An action of a group G on a set X is said to be quasi-n-transitive if the diagonal action of G on $X^n$ has only finitely many orbits. We show that branch groups, a special class of groups of automorphisms of rooted trees, cannot act quasi-2-transitively on infinite sets.

The Vlasov–Maxwell equations provide kinetic simulations of collisionless plasmas, but numerically solving them on classical computers is often impractical. This is due to the computational resource constraints imposed by the time evolution in the six-dimensional phase space, which requires broad spatial and temporal scales. The novelty of this study is to implement a quantum–classical hybrid Vlasov–Maxwell solver and the rigorous numerical scheme evaluation by numerical simulations. Specifically, the Vlasov solver implements the Hamiltonian simulation based on quantum singular value transformation, coupled with a classical Maxwell solver. We perform numerical simulation of a one-dimensional advection test and a one-spatial-dimension, one-velocity-dimension two-stream instability test on the Qiskit-Aer-GPU quantum circuit emulator with an A100 GPU. The computational complexity of our quantum algorithm can potentially be reduced from the classical $\mathcal{O}(N^6T^2/\epsilon )$ to $\mathcal{O}\left (\text{poly}(\log {N})\left (NT+T\log \left (T/\epsilon \right )\right )\right )$ for the $N$ grid system, simulation time $T$ and error tolerance $\epsilon$ in the limit where the number of queries is large enough and the error is small enough. Furthermore, the numerical analysis reveals that our quantum algorithm is robust under larger time steps compared with classical algorithms with the constraint of Courant–Friedrichs–Lewy condition.

Two desert cyanobacterial strains, Chroococcidiopsis sp. CCMEE 010 and CCMEE 130, capable far-red light photoacclimation (FaRLiP), were investigated for the stability of biosignatures after six years of desiccation. Biosignature detectability was demonstrated by confocal laser scanning microscopy and Raman spectroscopy thus highlighting that these two FaRLiP cyanobacteria are a novel reservoir of an array of pigments, encompassing canonical chlorophyll a, far-red shifted chlorophylls, phycobilins and carotenoids. The recorded signals were comparable to those of dried cells of Chroococcidiopsis sp. CCMEE 029, CCMEE 057 and CCMEE 064, not capable of FaRLiP acclimation and previously reported for biosignature stability and survivability after exposure to space and Mars-like conditions during the BIOMEX (BIOlogy and Mars EXperiment) and BOSS (Biofilm Organisms Surfing Space) low Earth orbit missions. Since infrared-light driven photosynthesis has implications for the habitability of Mars as well as exoplanets, the stability of far-red shifted chlorophylls in dried Chroococcidiopsis is a prerequisite for future experimentations under simulated planetary conditions in the laboratory or directly into space. It is anticipated that post-flight investigations of FaRLiP cyanobacteria as part of the BioSigN (Bio-Signatures and habitable Niches) space mission will contribute to gather novel insights into biosignature degradation/stability and thus prepare future planetary exploration missions to Mars. In addition, the scored viability of strains CCMEE 010 and CCMEE 130 after prolonged desiccation is relevant to investigate life endurance under deep space conditions, as planned by the BioMoon mission that aims to expose dried and rehydrated extremophiles on the Moon surface after exposure to deep space.

The pulse duration is a critical parameter of picosecond-petawatt laser systems because it directly affects the results of high-energy-density physics experiments. This study systematically investigated the effects of the spectral width, central wavelength and beam-pointing deviations on pulse duration stability at the SG-II facility. A theoretical analysis of the relationship between spectra and pulse duration is conducted to quantify the impact on pulse duration stability, and the results are further validated through experimental measurements. In addition, beam-pointing deviations at the stretcher significantly affect the pulse duration. For example, a 27 μrad deviation can induce a 30% pulse duration variation. In contrast, the compressor exhibits greater robustness. Based on simulation and experimental results, we identify operational tolerance ranges for spectral width and beam-pointing deviation to maintain pulse duration stability within 5% at the SG-II facility. These findings provide critical guidance for optimizing the performance and reliability of chirped-pulse amplification/optical parametric chirped-pulse amplification-based high-power laser systems.

We present a number of measures and techniques to characterise and effectively construct quasi-isodynamic stellarators within the near-axis framework, without the need to resort to the computation of global equilibria. These include measures of the reliability of the model (including aspect-ratio limits and the appearance of ripple wells), quantification of omnigeneity through $\epsilon _{\mathrm{eff}}$, measure and construction of MHD-stabilised fields, and the sensitivity of the field to the pressure gradient. The paper presents, discusses and gives examples of all of these, for which expansions to second order are crucial. This opens the door to the exploration of how key underlying choices of the field design govern the interaction of desired properties (‘trade-offs’) and provides a practical toolkit to perform efficient optimisation directly within the space of near-axis quasi-isodynamic configurations.

Developing reduced-order models for the transport of solid particles in turbulence typically requires a statistical description of the particle–turbulence interactions. In this work, we utilize a statistical framework to derive continuum equations for the moments of the slip velocity of inertial, settling Lagrangian particles in a turbulent boundary layer. Using coupled Eulerian–Lagrangian direct numerical simulations, we then identify the dominant mechanisms controlling the slip velocity variance, and find that for a range of Stokes number ${S{\kern-0.5pt}t}^+$, Settling number ${S{\kern-0.5pt}v}^+$ and Reynolds number $\textit{Re}_\tau$ (based on frictional scales),the slip variance is primarily controlled by local differences between the ‘seen’ variance and the particle velocity variance, while terms appearing due to the inhomogeneity of the turbulence are subleading until ${S{\kern-0.5pt}v}^+$ becomes large. We also consider several comparative metrics to assess the relative magnitudes of the fluctuating slip velocity and the mean slip velocity, and we find that the vertical mean slip increases rapidly with ${S{\kern-0.5pt}v}^+$, rendering the variance relatively small – an effect found to be most substantial for ${S{\kern-0.5pt}v}^+\gt 1$. Finally, we compare the results with a model of the acceleration variance (Berk & Coletti 2021 J. Fluid Mech. 917, A47) based the concept of a response function described in Csanady (1963 J. Atmos. Sci. 20, 201–208), highlighting the role of the crossing trajectories mechanism. We find that while there is good agreement for low ${S{\kern-0.5pt}v}^+$, systematic errors remain, possibly due to implicit non-local effects arising from rapid particle settling and inhomogeneous turbulence. We conclude with a discussion of the implications of this work for modelling the transport of coarse dust grains in the atmospheric surface layer.

Powerful lasers may be used in the future to produce magnetic fields that would allow us to study turbulent magnetohydrodynamic inverse cascade behaviour. This has so far only been seen in numerical simulations. In the laboratory, however, the produced fields may be highly anisotropic. Here, we present corresponding simulations to show that, during the turbulent decay, such a magnetic field undergoes spontaneous isotropisation. As a consequence, we find the decay dynamics to be similar to that in isotropic turbulence. We also find that an initially pointwise non-helical magnetic field is unstable and develops magnetic helicity fluctuations that can be quantified by the Hosking integral. It is a conserved quantity that characterises magnetic helicity fluctuations and governs the turbulent decay when the mean magnetic helicity vanishes. As in earlier work, the ratio of the magnetic decay time to the Alfvén time is found to be approximately $50$ in the helical and non-helical cases. At intermediate times, the ratio can even reach a hundred. This ratio determines the endpoints of cosmological magnetic field evolution.

Understanding microbial adaptations to the extreme conditions of space is crucial for both astronaut health and the integrity of spacecraft materials. This study comparatively analyses the cosmic radiation resistance and growth responses to simulated microgravity (SMG) of a wild-type strain and an International Space Station (ISS) isolate of Penicillium rubens. Resistance to helium- and iron-ion radiation was determined, alongside growth under SMG using clinorotation. The results revealed that the ISS isolate exhibited higher resistance to both helium- and iron-ion radiation than the wild-type strain, suggesting adaptive mechanisms that enhance survival in space environments. Additionally, while the ISS isolate demonstrated significantly increased growth in SMG compared to normal gravity conditions, the wild-type strain showed no difference between the two conditions. These findings indicate that prolonged exposure to the space environment may select for traits that enhance resistance to cosmic radiation and alter growth dynamics under microgravity. Such adaptations could have implications for microbial monitoring in space habitats, planetary protection policies, and potential biotechnological applications in space. Further investigations into the genetic and metabolic differences between both strains may provide deeper insights into fungal adaptation to space environments.

Presented here is a novel formulation of the mean-field dynamo as a modulational instability of magnetohydrodynamic (MHD) turbulence. This formulation, termed mean-field wave kinetics (MFWK), is based on the Weyl symbol calculus and allows describing the interaction between the mean fields (magnetic field and fluid velocity) and turbulence without requiring scale separation that is commonly assumed in the literature. The turbulence is described by the Wigner–Moyal equation for the spectrum of the two-point correlation matrix (Wigner matrix) of magnetic-field and velocity fluctuations and depicts the turbulence as an effective plasma of quantum-like particles that interact via the mean fields. Eddy–eddy interactions, which serve as ‘collisions’ in this effective plasma, are modelled within the standard minimal tau approximation to aid comparison with existing theories. Using MFWK, the non-local electromotive force is calculated for generic turbulence from first principles, modulo the limitations of MFWK. This result is then used to study, both analytically and numerically, the modulational modes of MHD turbulence, which appear as linear instabilities of the said effective quantum-like plasma of fluctuations. The standard $\alpha ^2$-dynamo and other known results are reproduced as special cases. A new dynamo effect is predicted that is driven by correlations between the turbulent flow velocity and the turbulent current.

Non-autonomous self-similar sets are a family of compact sets which are, in some sense, highly homogeneous in space but highly inhomogeneous in scale. The main purpose of this paper is to clarify various regularity properties and separation conditions relevant for the fine local scaling properties of these sets. A simple application of our results is a precise formula for the Assouad dimension of non-autonomous self-similar sets in $\mathbb{R}^d$ satisfying a certain “bounded neighbourhood” condition, which generalises earlier work of Li–Li–Miao–Xi and Olson–Robinson–Sharples. We also see that the bounded neighbourhood assumption is, in few different senses, as general as possible.

The heating effect of electromagnetic waves in ion cyclotron range of frequencies (ICRFs) in magnetic confinement fusion device is different in different plasma conditions. In order to evaluate the ICRF heating effect in different plasma conditions, we conducted a series of experiments and corresponding TRANSP simulations on the EAST tokamak. Both simulation and experimental results show that the effect of ICRF heating is poor at low core electron density. The decrease in electron density changes the left-handed electric field near the resonant layer, resulting in a significant decrease in the power absorbed by the hydrogen fundamental resonance. However, quite a few experiments must be performed in plasma conditions with low electron density. It is necessary to study how to make ICRF heating best in low electron density plasma. Through a series of simulation scans of the parallel refractive index (n//) of the ICRF antenna, it is concluded that the change of the ICRF antenna n// will lead to the change of the left-handed electric field, which will change the fundamental absorption of ICRF power by the hydrogen minority ions. Fully considering the coupling of ion cyclotron wave at the tokamak boundary and the absorption in the plasma core, optimizing the ICRF antenna structure and selecting appropriate parameters such as parallel refractive index, minority ion concentration, resonance layer position, plasma current and core electron temperature can ensure better heating effect in the ICRF heating experiments in the future EAST upgrade. These results have important implications for the enhancement of the auxiliary heating effect of EAST and other tokamaks.

This paper presents a Hammir tandem mirror confinement performance analysis based on Realta Fusion’s first-of-a-kind model for axisymmetric magnetic mirror fusion performance. This model uses an integrated end plug simulation model including, heating, equilibrium and transport combined with a new formulation of the plasma operation contours (POPCONs) technique for the tandem mirror central cell. Using this model in concert with machine learning optimization techniques, it is shown that an end plug utilizing high temperature superconducting magnets and modern neutral beams enables a classical tandem mirror pilot plant producing a fusion gain Q > 5. The approach here represents an important advance in tandem mirror design. The high-fidelity end plug model enables calculations of heating and transport in the highly non-Maxwellian end plug to be made more accurately. The detailed end plug modelling performed in this work has highlighted the importance of classical radial transport and neutral beam absorption efficiency on end plug viability. The central cell POPCON technique allows consideration of a wide range of parameters in the relatively simple near-Maxwellian central cell, facilitating the selection of more optimal central cell plasmas. These advances make it possible to find more conservative classical tandem mirror fusion pilot plant operating points with lower temperatures, neutral beam energies and end plug performance requirements than designs in the literature. Despite being more conservative, it is shown that these operating points have sufficient confinement performance to serve as the basis of a viable fusion pilot plant provided that they can be stabilized against magnetohydrodynamic and trapped particle modes.

This study presents high-power mode-selective operation in a large-mode-area (LMA) fiber laser. A spatial mode-adaptive control system incorporating a 5×1 photonic lantern was employed to facilitate mode conversion between the LP01 and LP11 modes. The coherence length between the five single-mode arms and the stimulated Brillouin scattering threshold in the amplifier were well balanced by tuning the seed linewidth. In addition, the specific design of the fiber amplifier’s bending radius enabled stable mode-selective output with high mode purity. Consequently, a near-fundamental mode control was achieved in a 42-μm LMA fiber laser, yielding a beam quality M2 factor of 1.97 at an output power of 1 kW. Subsequently, a stable LP11 mode laser output with an output power of 219 W and an optical conversion efficiency of 75% was obtained. This research provides a significant technical foundation for the mode-selective operation of high-power LMA fiber lasers.

In the design and construction of ultra-high-peak-power laser systems, it is necessary to control the accumulated B-integral of the laser pulse, but currently there are no reasonable B-integral control standards for picosecond and femtosecond lasers. We systematically evaluate the influence of the B-integral on the output capability of picosecond and femtosecond laser systems for the first time, to our knowledge, taking Nd:glass lasers and Ti:sapphire lasers as examples. For picosecond lasers, the temporal domain compressibility and the small-scale self-focusing effect restrict the B-integral to 1.7 and 1.9, respectively. For femtosecond lasers, the B-integral is mainly restricted by the small-scale self-focusing effect and the far-field focusability, which limit the B-integral to 1.5 and 1.7, respectively. The restriction made by far-field focusability can be largely relaxed by inserting a deformable mirror. The study of the factors restricting the B-integral will provide guidance for the design of ultra-high-peak-power laser systems.

Accurate redshift measurements are essential for studying the evolution of quasi-stellar objects (QSOs) and their role in cosmic structure formation. While spectroscopic redshifts provide high precision, they are impractical for the vast number of sources detected in large-scale surveys. Photometric redshifts, derived from broadband fluxes, offer an efficient alternative, particularly when combined with machine learning techniques. In this work, we develop and evaluate a neural network model for predicting the redshifts of QSOs in the Dark Energy Spectroscopic Instrument (DESI) Early Data Release spectroscopic catalogue, using photometry from DESI, the Widefield Infrared Survey Explorer (WISE), and the Galactic Evolution Explorer (GALEX). We compare the performance of the neural network model against a k-Nearest Neighbours approach, these being the most accurate and least resource-intensive of the methods trialled herein, optimising model parameters and assessing accuracy with standard statistical metrics. Our results show that incorporating ultraviolet photometry from GALEX improves photometric redshift estimates, reducing scatter and catastrophic outliers compared to models trained only on near infrared and optical bands. The neural network achieves a correlation coefficient with spectroscopic redshift of $0.9187$ with normalised median absolute deviation of $0.197$, representing a significant improvement over other methods. Our work combines DESI, WISE, and GALEX measurements, providing robust predictions which address the difficulties in predicting photometric redshift of QSOs over a large redshift range.