar Probe mission. Here we show, in a synergistic approach to studying solar wind dynamics, that the large-scale magnetic topology of the Parker spiral can also be created and studied in the laboratory. By generating a rotating magnetosphere with Alfvénic flows, magnetic field lines are advected into an Archimedean spiral, giving rise to a dynamic current sheet that undergoes magnetic reconnection and plasmoid ejection. These plasmoids are born at the tip of the streamer cusp, driven by non-equilibrium pressure gradients, and carry blobs of plasma outwards at super- Alfvénic speeds, mimicking the observed dynamics of coronal helmet streamers. Further more, a simple heuristic model based on a critical plasmoid length scale and sonic expansion time is presented. This model explains the frequencies observed in the experiment and simulations (10s of KHz) and is consistent with the 90 minute plasmoid ejection period of full-scale coronal streamers as observed by the LASCO and SECCHI instrument suites.
Speaker: Noah Mandell, Princeton University, USA
Date/time: Wednesday 29th April 2020, 4pm BST/11am EDT
Abstract: Understanding turbulent transport physics in the tokamak edge and scrape-off layer (SOL) is critical to developing a successful fusion reactor. The dynamics in these regions plays a key role in determining the L-H transition, the pedestal height and the heat load to the vessel walls. Large-amplitude fluctuations, magnetic X-point geometry, and plasma interactions with material walls make modeling turbulence in the edge/SOL more challenging than in the core region, requiring specialized gyrokinetic codes. Electromagnetic effects can also be important in the edge/SOL region due to steep pressure gradients, and coupling of perpendicular dynamics with kinetic shear Alfven waves can result in line bending. However, all gyrokinetic results in the SOL to date have assumed electrostatic dynamics, due in part to numerical challenges like the Ampere cancellation problem. We present the first nonlinear electromagnetic gyrokinetic results of turbulence on open field lines in the tokamak SOL, obtained using the Gkeyll full-f continuum gyrokinetic code. The results, which use a model helical SOL geometry and NSTX-like parameters, show magnetic fluctuations of up to delta B_perp/B ~ 1%. Line-tracing visualizations show that field lines are pushed and bent by radially-propagating blobs. Comparisons to electrostatic simulations show that including electromagnetic effects can produce larger relative density fluctuations and more intermittent transport.
Featured paper in JPP: Electromagnetic full- f gyrokinetics in the tokamak edge with discontinuous Galerkin methods
Speaker: Prof Stanislav Boldyrev, University of Wisconsin-Madison, USA
Date/time: Wednesday 22nd April 2020, 4pm BST/11am EDT
Abstract: Solar wind provides an example of a weakly collisional plasma expanding from a thermal source in the presence of spatially diverging magnetic field lines. Observations show that in the inner heliosphere, the electron temperature declines with the distance approximately as Te(r) ~ r^−0.3 . . . r^−0.7, which is significantly slower than the adiabatic expansion law r^−4/3. Motivated by such observations, we propose a kinetic theory that addresses the non-adiabatic evolution of a nearly collisionless plasma expanding from a central thermal source. We concentrate on the dynamics of energetic electrons propagating along a radially diverging magnetic flux tube. Due to the conservation of their magnetic moments, the electrons form a beam collimated along the magnetic field lines. Due to weak energy exchange with the background plasma, the beam population slowly loses its energy and heats the background plasma. We propose that no matter how weak the collisions are, at large enough distances from the source a universal regime of expansion is established where the electron temperature declines as Te(r) ~ r^−2/5. This is close to the observed scaling of the electron temperature in the inner heliosphere. Our first-principle kinetic derivation may thus provide an explanation for the slower-than-adiabatic temperature decline in the solar wind.
Speaker: Prof Per Helander, Max Planck Institute for Plasma Physics, Greifswald, Germany
Date/time: Wednesday 15th April 2020, 4pm BST/11am EDT
Abstract: Stellarators, tokamaks, and other devices for fusion plasma confinement use electromagnets to create the magnetic field. In the case of stellarators, the required magnetic-field coils can be very complicated and contribute significantly to the overall cost of the device. We shown that the coils can, at least in principle, be substantially simplified by the use of permanent magnets. Such magnets cannot create toroidal magnetic flux, but they can be used to shape the plasma and thus to create poloidal flux and rotational transform, thereby easing the requirements on the magnetic-field coils. As a proof of principle, a couple of examples of quasi-axisymmetric stellarator design with permanent magnets are shown.
Speaker: Aaron Bader, University of Wisconsin, USA
Date/time: Wednesday 8th April 2020, 4pm BST/11am EDT
Abstract: Stellarators offer an inherently steady state reactor concept with low recirculating power. Because stellarators do not rely on plasma current for confinement, they are not susceptible to current driven disruptions. Stellarators are also capable of operating at high density, and can perform stably beyond ideal MHD stability limits. Because stellarator configurations have magnetic fields imposed mainly by external coils, there is significant freedom to tailor the confinement properties to the device needs. Only in the last few decades has theoretical knowledge of stellarator confinement advanced so as to produce optimized configurations. This talk will focus on how these devices are optimized, both for current experiments that exist today, and for future experiments, pilot plants, and reactor concepts. Four topical areas, identified as key physics gaps for stellarators, are discussed: turbulent transport optimization by design, energetic particle transport, divertor performance, and coil design.
Featured paper in JPP: Stellarator equilibria with reactor relevant energetic particle losses
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