JPP invites you to join us for the JPP Frontiers of Plasma Physics Colloquium
Organisers: Bill Dorland, Cary Forest and Alex Schekochihin
For information on how to join the Colloquium please sign up here
For details of upcoming talks, please see here
For details of past talks and recordings (where available), please see here
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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