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Physics Physics Plasma Physics

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Wednesday 12 August

Plasma Physics · Cross-listed from astro-ph.SR

Large-Scale Dynamos Driven by Shear-Flow-Induced Jets

B. Tripathi, A. E. Fraser, P. W. Terry et al. (+3 more)

At every scale they occupy, magnetic fields affect various phenomena, including star formation, cosmic ray transport, charged particle acceleration, space weather, transport in planetary atmospheres, and laboratory plasmas. These fields are often generated and sustained by turbulent flows in a process called the dynamo. In 1955, E. N. Parker parameterized the effects of small-scale turbulence to propose a mean-field dynamo theory. The widely used theory reproduces observed large-scale fields but suffers from difficulty in tuning parameters as they are not justified from first principles: Studies of turbulent flows show tangled magnetic fields, which are folded and fragmented into small-scale structures due to shear-flow straining. Here, considering a shear flow that is unstable and driven, we develop analytic theory and perform three-dimensional (3D), advanced computer simulations of turbulence with up to 4096 x 4096 x 8192 grid points, showing ab initio generation of quasi-periodic, large-scale magnetic fields. The generation occurs via the mean-vorticity effect—an additional mean-field dynamo process postulated in 1990. Crucial to this dynamo is the prior generation of large-scale 3D jets, robustly produced as topologically protected and exact nonlinear solutions of the magnetohydrodynamic equations. The jet-driven dynamo applies to shear-driven laboratory and astrophysical systems. These include binary neutron star mergers, where the reported dynamo likely operates on microsecond timescales to produce in milliseconds some of the strongest magnetic fields in the Universe, providing signals for multimessenger astronomy.
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All authors B. Tripathi, A. E. Fraser, P. W. Terry, E. G. Zweibel, M. J. Pueschel, R. Fan

Plasma Physics

Evaluation of the Ambipolar Diffusion Approximation in Partially Ionized Rarefied Hypersonic Flows

Marisa Petrusky and Iain D. Boyd

Accurate numerical simulation of rarefied hypersonic plasmas is increasingly important for optimization of re-entry spacecraft design and the development of advanced aerospace technologies. For kinetic simulation methods, it is convention to enforce ions and electrons to diffuse at the same rate, known as the ambipolar diffusion approximation. This approach circumvents costly resolution of fast electron motion, but neglects the complex plasma dynamics of ions and electrons. Almost all studies that investigated the efficacy of the ambipolar diffusion approximation in hypersonics report noticeable differences in flowfield properties when electrostatic modeling is used, including increases in vehicle surface heat flux and decreases in electron temperature. However, it is unknown whether these reported differences originate directly from acceleration and deceleration of charged species through the electric fields and momentum-exchange collisions between charged and neutral species, defined as first-order effects, or from subsequent interactions with particles experiencing first-order effects, defined as second-order effects. Kinetic hypersonic flow simulations with electrostatic modeling are performed with argon to quantify the validity of the ambipolar diffusion approximation in terms of capturing first-order plasma effects along a one-dimensional stagnation streamline. Three different plasma diffusion regimes are studied under two sets of rarefied freestream flow conditions. The approximation is evaluated in terms of predicting plasma density distributions, electron temperature, and stagnation point heat flux. New criteria are proposed for identification of plasma diffusion regimes in hypersonic flows and use of the ambipolar diffusion approximation.
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Plasma Physics

Derivation and application of sheath boundary conditions for drift-kinetic simulations in a linear plasma device based on a gyromoment approach

Jacob Emil Mencke, Thomas Stucker and Paolo Ricci

Boundary conditions for a drift-kinetic model at the collisional presheath entrance with perpendicular incidence of the magnetic field to the wall are derived and numerically implemented. The drift-kinetic model for the plasma is based on the expansion of the ion distribution function on a Hermite-Laguerre basis, and the evolution of the resulting gyromoments. A linear-plasma-device geometry is considered. Comparison with simpler simulations with previously used ad hoc boundary conditions is presented. For the new set of boundary conditions, a significant increase of the plasma outflow to the wall is observed, leading to a significantly smaller plasma density in the whole volume of the device.
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Plasma Physics · Cross-listed from physics.med-ph

A 2D Hydrothermodynamic Analytical Model for Rapid Tumor Ablation using High-Intensity Focused Ultrasound

D. Tsiklauri

We establish a self-consistent 2D hydrothermodynamic analytical model for high-intensity focused ultrasound tumor ablation. Expanding compressible Navier-Stokes equations to second order demonstrates that a stationary cellular matrix suppresses acoustic streaming ($\mathbf{v}_2 = 0$). This constraint forces the absorbed wave momentum flux to convert entirely into localized, time-averaged static pressure gradients ($\nabla \langle p_2 \rangle = \mathbf{F}_2$), bridging non-linear hydrodynamics with thermodynamic dissipation. Solving the non-diffusive Pennes bioheat equation under a $1.0\,\text{s}$ top-hat pulse reveals that a spherically focusing geometry ($\propto 1/r^2$) overrides exponential damping past a critical geometric threshold ($r_{\text{crit}} = 2x_0$), preventing upstream skin overheating. We derive an optimization criterion where the absorption coefficient matches half the inverse target depth ($\alpha = 1/2x_0$). Solving the non-isothermal Arrhenius integral yields a sharp lesion boundary radius at $r_b = 0.75\,w_0$, where the volume average reaches $72.1^\circ\text{C}$ while the core peaks at $90.0^\circ\text{C}$. Post-pulse 2D free-space Green's function convolution confirms immediate monotonic thermal decay ($\partial \theta/\partial t' < 0$) outside this boundary. This closed-form framework provides explicit scaling laws for non-invasive wave-matter thermal confinement, bypassing computationally heavy numerical simulations.
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Plasma Physics

The Ambipolar electric field in multispecies plasma atmospheres: effects of alpha particles and stochastic heating

Luca Barbieri, Pascal Démoulin and Daniel Verscharen

We investigate stationary states of a collisionless, gravitationally stratified plasma atmosphere composed of electrons, protons, and alpha particles by extending Pannekoek–Rosseland theory to multispecies and multi-temperature plasmas. Starting from Liouville's theorem, we derive the self-consistent ambipolar electric field from kinetic equilibrium and charge neutrality. For a single-temperature atmosphere, we obtain analytical expressions for the ambipolar field, show its dependence on alpha-particle abundance, and determine the relative stratification of the three species. A first-order analytical approximation to the electrostatic potential accurately reproduces the numerical solution. We then generalize the formalism to multi-temperature plasmas generated by stochastic boundary heating, representing the stationary distribution as a superposition of Maxwellian populations. Gravitational filtering produces non-exponential density profiles and increasing temperatures with altitude, while preserving the relative species stratification, with alpha particles most strongly stratified and protons least. The ambipolar electric field contains a dominant gravitational contribution, corresponding to the generalized Pannekoek–Rosseland field, and a thermoelectric contribution arising from species-dependent temperature gradients, which accounts for its non-monotonic structure. These results provide a framework for studying the combined effects of plasma composition and stochastic heating in gravitationally stratified astrophysical plasmas.
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Plasma Physics

Measurements of Laser-Driven Plasma Expansion into Hohlraum-Relevant Background Gas

S. Hilsabeck, S. Dannhoff, C. A. Walsh et al. (+3 more)

Experiments at the OMEGA EP laser facility were designed and executed to study plasma expansion into hohlraum-relevant gas fills (0.3-0.6 mg/cc of helium), providing a surrogate platform for investigating hohlraum wall blow-off, non-local transport, and magnetized plasma effects. We observe well-defined density features and filamentary structures as laser-driven copper plasma expands into a low-Z background gas. Shadowgraphy resolves sharp density features over time and reveals fine-scale filamentation in the laser spot region with characteristic transverse scales of 10-100 microns near the foil surface. Proton radiography provides sensitivity to path-integrated magnetic fields and density modulations throughout the bubble volume. We extract the bubble expansion as a function of time for two gas pressures, 350 psi (producing 0.3 mg/cc equivalent conditions) and 700 psi (0.6 mg/cc equivalent conditions), and compare the measured propagation to magnetohydrodynamic simulations performed with Gorgon and HYDRA. While the large-scale shape of the bubble is well reproduced by both codes, the time-dependent expansion rate shows significant discrepancies (20-50% faster) compared to experimental observations between 1 and 3 ns. This leads to increasingly larger differences in bubble sizes at later times. The optical measurements of bubble expansion and evolution of small-scale structures point to additional constraints required for Biermann-battery field generation, thermal transport, and instability growth in hohlraum-relevant plasmas, to ensure accurate, predictive modeling of gas-filled hohlraums.
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All authors S. Hilsabeck, S. Dannhoff, C. A. Walsh, M. Sherlock, G. D. Sutcliffe, E. R. Tubman

Plasma Physics

Reconstructive AI Spectroscopy of Charged Particle Beams

Vasily Kozhevnikov, Andrey Kozyrev, Elena Klepalova et al. (+2 more)

This work introduces a physics-informed neural network (PINN) framework for reconstructing electron energy spectra from sparsely sampled attenuation-curve data. Leveraging the NVIDIA PhysicsNeMo platform, the proposed mesh-free methodology operates directly on raw experimental datasets while explicitly incorporating all experimental uncertainties. Validation on subnanosecond electron beam measurements demonstrates that the approach accurately resolves complex, multi-peaked spectral features of energy distribution. The framework enforces physical consistency through embedded governing principles and exhibits substantial predictive capability for energy spectrum reconstruction from noisy, low-precision, and sparse experimental data.
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All authors Vasily Kozhevnikov, Andrey Kozyrev, Elena Klepalova, Victor Tarasenko, Evgenii Baksht

Tuesday 11 August

Plasma Physics

Data-Driven Generation of Compact Quasi-Isodynamic Stellarators

Yang Han, Hanlin Chen. Shuai Cao, Zhiyuan Lu et al. (+3 more)

Stellarator design explores a vast space of three-dimensional plasma boundaries, only a small fraction of which yields usable equilibria. Data-driven models can narrow this search by learning from existing optimized configurations. Building on the ConStellaration database, we extend conditional boundary generation to four-field-period QI configurations, focusing on the sparsely sampled low-aspect-ratio regime. The approach learns the common geometric structure of known QI equilibria and then adapts it using a small high-fidelity compact dataset, allowing target magnetic properties to guide generation beyond the original data distribution. This adaptation reduces the compact-domain test loss by approximately 87% and yields converged ultra-compact candidates consistent with the prescribed conditions. Several candidates show favorable confinement indicators, and one provides a useful seed for further QI optimization and finite-beta assessment. The method therefore serves as a data-informed front end to high-fidelity physics and optimization.
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All authors Yang Han, Hanlin Chen. Shuai Cao, Zhiyuan Lu, Dehong Chen, Guosheng Xu, Baonian Wan

Plasma Physics

Machine-learning surrogate models for nonlinear energetic-particle transport predictions in ITER

Yashika Ghai, Donald A. Spong, Jacobo Varela et al. (+1 more)

Fast and accurate prediction of energetic-particle transport driven by Alfvén eigenmode (AE) instabilities is essential for integrated modeling workflows used in the design and optimization of burning plasma fusion reactors. In this work, we develop machine-learning-based surrogate models for rapid prediction of energetic beam and alpha-particle transport fluxes, together with predictive uncertainty estimates, for an ITER steady-state scenario. Two complementary surrogate methodologies, Gaussian process (GP) regression and hierarchical neural networks (NNs), are trained using nonlinear FAR3d gyrofluid simulations of energetic-particle transport. A flux-variability analysis demonstrates that the selected plasma-state representation provides a sufficiently unique parameterization of the nonlinear transport response over most of the sampled feature space, thereby justifying the surrogate formulation. Both surrogate models reproduce the nonlinear transport fluxes with high predictive accuracy while reducing the computational cost of transport evaluation by approximately five to six orders of magnitude relative to direct nonlinear FAR3d simulations. Although the two approaches achieve comparable predictive accuracy, they exhibit distinct uncertainty characteristics: the GP provides more consistent global uncertainty estimates, whereas the NN more clearly distinguishes between different transport regimes. This work establishes a proof of concept for developing machine-learning surrogate models of energetic-particle transport that are sufficiently accurate and computationally efficient to be incorporated into future integrated modeling workflows.
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All authors Yashika Ghai, Donald A. Spong, Jacobo Varela, Luis Garcia

Plasma Physics · Cross-listed from physics.acc-ph

High-energy electron-positron beam collisions with large-angle disruptions

W. Zhang, T. Grismayer and L. O. Silva

We show that the beam and field dynamics in high-energy electron-positron ($e^-e^+$) collisions are characterized by a new dimensionless parameter introduced as $\varepsilon$ in this study. The disruption effect deflects the particles transversely at angles equal to $\varepsilon$. The particles simultaneously undergo deceleration of longitudinal velocities (a “braking effect"). The deceleration scales as $\propto \varepsilon^2$. A longitudinal electric field is further provoked, whose amplitude scales as $\propto \varepsilon$. We identify $\varepsilon \gtrsim 1$ (with large-angle disruptions) as a novel extreme regime, where the transverse motion becomes strongly relativistic. The braking effect completely stops and further reverses the beam propagation. Our theoretical model is in excellent agreement with electromagnetic particle-in-cell simulations. The previous beam-beam studies, including legacy numerical codes, apply only to the $\varepsilon \ll 1$ regime. They fail to capture the correct beam features and collision luminosities, and overestimate beam-beam effects (including beamstrahlung and pair production) for considerable $\varepsilon$, thus demonstrating the need for fully electromagnetic particle-in-cell codes to study these regimes.
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Plasma Physics

Gas Flow Rate Influence on Gas Temperature Regulation in a Reinforced Radio-Frequency Cross-Field Atmospheric Plasma Jet

Radhika T. P. and Satyananda Kar

This study investigates the effect of gas flow rate on the gas temperature and discharge characteristics of a reinforced radio frequency cross-field atmospheric pressure plasma jet (APPJ) with an additional floating electrode. The plasma jet length, electron excitation temperature, electron density, and reactivity were enhanced by introducing copper floating electrodes of varying widths. However, this enhancement was accompanied by an undesired rise in gas temperature, limiting the plasma's application for heat- sensitive materials. To control this temperature rise, the gas flow rate varied from 1.5 to 9 lpm, showing a significant reduction in gas temperature from 438 K to 402 K as the flow rate increased, particularly at higher input powers. The study reveals that while an increase in gas flow rate initially improves ionization and reactivity by increasing electron excitation temperature and density, the insufficient input power for ionization at higher flow rates causes a decline in these parameters due to reduced ionization efficiency. Further optimization was achieved by increasing input power, which allowed better utilization of neutral atoms and improved plasma reactivity even at higher flow rates. The findings highlight the importance of tuning both gas flow rate and input power to maintain optimal plasma performance for various applications, particularly where controlled gas temperature and high reactivity are essential.
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Plasma Physics

Potential of Atmospheric Pressure Thermal Plasma Technology towards Waste Processing: A Comprehensive Review

Tejashwi Rana, Aishik Basu Mallick, Radhika T P et al. (+3 more)

The enhancement of living standards has significantly contributed to the rapid growth of urban populations, resulting in a substantial increase in municipal solid waste (MSW) generation. This trend underscores the critical need for sustainable, environmentally friendly, cost-effective, and highly efficient waste management solutions. This study highlights the pressing necessity for effective MSW management and examines plasma pyrolysis/gasification as an emerging technology to address this challenge. The article provides a detailed analysis of thermal plasma generation techniques employing diverse power sources, including direct current, alternating current, radiofrequency inductively coupled, and microwave-based systems. A comparative evaluation of various plasma torch designs is conducted, emphasizing their applicability in waste-to-energy and waste treatment processes. A comprehensive overview of the treatment of a broad spectrum of waste materials, such as MSW, sewage sludge, coal, wood, plastics, tyres, and rubber, using thermal arc plasma technology is presented. The process predominantly converts waste into a combustible gas (syngas) with a calorific value ranging from 5 to 15 MJ/Nm3 and produces vitrified slag or ash as a by-product. The findings suggest that thermal plasma pyrolysis/gasification offers a promising approach to waste management, facilitating energy generation and material recovery while addressing the challenges of increasing MSW generation.
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All authors Tejashwi Rana, Aishik Basu Mallick, Radhika T P, Suryasunil Rath, Pratyay Chattopadhyay, Satyananda Kar

Plasma Physics

Development and Characterization of a Microwave Atmospheric Pressure Plasma Jet

Suryasunil Rath and Satyananda Kar

Over the past decade, atmospheric pressure discharges in the microwave frequency range have gained significant attention due to their promising applications in material processing, CO2 dissociation, waste management, hydrogen production, water treatment, and more. This study presents the development and characterization of a waveguide-based microwave atmospheric pressure plasma jet (MW-APPJs), focusing on its design, diagnostics, and operational parameters. The setup incorporates a microwave power source, microwave waveguide networks, including the applicator section, and diagnostic tools for measuring plasma properties. Optical emission spectroscopy (OES) is employed to analyze the reactive species and determine plasma parameters which include electron excitation temperature (Texc) and electron number density (ne). The characterization highlights the influence of spatial and temporal gradients, gas flow rates, and power input on plasma behaviour. From OES, the Texc and ne variations were against the power increment. The thermocouple variations are also plotted with power.
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Monday 10 August

Plasma Physics

Steady state, core, operational optimization of an ARC-like tokamak via plasma composition and shape

A. Saltzman, P. Rodriguez-Fernandez, A. Ho et al. (+8 more)

Impurity composition, plasma shape, and pedestal density all provide strong levers on fusion power. Here, we explore the ways in which their variation changes fusion power and seek to find the optimum of these parameters. The key impacts of these variables are through changes in the core turbulent transport, the density of the fuel species, the pedestal pressure, and the plasma volume. ITG stabilization due to increased amounts of impurities is observed. The dependence of all of these parameters on the pedestal pressure is especially complicated because of the separate impacts on the peeling and ballooning modes, which can each limit the pedestal. Optimization of this multidimensional operating space is enabled by the use of Bayesian optimization, resulting in an operating point similar to ARC V3A with ~30% more fusion power and a higher fusion power density. Increased shaping parameters, including elongation, triangularity, and squareness are all beneficial, as is high Zeff. When elongation is also allowed to vary, a ~65% increase in fusion power can be achieved. While not commonly considered, we find squareness is an important lever on fusion power. The plasma performance is limited by the Greenwald density limit constraint. This workflow developed here and demonstrated with the example of ARC V3A can readily be applied to other tokamak designs.
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All authors A. Saltzman, P. Rodriguez-Fernandez, A. Ho, G. Snoep, J. Han, J. Hall, M.S. Anastopoulos Tzanis, J. Hillesheim, A.J. Creely, P. Snyder, N.T. Howard

Sunday 9 August

Plasma Physics

Energy-optimized scaling laws for self-guided laser wakefield accelerators

Petr Valenta, Marcel Lamač, Kyle G. Miller et al. (+4 more)

Laser wakefield acceleration promises compact electron accelerators for applications in medicine, industry, and fundamental science. Yet, despite rapid progress, accurately predicting the electron energy attainable in a given experimental configuration and the acceleration length required to reach it remains an open challenge. Here we use Bayesian optimization combined with advanced particle-in-cell simulation techniques to determine the maximum electron energy that a self-guided laser wakefield accelerator driven by a laser of a given energy and wavelength can produce. By systematically optimizing the accelerator performance across a range of laser energies and wavelengths, we derive energy-optimized scaling laws. These scaling laws yield the highest electron energy over the shortest acceleration length possible, are expressed solely in terms of laser energy and wavelength, and are accompanied by the complete set of laser and plasma parameters required to enable the scaling. The resulting scaling laws provide practical guidance for designing state-of-the-art laser wakefield acceleration experiments operating at their fundamental performance limits.
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All authors Petr Valenta, Marcel Lamač, Kyle G. Miller, Brandon K. Russell, Gabriele M. Grittani, Alec G. R. Thomas, Sergei V. Bulanov

Plasma Physics

Transitional evolution of the wave dispersion relation during helicon discharge ignition: an analytical theory from vacuum to steady operation

Lei Chang

The dispersion relations of the helicon and Trivelpiece–Gould (TG) waves in a steady, fully developed magnetised plasma column are well established, but the discharge does not begin in that state. During ignition the electron density rises through five to seven orders of magnitude and the electron–neutral collision frequency falls by two to three, so the dispersion relation is itself a time-dependent object whose very existence must be justified. This paper develops a closed analytical theory of that transition for a cylindrical column in a uniform axial field, using a cold, collisional, two-species description that remains regular in the vacuum limit. Three structural results emerge. First, the vacuum limit is a degenerate double root of the quartic governing the perpendicular wavenumber; the helicon and TG branches are not independent modes that appear at some threshold but the two lobes of a single vacuum root, split by the gyrotropic and anisotropic parts of the dielectric tensor. The leading-order contribution to the discriminant cancels identically, so the branches separate linearly in density rather than with the square-root behaviour generic to a perturbed repeated root; the separation is correspondingly suppressed, and the helicon root remains pinned at its vacuum value until the electron density reaches about 10$^{15}$ m$^{-3}$ while the TG root departs immediately.
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Plasma Physics · Cross-listed from physics.acc-ph

Terahertz-based longitudinal phase space diagnostics of laser wakefield accelerated electron beams

Zhihao Chen, Yu Fang, Ran Li et al. (+5 more)

Femtosecond relativistic electron beams are key probes of ultrafast dynamics, and their pulse duration directly limits the achievable temporal resolution. Laser wakefield acceleration (LWFA) provides a compact source of such beams, but the injection-induced energy spread makes bunch compression sensitive to nonlinear longitudinal transport, motivating direct longitudinal phase space (LPS) measurements. Here, a terahertz transverse-deflecting cavity (THz-TDC) combined with a dipole magnet is used to reconstruct the nonlinear LPS of LWFA electron bunches compressed in a double-bend achromat (DBA), resolving a characteristic C-shaped distribution associated with higher-order longitudinal transport. At an average energy of approximately 4.55 MeV, the diagnostic achieves a temporal resolving power of 1.8 fs and an energy resolution of 6.0 keV, corresponding to a relative energy resolution of 0.13%. For comparable energy spreads of approximately 2.9%, shifting the transmitted energy-window center from 4.574 MeV to 4.532 MeV moves the selected beam away from a low-slope region of the nonlinear LPS and increases the root-mean-square bunch length from 26 fs to 42 fs; with the window center held near 4.553 MeV, increasing the energy spread from 2.0% to 4.4% lengthens the bunch from 27 fs to 44 fs. These results show that the final bunch duration is governed by both the position and width of the transmitted energy window within the nonlinear LPS, establishing an LPS-guided strategy for optimizing DBA-compressed LWFA electron bunches and providing a basis for future higher-order phase-space correction.
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All authors Zhihao Chen, Yu Fang, Ran Li, Xiancong Yang, Tianliang Zhang, Jianfei Hua, Fei Li, Wei Lu

Plasma Physics · Cross-listed from astro-ph.HE

Plasmoid-Mediated 2D Magnetic Reconnection in Partially Ionized Plasmas

Yue Hu, Siyao Xu, Grzegorz Kowal et al. (+3 more)

Magnetic reconnection in partially ionized plasmas is an important channel for energy release. While the plasmoid instability is well characterized in 2D fully ionized plasmas, its behavior in the presence of neutral-dominated plasma remains poorly understood in the nonlinear, high-Lundquist-number ($S = 10^5$) regime. We present high-resolution ($16384 \times 4096$ cells) two-dimensional two-fluid (ion $+$ neutral) simulations of Harris-sheet reconnection with upstream plasma beta $\beta = 2$, comparing fully ionized and partially ionized (ionization fraction $\xi = 10^{-1}$ and $10^{-2}$) regimes. Neutral-ion decoupling accelerates the linear tearing stage and alters the plasmoid hierarchy: the large-scale “monster” plasmoid that dominates the fully ionized case is suppressed, and the sheet instead fragments into a dense chain of sub-scale plasmoids. Below the neutral-ion decoupling scale $\ell_{\rm dec}$, ions concentrate into the plasmoids, reaching peak overdensities $\rho_i/\rho_{i,0} \approx 10$ ($\xi = 10^{-1}$) and $3-5\times10^{3}$ ($\xi = 10^{-2}$), while the neutrals remain comparatively smooth. This local pile-up raises the ionization fraction and recouples the two fluids within the plasmoids. Measured from the out-of-plane electric field at the reconnection sites, the reconnection rate in the $\xi = 10^{-2}$ case achieves $R_{\rm rec}\approx0.01$, whereas the $\xi = 10^{-1}$ case rises to a rate $\approx0.02$ and further $0.035$ when apparent coalescence occurs. In the $\xi = 10^{-2}$ case, the ambipolar drift drives a rapid ion inflow $\sim0.5\,v_{A,0}$ into the layer at the same reconnection sites, far above the neutral inflow velocity $\sim0.1\,v_{A,0}$. Here, $v_{A,0}$ is the upstream total Alfvén speed.
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All authors Yue Hu, Siyao Xu, Grzegorz Kowal, James M. Stone, Alex Lazarian, Hui Li

Saturday 8 August

Plasma Physics

Dipole-Field Magnetic Windows for Radio-Frequency Transmission Through Hypersonic Plasma Sheaths: A Reduced-Order Scaling Model

Richard Lieu

Hypersonic vehicles and atmospheric-entry bodies can experience radio-frequency communication blackout when shock-heated gas surrounding the vehicle becomes sufficiently ionized that plasma cutoff and collisional attenuation restrict electromagnetic transmission. Magnetic-window approaches attempt to reduce this loss by exploiting the anisotropic dispersion of a magnetized plasma, in which selected right-hand or whistler-like modes may propagate along preferred directions. This paper develops a reduced-order scaling model for magnetic-window transmission through a finite-thickness hypersonic plasma sheath when the magnetic source is represented as an onboard axial dipole. The model gives a closed-form estimate of the collisionless angular aperture, compares the underlying projected-cyclotron criterion with full cold-plasma dispersion roots, and extends the aperture estimate to a loss-limited cone using a collisional optical-depth approximation. A simplified neutral-density, speed, and ionization closure is used only to generate qualitative parametric maps and sensitivity trends. The results clarify how dipole-field decay, sheath thickness, radio frequency, vehicle scale, electron density, and collisions jointly constrain the candidate transmission window. The contribution is intended as a screening framework for selecting cases for full-wave electromagnetic simulation, nonequilibrium aerothermochemistry, antenna-coupling analysis, and laboratory validation, rather than as a demonstrated engineering solution to plasma blackout.
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Plasma Physics

Beam experiments for reactive ion etching of silicon (Si)-based materials by silicon halide ions

Kazuhiro Karahashi, Tomoko Ito and Satoshi Hamaguchi

Etching yields of Si, SiO2, and Si3N4 have been determined for silicon ion (Si+), halogen ions (F+, Cl+, and Br+) and silicon halide ions (SiF+, SiF3+, SiCl+, SiCl3+, SiBr+, and SiBr3+) irradiation in 300 to 1000 eV using a mass-selected ion beam apparatus that can irradiate a single species ion to sample surfaces under an ultra-high vacuum condition. Si+ irradiation below 1000eV deposits silicon atoms on Si, SiO2, and Si3N4 surfaces. The etching yields of silicon tri-halide ions (SiF3+, SiCl3+, and SiBr3+) above 1000 eV are larger than those of halogen ions, respectively, and these etching yields depend on the incident ion energy and the etching material (especially Si3N4). At low incident energy, silicon mono-halide ions (SiF+, SiCl+, and SiBr+) deposit silicon on substrates, and the etching threshold energy depends on the halogen species. This information contributes to a database of experimental values needed to increase the precision of an etching process and a profile simulator.
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Plasma Physics

Simulations of self-accelerating electron phase space holes in an applied electric field

Ran Guo

The self-acceleration of electron phase space holes in an applied electric field is investigated via one-dimensional electrostatic Vlasov simulations. The electron holes (EHs) are initialized in a self-consistent manner with immobile ions, and the ion response is enabled at the beginning of simulations. A benchmark simulation is conducted to confirm the EH self-acceleration in the absence of the external electric field. Then, we investigate the EH behaviors by applying the uniform and sinusoidal electric fields, respectively. The effects of different strengths and durations of these external electric fields are studied. It is found that the uniform electric field applied in the direction of the self-acceleration can delay the onset of this process and change the final speed of EHs. The applied sinusoidal electric field can fix the EHs at their initial positions and suppress the self-acceleration if the electric field amplitude and duration are appropriate. In addition, it is observed that these external electric fields can induce the splitting of EHs and the generation of secondary EHs. The physical mechanisms of these phenomena are discussed in detail.
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Friday 7 August

Plasma Physics

PASCHEN-1D: A one-dimensional fluid plasma solver with multi-mechanism surface emission and flexible external circuit coupling

Asif Iqbal, Yves Heri, Bingqing Wang et al. (+3 more)

We present PASCHEN-1D (Plasma Advanced Solver with Coupled High-fidelity Emission and external Network), a one-dimensional time-dependent fluid plasma solver developed for self-consistent simulation of gas discharges and plasma breakdown with coupled electrode surface emission and flexible external circuit networks. The code solves drift-diffusion continuity equations for electrons and ions together with Poisson's equation. It is dynamically coupled to lumped RLC circuits, which self-consistently treat plasma transport, plasma-surface interaction, dielectric effects, and circuit response within a single framework. The electrode emission module includes ion-induced secondary electron emission, Fowler-Nordheim and Murphy-Good field emission, Richardson-Dushman thermionic emission, and photoemission based on a general, exact quantum mechanical emission theory. A finite-volume formulation with Kurganov-Tadmor fluxes, explicit diffusion, and fourth-order Runge-Kutta time integration is employed to ensure stable transient (sometimes ultrafast) evolution across breakdown and glow regimes. The solver is validated against multiple benchmark cases, including nanosecond pulsed dielectric-barrier discharges, DC breakdown and glow transitions, and Paschen curve construction for argon and nitrogen, with results consistent with published studies. With high-fidelity emission physics and a flexible circuit-coupling framework, PASCHEN-1D provides a versatile and efficient tool for modeling breakdown and transient discharge phenomena.
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All authors Asif Iqbal, Yves Heri, Bingqing Wang, Lan Jin, Md Arifuzzaman Faisal, Peng Zhang

Plasma Physics

Characterization of a prototype parallel-plate $^{238}$U fission chamber with DD and DT fusion neutron sources

V. Hagenlocker, R.A. Tinguely, X. Wang et al. (+5 more)

The SPARC tokamak will employ $^{238}$U-based fission chambers (FCs) to monitor high-performance deuterium-tritium (DT) plasma operations, spanning neutron yield rates from ${\sim}10^{15}$ to ${>}10^{19}$ n/s. This work validates the $^{238}$U FC design, which utilizes a parallel-plate detector geometry and borated polyethylene collimation to prioritize unscattered DD and DT fusion neutrons. Experimental testing with both DD and DT neutron generators corroborates vendor-specified efficiencies and demonstrates excellent detector linearity, with measured count rates showing good agreement with OpenMC neutronics simulations. Further characterization confirms the $^{238}$U FC's robustness against SPARC-relevant environmental challenges, including stray magnetic fields up to 14 mT and possible signal degradation risks associated with $\sim$30 m long cable runs. These results confirm that the $^{238}$U FC, supported by indirect neutron shielding as well as appropriate pulse height thresholds, provides a reliable solution for fusion power measurements as part of the SPARC neutron diagnostics suite.
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All authors V. Hagenlocker, R.A. Tinguely, X. Wang, J.L. Ball, B. Buschmann, M. Gatu-Johnson, R. Gocht, P. Raj

Plasma Physics

Electron-Scale-Driven Turbulence by Negative-Density-Gradient in NSTX

C. Clauser, J. Candy, J. Parisi et al. (+7 more)

Gyrokinetic simulations of an NSTX spherical-tokamak plasma reveal novel negative-density-gradient (NDG) drift waves at electron-gyroradius-scale that drive turbulent transport comparable to the experimental power flow. Linear simulations indicate that the dominant instability is a trapped-electron electron-scale tearing-parity mode driven mainly by negative electron density gradient, $a/L_{ne} \equiv - a/n_e \, (dn_e/dr) < 0$. Although thermal transport is mainly carried by transverse magnetic, $\delta A_{\parallel}$, fluctuations, the growth rate is sensitive to compressional magnetic fluctuations, $\delta B_{\parallel}$, differentiating these from standard microtearing modes (MTMs). Linear sensitivity analysis also reveals that the modes are most unstable at lower collisionality, consistent with the trapped-electron character. Electron-scale and multi-scale nonlinear simulations show that these modes can account for several megawatts of experimental power, with the latter suggesting that experimental gradients may lie at a bifurcation between distinct turbulence regimes.
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All authors C. Clauser, J. Candy, J. Parisi, T. Rafiq, W. Guttenfelder, G. Avdeeva, E. Belli, J. Berkery, I. Sfiligoi, E. Schuster

Plasma Physics

Linear and nonlinear benchmark of gyrokinetic simulation of energetic particle driven toroidal Alfven eigenmodes in ITPA TAE benchmark case

Youjun Hu, Yang Chen, Lei Ye et al. (+2 more)

A new gyrokinetic code, TEK, was benchmarked in simulating energetic particle (EP) driven toroidal Alfven eigenmodes (TAEs) in the simple tokamak configuration chosen by the ITPA-EP group for code benchmarking purpose. Linear benchmark has been well established by other codes, whereas nonlinear benchmark for this case is lacking. This paper presents, besides the linear benchmark, nonlinear results for both single-n and multiple-n simulations (n is the toroidal mode number). The nonlinear results are in good agreement with an analytical theory on zonal field beat-driven by Alfven eigenmodes, partially verifying correctness of the nonlinear simulations. The saturation level and the resulting EP transport are examined. This provides data for future inter-code nonlinear benchmarking. In TEK, all species (electrons, thermal ions, EPs) are treated on the same footing using the gyrokinetic model (with electrons in the zero Larmor radius limit). The electromagnetic cancellation problem is mitigated by using the mixed-variable pullback method. Numerical details related to electromagnetic gyrokinetic simulation are discussed.
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All authors Youjun Hu, Yang Chen, Lei Ye, Zhiyong Qiu, Youwen Sun

Thursday 6 August

Plasma Physics

The Equations of Reduced Magnetohydrodynamics in Dipole Coordinates

Ian M. DesJardin, Christopher Bard, Natalia Y. Buzulukova et al. (+1 more)

The equations of reduced magnetohydrodynamics (RMHD) isolate the Alfvénic energy transfer and turbulence dynamics from MHD in a computationally tractable way. In this study, we derive the equations of RMHD for a low $\beta$ plasma in a dipole coordinate system aligned with the background magnetic field using a multiscale analysis. The Kreiss theorem is used to derive the equilibrium conditions that the background conditions must satisfy on the time scale of MHD turbulence. From these, we find a connection between plasma flow along flux tubes and changes in the Alfvén speed that may drive nonlinear wave effects. The final equations demonstrate an intimate coupling between field aligned currents and plasma vorticity including the effects of nonuniform flux tube area and realistic plasma density profiles. This work has immediate application to the magnetosphere-ionosphere coupling problem in the Earth's magnetosphere as it provides a way to link dynamically evolving field aligned currents from the magnetosphere, especially important during geomagnetic storms and substorms, with the development of magnetohydrodynamic turbulence at low altitudes in a self-consistent manner. This also clarifies the role of Alfvén waves as a transfer mechanism of energy under these inhomogeneous circumstances while remaining simple enough to make predictions. Furthermore, this study makes use of a novel methodology, a computer algebra system, in performing the brunt of algebraic work under complicated coordinate systems. We hope this approach serves as a template for performing reproducible and verifiable multiscale perturbation analysis under arbitrarily complex geometries.
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All authors Ian M. DesJardin, Christopher Bard, Natalia Y. Buzulukova, John C. Dorelli

Plasma Physics · Cross-listed from astro-ph.SR

Multi-wavelength synthesis of a flux rope-trapped mini-prominence eruption and post-flare coronal rain

Samrat Sen, Alexander G. M. Pietrow, Veronika Jercic et al. (+1 more)

Small-scale eruptive phenomena in the solar corona including miniature flux ropes and associated cool plasma condensations are not fully understood despite increasing high-resolution observations. We perform forward modeling based on a 2.5D MHD simulation capturing homologous flux rope eruptions, in-situ condensation leading to a mini-prominence, and subsequent post-flare coronal rain. Synthetic diagnostics are obtained using optically-thin EUV and UV emissions, and non-LTE radiative transfer treatment for the H$\alpha$ line. The synthetic EUV emission reveals the flux ropes as bright rim-like structures. The corresponding UV diagnostic shows bright region, which is co-spatial with the dark core due to embedded cool plasma ($\sim$ tens of kK) inside the flux rope, identifying an erupting mini-prominence. Spectral synthesis of Si IV 1402.77 A indicates an upward motion of the mini-filament, and reveals the presence of two predominant velocity components during eruption. At a later stage, thermal instability in post-flare arcades produces coronal rain with temperatures of $\approx 10^4$ K. The EUV diagnostics reveal brightening at the downstream of the rain blob, indicating localized heating associated with compressional effects. The H$\alpha$ spectral synthesis shows enhanced absorption signatures and red-shifted profiles corresponding to downflows of the coronal rain blobs up to $\approx 23$ km s$^{-1}$, whereas the Si IV 1402.77 A spectral profile shows the maximum downflow velocity of $\approx50$ km s$^{-1}$, highlighting the evidence of thermodynamic and kinematic structuring within the falling rain blobs. The synthetic diagnostics provide clear, multi-wavelength signatures that can guide future high-resolution observations, and highlight the importance of small-scale reconnection-driven processes in shaping the multi-thermal structure (between MK to kK) of the solar corona.
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All authors Samrat Sen, Alexander G. M. Pietrow, Veronika Jercic, Patrick Antolin

Plasma Physics

Polarization-Conditioned Fourier-enhanced DeepONet for Electric Field Reconstruction from EFISH Measurements

Zhijian Yang, Edwin Setiadi Sugeng, Yaqi Zhang et al. (+2 more)

Electric-field-induced second-harmonic generation (EFISH) is an established laser diagnostic for quantifying electric fields in plasmas, yet ensuring field accuracy remains challenging given the coherent, path-integrated nature of the signal. We address this via machine learning, developing a Polarization-Conditioned Fourier-enhanced Deep Operator Network (PC-FDON) – a unified operator-learning model that reconstructs field profiles from EFISH measurements across both polarizations and various optical parameters. Its architecture incorporates three advances: (i) a Fourier-enhanced branch providing inductive bias for the Gouy phase shift and wave-vector mismatch; (ii) a polarization-conditioning branch encoding signal polarization via Feature-wise Linear Modulation (FiLM) and gated units, enabling one model to handle both polarizations; and (iii) a physics-informed loss enforcing self-consistency with the governing EFISH equation. Trained on data spanning multiple function families, polarization states, and phase-mismatch values, PC-FDON achieves promising reconstruction under noise-free, incomplete, and noisy inputs, with generalizability comparable to our previous polarization-specific model. Pointwise epistemic uncertainty estimates via Monte Carlo dropout reflect model confidence and enable out-of-distribution (OOD) detection through a location-dependent exceedance fraction metric. Validation is performed on realistic electrode configurations under both polarizations and varying Rayleigh ranges, including a simulated surface dielectric barrier discharge where the framework correctly flags OOD inputs, and experimental data showing good agreement with simulations. The architecture – spectral inductive bias, conditional modulation, and dataset-specific uncertainty – shows strong potential for broader application beyond plasma diagnostics.
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All authors Zhijian Yang, Edwin Setiadi Sugeng, Yaqi Zhang, Anbang Sun, Tat Loon Chng

Plasma Physics · Cross-listed from hep-ph

Net electron spin rotation in a plane-wave pulse: Holonomy set by the anomalous magnetic moment

N. S. Akintsov, A. P. Nevecheria, S. N. Andreev et al. (+1 more)

We compute the spin rotation that survives after a relativistic electron has crossed a plane-wave laser pulse of finite duration. In the interaction picture built on the exact $g=2$ evolution, the Thomas-Bargmann-Michel-Telegdi equation becomes parallel transport by a connection with constant coefficients on the polarization plane, and the pulse enters only through the closed curve that the transverse vector potential traces there. The net rotation is the holonomy of that connection: an angle $-\frac{1}{2}a_e^2\mathcal{A}$ about the propagation direction, with $a_e=(g-2)/2$ the anomaly and $\mathcal{A}$ twice the signed area enclosed by the curve. That area is the spin angular momentum the pulse carries per unit area, so the rotation measures the helicity of the light. Reduction of the residual dynamics to a rotation coupled through the anomaly alone is an exact result of the 1960s [Ternov, Bagrov, and Klimenko, Sov. Phys. J. 11, 29 (1968); Bagrov and Gitman, The Dirac Equation and its Solutions (De Gruyter, Berlin, 2014), Sec. 5.3], which yields two closed-form cases; the area law is the general second order that those two cases bound. The same area governs the orientation memory of a neutral magnetic dipole [Oblak and Seraj, Phys. Rev. D 109, 044037 (2024)] with a coupling of order unity. For a charged electron on a Volkov orbit the coupling $g/2$ cancels identically, which suppresses the rotation by $1.3\times10^{-6}$ and leaves a channel with no $g$-independent part. We verify the cancellation at $g=2$ over 180 pulse configurations and the area law over 89 more. Finite focusing restores that part at second order in $1/kw_0$, and it exceeds the anomalous signal unless $w_0\gtrsim16\lambda$ at $\gamma=10$, or $270\lambda$ at $\gamma=1$.
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All authors N. S. Akintsov, A. P. Nevecheria, S. N. Andreev, Qing-Hua Qin

Plasma Physics

SafeDivertor: Faithful Divertor Heat Flux Reconstruction from Macroscopic Plasma State Signals via Time-Frequency Prior Exploitation

Hao Si, Zehua Chen, Qingquan Yang et al. (+8 more)

Divertor heat-flux analysis is essential for understanding plasma-wall interactions and protecting plasma-facing components in magnetic-confinement fusion devices, while conventional infrared-based inversion is usually performed after discharge and requires heat-conduction modeling with device-specific material properties, divertor geometry, and boundary conditions. Rather than accelerating this conventional infrared-based inversion paradigm, we introduce a new online-oriented signal-based reconstruction paradigm that directly reconstructs time-resolved radial heat-flux profiles from multi-source macroscopic plasma-state signals available during discharge. To enable systematic study of this task, we construct DivMPS2HF, a multi-source discharge dataset that provides the data foundation and benchmark for signal-based divertor heat-flux reconstruction. We further propose SafeDivertor, a task-driven framework designed to address the key challenges of signal-based heat-flux reconstruction. It employs physical prior-aware initialization to provide radial-distribution guidance for target channels, input perturbation to reduce over-reliance on specific heterogeneous signals, spectral-aware reconstruction optimization to exploit time-frequency priors and preserve transient dynamics, and progressive training to stabilize the optimization of these complementary objectives. Experiments on DivMPS2HF demonstrate that SafeDivertor achieves the best overall performance among the evaluated time-series baselines across all five metrics, establishing a new performance benchmark for signal-based divertor heat-flux reconstruction. The source code will be released on https://github.com/Event-AHU/OpenFusion
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All authors Hao Si, Zehua Chen, Qingquan Yang, Xiao Wang, Dengdi Sun, Wanli Lyu, Gaoting Chen, Guosheng Xu, Hang Su, Jin Tang, Jun Zhu