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Physics Physics Plasma Physics

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Friday 18 September

Plasma Physics

Mechanism of Ionization Avalanche in Tokamak Microwave Gas Breakdown

Jinwoo Gwak, Yeongsun Lee, Jeongwon Lee et al. (+5 more)

Microwave breakdown driven by electron cyclotron (EC) waves provides a non-inductive route to plasma initiation in reactor-scale tokamaks. We introduce a three-dimensional Monte Carlo simulation that, for the first time, self-consistently treats nonlinear wave-particle interactions, atomic collisions, and guiding-center transport. The Monte Carlo simulation unveils the key role of parallel Brownian motion in the ionization avalanche mechanism. The predicted breakdown boundary is validated against KSTAR experiments. This work concludes that microwave gas breakdown will be successful under ITER-relevant conditions at a D$_2$ prefill pressure near 2 mPa with 1 MW of injected EC power.
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All authors Jinwoo Gwak, Yeongsun Lee, Jeongwon Lee, Won Ik Jeong, Hyun-Tae Kim, Yong-Seok Hwang, Min-Gu Yoo, Yong-Su Na

Thursday 17 September

Plasma Physics

Inferring Thermal Dissociation Kinetics of Ion Clusters from Molecular Dynamics

Nicholas Laws, Adler Smith and Elaine Petro

Electrospray ionic liquid ion sources operating in the pure ion regime emit metastable, low solvation number ion clusters whose post-emission dissociation modifies plume composition, energy distributions, and diagnostic observables. While field-free measurements support first-order, thermally activated decay on microsecond timescales, the acceleration region is characterized by rapidly varying electric fields and nanosecond residence times such that dissociation becomes strongly field-enhanced. In this work, microcanonical ensemble molecular dynamics (MD) trajectories are used to infer dissociation kinetics of EMI-BF4 ion clusters under controlled internal energy and applied electric field. Positive and negative dimers, trimers, and tetramers are simulated at temperatures between 600 to 1000 K and uniform fields of 10^6 to 10^9 V/m. Dissociation lifetimes are extracted using a connectivity-based fragmentation criterion, and product channels are classified to obtain pathway-resolved branching probabilities. Across all solvation numbers, lifetimes are field independent in the weak field limit, but collapse by orders of magnitude above 10^8 V/m. Cluster polarity dependence is most prominent in the temperature-controlled regime and diminish as electrostatic work dominates. The MD simulations further reveal that field-driven changes in dissociation topology, including a transition in trimer breakup from neutral pair evaporation to charged core ion ejection, and three competing tetramer pathways involving single neutral loss, double neutral emission, and core ion ejection. The MD dataset is reduced to compact parameterizations of {\tau_n}(E,T), yielding transferable kinetic inputs for multiscale plume transport models.
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Plasma Physics · Cross-listed from astro-ph.HE

Energy Partition in Relativistic Electron-Positron-Ion Reconnection

Nilay Mancini, Luca Comisso and Lorenzo Sironi

Relativistic magnetic reconnection, a prime mechanism for particle acceleration, has been extensively studied in electron–ion and pair plasmas, but how the dissipated energy is shared among species in electron–positron–ion plasmas, and in particular how ions are energized within a pair-dominated layer, remains poorly understood. We develop an analytic theory that couples single-particle orbit dynamics to the collective plasma response, and we validate it with particle-in-cell simulations spanning pair-dominated to electron–ion compositions. Particles are directly energized in electric-dominated regions until the reconnected magnetic field deflects them into the outflow; the characteristic escape length is in turn set by the relativistic skin depth of the reconnection-heated mixture, including the ion contribution. Writing $\sigma_e^*$ for the magnetic energy available per particle in units of $m_ec^2$ and $\mu=m_i/m_e$, the mean lepton energy scales as $\sigma_e^*$, whereas the mean ion energy scales as $\mu^{1/3}(\sigma_e^*)^{2/3}$ when ions escape non-relativistically from the electric-dominated regions and approaches the lepton energy when their escape is ultrarelativistic. The predicted energy fractions agree with simulations across composition, mass ratio, and magnetization. The ion share of the dissipated energy can fall far below the commonly assumed ion–lepton equipartition, reducing the energy available for hadronic and neutrino emission in reconnection-powered astrophysical sources.
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Plasma Physics

Experimental setup for testing nanocalorimeter sensors as a plasma diagnostics tool

Carles Corbella, Feng Yi and Andrei Kolmakov

The application of microfabricated nanocalorimeters for real-time, highly sensitive, and selective monitoring of the energy fluxes from low-pressure plasma discharges has been reported recently. Here, we describe the experimental setup for complementary characterization of nanocalorimeters performance using plasma diagnostics techniques. Langmuir probe, retarding field energy analyzer (RFEA), optical emission spectroscopy (OES), quadrupole mass spectrometry (QMS), and thermocouple-based thermometry constitute the plasma monitor suite. We discuss two application examples of the plasma monitor suite: (1) discrimination between electron- and ion-induced heat fluxes via sensor direct current (DC) biasing, and (2) detection of fast temperature transients in pulsed discharges. The main goal of this work is to demonstrate a nanocalorimetry platform as a real-time metrology for plasma processing and process control.
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Plasma Physics

Gradient-Based Construction of Collisionless Steady-State Guiding-Center Distributions in Tokamaks and Stellarators

Jingyi Yu and Chang Liu

We present a matrix-free residual-minimization method for preparing guiding-center distribution functions on finite grids. The method minimizes a fixed conservative discrete residual in the physical Jacobian metric using Polak-Ribiere+ iterations with exact quadratic line searches. JAX/XLA evaluates the operator and its adjoint without assembling a four-dimensional matrix; independent magnetic-moment slices are batched and iterations are compiled into one execution graph. An axisymmetric hierarchy verifies invariant references and tests the workflow using a NUBEAM fast-ion distribution together with an EPCoM constants-of-motion representation constructed from it, before matched precise quasi-axisymmetric (precise-QA), HSX, and W7-X calculations. The three-dimensional endpoints reduce the discrete residual and the finite-time variation of the distribution sampled along independently integrated characteristics. Non-negative projection is treated as an optional constrained extension: it leaves the residual operator unchanged but changes the feasible zero-residual set and endpoint selection. Results are finite-grid, finite-time evidence and do not establish a continuous equilibrium, grid convergence, strict positivity, or a transport prediction.
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Plasma Physics · Cross-listed from physics.optics

Rapidity-Coupled Spin Dynamics in Pulsed Laser Fields from Physics-Informed Neural Networks

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

Carrier-envelope-phase (CEP) stabilized few-cycle pulses make the sub-cycle field structure a control parameter for laser-driven polarized electron sources, yet spin solvers for such pulses are rarely tested against exact results. We show that for an electron initially at rest in a linearly polarized plane-wave pulse the rest-frame polarization angle follows the instantaneous vector potential, $\Sigma = 2\arctan(a_x/2) + a_e a_x$ with $a_e$ the electron anomaly: the net rotation vanishes for every CEP, while the peak intra-pulse angle varies over the CEP by 3.02 degrees for a two-cycle and by 0.22 degrees for an eight-cycle pulse. The result follows from the Volkov orbit in longitudinal and transverse rapidities. We use it to validate a light-front reference integrator and a physics-informed neural network trained only on the light-front equations, which reaches $4\times10^{-5}$ in the spin sector and carries over to elliptical polarization, for which no closed-form solution is known. This gives validated tools for CEP-resolved spin dynamics in intense fields.
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All authors N. S. Akintsov, A. P. Nevecheria, S. N. Andreev, Qing-Hua Qin

Plasma Physics

Indirect-Drive Fusion Target Design for Commercial Fusion Energy

C. R. Weber, A. L. Kritcher, S. Bhandarkar et al. (+26 more)

This paper presents the physics basis for commercially relevant laser indirect-drive (LID) (radiation-driven) inertial fusion energy (IFE) using a 10 MJ laser driver. To date, this approach, proven at the National Ignition Facility (NIF), remains the first and only controlled fusion method to demonstrate the key physics required for fusion energy production, including a self-sustained burning plasma, substantially de-risking the path to commercial fusion energy. Building directly on these results, we present scaled designs to larger target sizes and fusion gains relevant for commercial power generation ($G\sim 26$–$43$). The designs remain close to experimentally demonstrated ignition physics, modifying target components to improve scalability, manufacturability, and cost-effectiveness for fusion energy applications while preserving ignition-relevant implosion physics and fusion power plant compatibility. The baseline platform retains a high-density carbon ablator and clean cryogenic DT fuel layering while extending ignition platforms to substantially larger fuel masses (exceeding 10 times that of current ignition experiments at the NIF) and higher burn fractions ($\sim$40%) with total areal densities at stagnation of $\sim$3~g/cm$^2$. Benchmarked simulations anchored to NIF ignition experiments (using HYDRA and LASNEX) predict that these designs achieve robust ignition and propagating burn at substantially higher fusion yields (265–427~MJ) with significant ignition margin (2–4$\times$ relative to NIF) against hydrodynamic instabilities and representative power-plant non-idealities, including low-mode asymmetry, polycrystalline DT ice roughness, HDC ablator voids, and target-support and fill-hole perturbations. We also show that implosion symmetry and laser-plasma interactions (LPI) can be controlled with our novel multi-beam configuration using thousands of laser beam-lines.
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All authors C. R. Weber, A. L. Kritcher, S. Bhandarkar, T. Briggs, T. Chapman, T. M. Fears, B. A. Hammel, D. D.-M. Ho, O. Hurricane, B. Kozioziemski, J. Milovich, A. Nikroo, A. Oudin, W. Riedel, A. Wray, N. Alexander, G. Cearley, M. Dunne, J. Gaffney, D. Hammond, J. Kilkenny, R. Lau, J. Lawson, J. Ludwig, B. MacGowan, R. Peterson, V. Smalyuk, R. Toro, Inertia Collaboration

Plasma Physics

Efficient laser ion acceleration in near-critical density plasmas in the picosecond pulse regime

Joshua Luoma, Andreas Kemp, Andrew Longman et al. (+2 more)

Kilojoule-class short-pulse (< 10 ps) lasers are excellent tools for generating relativistic ion beams, but achieving high laser-to-ion conversion efficiency remains an open challenge. Recent experiments show a significant increase in conversion efficiency when leveraging targets with an average plasma density approaching the relativistic-critical limit. We present an analytical model that explains this increase by extending the theory of target-normal sheath acceleration (TNSA) to include laser penetration of relativistic-critical density targets, enabling energy transfer to the ion front by heating the expanding electron sheath. This physical process increases the sheath field strength and significantly improves ion cutoff energy and laser-to-ion conversion efficiency relative to classical TNSA. The model predicts a maximum ion conversion efficiency of 37% by optimizing laser transmission and target areal density. Key scalings of the model agree with particle-in-cell (PIC) simulations and published experimental data. A series of 2D and 3D simulations of hydrocarbon plasmas confirm the robustness of the acceleration process, demonstrating a path for maximizing ion yields using relativistic-critical targets.
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All authors Joshua Luoma, Andreas Kemp, Andrew Longman, Dean Rusby, Gennady Shvets

Wednesday 16 September

Plasma Physics

Helicon wave propagation, plasma generation and interaction with low-frequency waves in toroidal magnetic configurations

Simon P. H. Vincent, Mounir Alfazzaa, Patrick Quigley et al. (+5 more)

Helicon waves are widely used for efficient plasma production in low-temperature devices and have recently attracted attention as a candidate for current drive in fusion plasmas. Yet experimental investigations of helicon waves in toroidal geometries, and of their interaction with plasma dynamics, remain extremely limited. In this work, we present, to our knowledge, the first detailed experimental characterization of helicon waves in a toroidal configuration. A birdcage resonant antenna operating at 13.56 MHz is used to launch helicon waves in the toroidal basic plasma physics device TORPEX, either into a pre-existing magnetron-generated plasma, or as the plasma source. Measurements are performed in pure toroidal and simple magnetized torus magnetic configurations, for both argon and hydrogen plasmas. Three-axis magnetic probe measurements enable clear identification of a dominant m=+1 helicon mode over our parameter space. The helicon amplitude is found to scale linearly with the antenna power, and decreases with the confining magnetic field amplitude. As the antenna power is increased the helicon amplitude exhibits a saturation, correlated with enhanced low-frequency fluctuations and turbulent transport. In addition, a strong interaction between helicon waves and low-frequency density fluctuations is observed, revealing a non-linear coupling between RF waves and plasma turbulence. These results provide the first detailed experimental characterization of helicon waves in a toroidal low-temperature plasma device and establish TORPEX as a unique testbed for studying toroidal helicon wave physics under controlled and well-diagnosed conditions.
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All authors Simon P. H. Vincent, Mounir Alfazzaa, Patrick Quigley, Cyrille Sepulchre, Philippe Guittienne, Rémy Jacquier, Marcelo Baquero-Ruiz, Ivo Furno

Plasma Physics

Kilojoule-scale laser acceleration enabling efficient generation of electron-positron and muon beams

R. Babjak, M. Pouyez, C. Badiali et al. (+2 more)

Despite many experimental attempts, laboratory pair-beam sources have not yet reached the charge, collimation, and density needed to observe kinetic pair-plasma instabilities. Here we show that an optimised direct-laser-acceleration (DLA) regime in a gas target can close this gap in the regime of kilojoule-class petawatt lasers. Quasi-3D PIC and Geant4 simulations show that DLA-accelerated electron bunches carry tens of nC at GeV-scale energies with low divergence. Converted in a high-Z target, they generate up to 3x10^12 positrons with tunable energy spectra and transverse sizes of several skin depths in the beam rest frame. A proof-of-principle PIC simulation of the pair beam propagating through background gas shows the growth of a current-filamentation instability. This provides direct numerical evidence that kinetic pair dynamics are within reach of near-term facilities such as ELI-L4. The same electron driver also generates 6.5x10^5 Bethe-Heitler muons per shot, at up to 580 muons/J. We derive a scaling law for the muon yield and validate it against Geant4 simulation. Above a modest driver-energy threshold, the yield depends on the total energy delivered to the electron beam rather than on its peak energy - a design principle for future laser-based muon sources. These results establish DLA-driven secondary sources as a practical, near-term pathway to laboratory pair plasmas, high-yield muon beams, and lepton-accelerator injectors.
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All authors R. Babjak, M. Pouyez, C. Badiali, T. Grismayer, M. Vranic

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

Heavy ion driven plasma wakefield acceleration with drift-like phase-shift acceleration scheme

Li Jiangdong, Xia Guoxing, Liu Jie et al. (+2 more)

Traditional plasma density modulation schemes can extend the dephasing length but suffer from wakefield degradation as plasma density increases. To address this limitation, a drift-like phase-shift acceleration scheme is proposed, introducing drift sections between plasma cavities to enable controlled phase shifting and avoid dephasing. Idealized simulation results show that electrons are accelerated from 16 MeV to 700 MeV within 1.65 m with an energy spread of 5%. Furthermore, segmented simulations introducing realistic active plasma lenses verify the practical feasibility of this scheme, electrons are accelerated up to 720.5 MeV with a reduced energy spread of 3.3% within 1.67 m. The plasma wakefield maintains a robust amplitude throughout the propagation, suggesting the potential for continued acceleration beyond the simulated region. This scheme demonstrates a promising way to enable the witness beam to gain energy continuously from the tail to the head of the driver beam and provides a new approach for generating high-energy, high-quality beams in heavy ion driven plasma wakefield acceleration.
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All authors Li Jiangdong, Xia Guoxing, Liu Jie, Li Guangxian, Yang Jiancheng

Plasma Physics

X-Ray Spectroscopy for Characterizing Stimulated Raman Scattering in Inertial Confinement Fusion

Tianluo Luo, Zeyang Li, Yunping Wang et al. (+4 more)

Accurate characterization of stimulated Raman scattering (SRS) remains a critical challenge in inertial confinement fusion (ICF), as SRS not only scatters laser energy but also generates suprathermal electrons that preheat the fuel and degrade implosion performance. Conventional backscatter diagnostics provide direct measurements of SRS but cannot collect the entire scattered-light signal, limiting the accurate characterization of SRS strength. Using a non-local thermodynamic equilibrium collisional-radiative model with a double-Maxwellian electron distribution, we systematically investigate how suprathermal electrons modify the titanium K-shell x-ray emission spectra. The spectra exhibit high sensitivity to the suprathermal-electron fraction at relatively low bulk electron temperatures, making them particularly suitable for diagnosing suprathermal electrons during the early stage of ICF, when even a small suprathermal-electron population can compromise fuel compression. The calculated spectra reproduce experimental measurements from the Nova Laser Facility with good agreement, and the inferred suprathermal-electron fractions show improved consistency with independently measured SRS losses compared with the original spectral analysis by Glenzer~\href{https://doi.org/10.1103/PhysRevLett.81.365} {\text{[S. H. Glenzer et al., Phys. Rev. Lett. 81, 365(1998)]}}. These results demonstrate that K-shell spectroscopy, combined with accurate NLTE collisional-radiative modeling, provides a reliable probe of SRS strength in laser-produced plasmas. Hence, this approach may be extended to spatially resolved diagnosis of SRS strength, offering a promising complement to conventional backscatter diagnostics.
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All authors Tianluo Luo, Zeyang Li, Yunping Wang, Zhihao Yang, Zhencen He, Dong Yang, Zhimin Hu

Tuesday 15 September

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

Effects of radial conductivity variation on the Ponomarenko dynamo

Shipra Verma and Kannabiran Seshasayanan

We study the effects of conductivity variation on the Ponomarenko dynamo. Taking monotonically increasing/decreasing and sinusoidally varying radial profiles, we study the kinematic dynamo problem. The threshold of the dynamo, given by the critical magnetic Reynolds number Rm$_c$, is found to strongly depend on the form of conductivity variation near the discontinuity of the velocity field where the shear is dominant. For monotonically varying profiles in the limit of sharp variation, this threshold maps to the problem of the dynamo with a jump in conductivity, while for the sinusoidal profile with large variations/wavenumbers, the threshold increases due to effective diffusivity arising from the rapid changes in conductivity. Far from the threshold, it is found that the growth rate depends only on the local gradient of the conductivity near the discontinuity, with the opposite sign of shear and conductivity gradient leading to a larger growth rate, thereby aiding the dynamo instability in regenerating B$_{\phi}$ from B$_r$. We calculate the expression for the growth rate in the limit of large Rm$(\gg 1)$ and find that the conductivity variation leads to a modification proportional to the local gradient of conductivity times Rm$^{-1/2}$. This correction is found to depend only on the local gradient of the conductivity at the discontinuity. Similar dependencies are found for realistic, smooth velocity profiles and for different magnetic boundary conditions.
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Plasma Physics

Landau Damping Beyond Smooth Velocity Distributions: A Dispersion-Free Lagrangian Time-Domain Framework

Huasheng Xie and Jinsong Zhao

The classic theory of Landau damping requires the velocity distribution function (VDF) to be analytically continued into the complex plane, implicitly requiring analyticity, which imposes constraints far more severe than infinite smoothness. Yet physical plasmas—encountered in discrete simulations, noisy spacecraft measurements, or truncated fusion distributions—are inherently non-analytic. This discrepancy poses a foundational “smoothness paradox”: why does Landau damping robustly persist in systems where the mathematical prerequisite of analyticity is profoundly violated? Here we resolve this paradox by demonstrating that wave-particle interaction is governed by a time-dependent resonance width $\Delta v_\mathrm{res}\sim 1/kt$. Using a dispersion-free Lagrangian time-domain solver, we show that this finite width kinematically coarse-grains microscopic VDF defects at early times, acting as a natural low-pass filter that validates smooth analytical proxies—explaining the observed robustness. However, as $t\to\infty$ the resonance width narrows, inevitably forcing the wave to resolve exact topological non-smoothness. This late-time resolution triggers three distinct breakdowns: VDF truncation arrests the resonant phase transition to yield undamped discrete Van Kampen modes; observational noise induces transient algebraic spikes via linear phase-space aliasing; and step-like gradients drive anomalously violent reactive instability growth. We establish the breakdown timescale $t_b\sim 1/k\delta v$, where $\delta v$ is the characteristic scale of the non-smooth defect, providing a quantitative criterion that redefines the validity boundaries of analytic continuation in kinetic theory.
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Plasma Physics

Pellet-Size Scaling of Quasi-Steady-State Plasma Performance in Wendelstein 7-X

Keisuke Fujii, Edgardo Villalobos Granados, Maryam Huck et al. (+11 more)

A continuous cryogenic-pellet injector has been used to realize long-pulse high-performance plasmas in Wendelstein 7-X (W7-X). A hydrogen ice pellet deposits particles directly in the confined region. If the particles are deposited sufficiently far inside the plasma, they produce a density gradient that suppresses ion-temperature-gradient turbulence and improves plasma confinement. However, after multiple pellets have been injected and the plasma density and temperature have increased, the plasma performance begins to saturate. In this work, we analyze multiple pellet-injection experiments conducted in 2024 and 2025, during which the injected pellet sizes varied unintentionally. This analysis reveals a positive correlation between pellet size and the quasi-steady-state stored energy of W7-X plasmas. Although this pellet-size dependence can be understood qualitatively from pellet-ablation physics, the measured deposition position differs quantitatively from the neutral-gas-shielding (NGS) model prediction. This discrepancy suggests significant inward transport of the pellet cloud. The trend identified here suggests that injection of even larger pellets could further improve plasma performance.
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All authors Keisuke Fujii, Edgardo Villalobos Granados, Maryam Huck, Jürgen Baldzuhn, Naoki Tamura, Steven Meitner, Larry Baylor, Golo Fuchert, Kai Jakob Brunner, Jens Knauer, Ekkehard Pasch, Jannik Wagner, Bart Lomanowski, W7-X Team

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

1D PIC Simulations of Resonant Scattering-Driven Pair Cascades in Magnetar Magnetospheres

Jens F. Mahlmann, Alexander A. Philippov, Adrien Soudais et al. (+2 more)

Hard X-rays in persistent magnetar emission originate from radiative processes in pair-loaded magnetospheric plasma, yet self-consistent kinetic simulations of such radiation-rich systems remain limited. We perform 1D particle-in-cell simulations of pair creation mediated by resonant inverse Compton scattering (RICS) along isolated field lines in twisted magnetospheres, capturing particle acceleration and radiative drag from first principles. The drag force, which strongly depends on the particle Lorentz factor and location, plays a key role in the magnetospheric circuit. Strong RICS drag impedes the flow of electrons and positrons, particularly along extended field lines. Nevertheless, the plasma sustains the circuit by self-organizing into a single accelerating gap near the star in one hemisphere. The accelerated plasma flow from the gap becomes loaded with copious electron-positron pairs in extended RICS zones, and also carries ions extracted from the star and accelerated in the gap. When radiative drag stops the pair-loaded flow at the magnetic equator, the ion component transfers momentum through streaming instabilities, delivering a small lepton population into the opposite hemisphere and sustaining the circuit. The gap confined to a single (anode) hemisphere implies asymmetric hard X-ray production, possibly detectable in phase-resolved magnetar spectra.
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All authors Jens F. Mahlmann, Alexander A. Philippov, Adrien Soudais, Andrei M. Beloborodov, Lorenzo Sironi

Plasma Physics

On energy conservation laws in the drift-reduced Braginskii model

Sergio García Herreros, Brenno de Lucca, Davide Mancini et al. (+5 more)

A revision of the drift-reduced Braginskii model is presented, focusing on its practical implementation in numerical codes, and systematically analyzing the effect of several commonly-used approximations on energy conservation. As a practical example, the effect of the approximations is quantified in the GBS code. By dropping the most important approximations and related energy sinks, the simulation shows increased transport levels and a target heat flux with closer agreement to experimental observations.
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All authors Sergio García Herreros, Brenno de Lucca, Davide Mancini, Zeno Tecchiolli, Micol Bassanini, Paolo Ricci, Louis Stenger, Christian Theiler

Plasma Physics

Electrons quench ion tails during evaporative cooling in hot ion mode plasmas

Eric Palmerduca, Hong Qin and Nathaniel J. Fisch

It is shown how evaporative cooling in hot plasma is essentially different from evaporative cooling in other media, such as neutral gases or Bose-Einstein condensates. The fundamental difference in plasmas arises both from the large mass ratio between electrons and ions in fully ionized plasma and the unusually sensitive dependence of plasma collisionality on speed. Thus, a hot ion mode plasma ($T_e < T_i$) is shown to support a distinctive evaporative cooling regime, where a strong reduction in the ion evaporation rate appears. This new regime may have application to approaches to economical nuclear fusion, where the ion tail plays an outsized role in the fusion reaction rate.
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Plasma Physics

Spatially-resolved methane decomposition in a short glow discharge: insights into suprathermal hydrogen and radical chemistry

Shurik Yatom

This work presents the spatially resolved mapping of methane decomposition chemistry in a short direct-current glow discharge, revealing segregated zones for dissociation and polymerization in an Ar-CH4 mixture at 400 mTorr. Laser-induced fluorescence (LIF), two-photon absorption LIF (TALIF), and optical emission spectroscopy (OES), are used to map the absolute number densities of atomic hydrogen (H), methylidyne (CH), and dicarbon (C2) radicals. The results reveal a segregated chemical environment driven by the non-local electron kinetics. The primary dissociation of methane is confined to the cathode sheath (< 4 mm), where the H atom density peaks at a value (). Analysis of the Halpha} line profile identifies two distinct suprathermal H atom populations with peak kinetic energies of ~110 eV and ~17.5 eV, attributed to ion-surface reflection and electron-impact dissociation, respectively. The densities of secondary radicals CH and C2 peak further from the cathode at the sheath-negative glow boundary (y=4-6 mm), identifying this region as the primary zone for polymerization, with the peak C2 density higher than that of CH by a factor of ~370. The discharge operates under strong non-equilibrium conditions, confirmed by the disparity between the low bulk gas temperature (~570 K) and the high vibrational temperature (3300-4700 K) of the emitting CH(A) radicals, the latter serving as a signature of a specific high-energy dissociative excitation pathway. This work demonstrates how the structured energy landscape of a short glow discharge spatially separates methane dissociation from subsequent polymerization. Kinetic estimates suggest that suprathermal H atoms may play a role in bulk plasma chemistry via abstraction reactions, with predicted CH4 activation rates exceeding those of the thermal H population.
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Plasma Physics

Nonlinear kinetic closures for linear instabilities

Elizabeth J. Paul, Archis Joglekar and Alexey Knyazev

Landau fluid closures are linear in the evolved moments. We examine the consequences of relaxing linearity. As an example, we introduce the nonlinear exact kinetic response closure, exact on an eigenmode but not on superpositions. A counting argument shows that a closure on $N$ instantaneous moments and the field can represent at most $(N+1)/2$ superposed eigenmodes. Guided by this, we train an equivariant neural-network and a learned Padé closure on analytic data. For bump-on-tail and slab ITG instabilities they reduce growth-rate errors by up to three orders of magnitude over Landau closures.
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Plasma Physics

Strong Nonlinear Alfvén Wave Interactions in a Laboratory Plasma

C. H. K. Chen, S. Dorfman, S. Boldyrev et al. (+6 more)

Alfvén waves and their nonlinear interactions are ubiquitous in space and astrophysical plasmas, and are thought to play important roles in the dynamics of these systems, yet their nature remains to be fully understood. We describe experiments performed on the Large Plasma Device to study the nature of counter- and co-propagating wave interactions relevant to strong Alfvénic turbulence. Both interactions were found to produce a broad spectrum of nonlinear modes as a result of a dominant quadratic nonlinearity. The counter-propagating interaction can be explained through the standard reduced MHD nonlinearity, and the co-propagating interaction can be explained through a recently-proposed model that includes second-order nonlinear terms from Hall MHD that dominate at large imbalance and scale with the ion inertial length. The predictions of the latter model were tested in both the experiment and in 3D hybrid simulations, where the nonlinear mode growth rate and ion inertial scale dependence were found to be consistent. Finally, at the obtained interaction strengths, energy was seen to be transferred to progressively smaller perpendicular scales, consistent with a local cascade, although not a state of fully-developed turbulence. These results reveal and verify the mechanisms occurring in balanced and imbalanced turbulence (as well as other Alfvénic nonlinear processes), and represent an important step towards the generation of controlled Alfvénic turbulence in the laboratory.
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All authors C. H. K. Chen, S. Dorfman, S. Boldyrev, L. Franci, A. Mallet, M. Abler, S. Vincena, S. Greess, T. A. Carter

Plasma Physics

JC-PIC: kinetic (PIC-MCC) simulation of lowtemperature plasmas with a documented library of one hundred test cases

Jean-Pierre Boeuf

Over the last twenty years, particle-in-cell simulations with Monte Carlo collisions (PIC-MCC) have given the low-temperature plasma community much of what it knows about electron heating in RF discharges, sheaths and secondary emission, striations, and the instabilities of magnetised plasmas. But these results remain in papers, and in codes that only their authors can run. JC-PIC is a free Windows program: a one-dimensional, three-velocity (1D3V) electrostatic PIC-MCC code with a parallel Fortran engine (OpenMP) and a complete graphical interface. It is built around a library of about one hundred ready-to-run cases, each documented, referenced and explained. About half of them reproduce a published result under the original conditions; the others compare the code with a classical analytical result. This overview presents the program, its companion book, and six of the cases from the physics point of view: Langmuir waves from Landau damping to electron trapping, the heating-mode transition in argon RF discharges, the electrical asymmetry effect, the Franck-Hertz experiment treated as a swarm, the striations of a positive column, and the ExB drift instability of Hall thrusters.
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Plasma Physics

KEMPIC-3D: A transparent and extensible electromagnetic Particle-in-Cell framework for kinetic plasma simulations

Jesús E. López, Keren C. Vanegas and Eduardo A. Orozco-Ospino

KEMPIC-3D, a fully electromagnetic three-dimensional particle-in-cell (PIC) code, has been developed in C/C++ and systematically verified for the self-consistent simulation of kinetic plasma phenomena. The numerical framework combines a staggered Yee finite-difference time-domain solver for Maxwell's equations with a relativistic Boris particle pusher, trilinear field interpolation, and charge-conserving current deposition. Its modular architecture emphasizes algorithmic transparency and extensibility, allowing the principal numerical routines to be directly inspected and modified to incorporate additional physical models and computational capabilities. The implementation is assessed through a hierarchy of benchmark problems addressing both the individual numerical components and the self-consistent particle–field coupling. The particle pusher is verified against analytical solutions for relativistic charged-particle motion, while the electromagnetic field solver is evaluated through guided-wave propagation and grid-convergence analysis. The complete PIC algorithm is subsequently verified through plasma oscillations, global energy conservation, and discrete charge continuity, demonstrating the numerical accuracy, conservation properties, and self-consistency of the coupled formulation. The capabilities of KEMPIC-3D are further demonstrated through a representative simulation of laser-driven plasma wakefield generation in an underdense plasma. The simulations reproduce the characteristic nonlinear wakefield structure and exhibit stable numerical behavior across different particle discretizations without additional spatial filtering. Together, these results establish KEMPIC-3D as a reliable, transparent, and extensible computational framework for multidimensional kinetic plasma simulations.
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Monday 14 September

Plasma Physics

Characteristic Mapping Method for Vlasov-Poisson with BGK-collisions

Xi-Yuan Yin, Philipp Krah, Zetao Lin et al. (+2 more)

This work presents the first steps for simulating kinetic plasmas with collisions using the characteristic mapping method (CMM). The CMM is a semi-Lagrangian method that explores a semi-group structure to store diffeomorphic flow maps efficiently. Using the semi-group structure, individual submaps can be composed to relate the flow backward in time to its initial food point. The novelty of the presented work is handling the source term by storing sub-integrals that correspond to the individual sub-maps and allow efficient integration. Furthermore, we use the Lagrangian structure to avoid implicit time integration in the hydrodynamic regime, which is known to be stiff. We benchmark our method on the Boltzmann-BGK and Vlasov-Poisson-BGK equations and consider different test cases, the Sod shock tube problem and nonlinear Landau damping for different Knudsen numbers. We show third-order spatial and temporal convergence and illustrate the fine-scale zoom property of CMM for the bump-on-tail instability.
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All authors Xi-Yuan Yin, Philipp Krah, Zetao Lin, Jean-Christophe Nave, Kai Schneider

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

Filament Formation via Collision-induced Magnetic Reconnection – Kinematic Features

Shuo Kong, Griselda Arroyo-Chávez, Volker Ossenkopf-Okada et al. (+4 more)

The Collision-induced Magnetic Reconnection (CMR) mechanism has recently been proposed as a filament formation model that emphasizes the active role of magnetic fields. To enable better observational tests with data from modern telescopes, we provide more specific observable predictions for filaments formed via CMR. Two types of CMR kinematics are revealed in position-velocity (PV) diagrams. First, due to the nature of CMR, the midplane contains a converging motion close to the filament and two diverging motions farther from the filament. They exhibit a blueshift, redshift, blueshift, redshift (BRBR) velocity pattern along the line-of-sight when the collision midplane is inclined, giving rise to a special pattern in the PV-diagram. Second, the longitudinal PV-diagram along the filament spine exhibits a velocity oscillating pattern, which is currently attributed to the magnetic transport of gas clumps in the converging motion. The first type of pattern involves emission primarily outside the filament, which can be confused with the environment. The second type, however, mainly emerges from gas inside the filament, thus more robust for observational tests. Both the integral-shaped filament and the Stick filament in the Orion A cloud show the velocity oscillation in PV-diagrams, although the spatial oscillation frequency and their velocity spread differ.
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All authors Shuo Kong, Griselda Arroyo-Chávez, Volker Ossenkopf-Okada, Héctor G. Arce, Ralf S. Klessen, Duo Xu, Szu-Ting Chen

Plasma Physics

Direct observation of electron shedding from a laser-plasma accelerator

Sheroy Tata, Salome Benracassa, Heychal Davidovich et al. (+6 more)

Laser-plasma accelerators have demonstrated the ability to produce ultrashort relativistic electron bunches with peak currents suitable for compact light sources, ultrafast diffraction, and strong-field studies. However, their performance critically depends on preserving the longitudinal phase-space density of the beam as it exits the plasma accelerator. Here, we report the first direct observation of a previously unresolved process in which a highly charged electron bunch undergoes significant longitudinal expansion and progressively loses electrons during extraction from a laser-driven wakefield accelerator, a phenomenon we refer to as electron shedding. Using femtosecond relativistic electron microscopy, we tracked the evolution of the beam far beyond the accelerator exit and observed the bunch stretching over many plasma wavelengths before shedding electrons during propagation. Particle-in-cell simulations reproduce the observed behavior and reveal that it originates from a combination of effects when a high-charge-density beam exits the accelerator. These dynamics redistribute energy from the beam head into a low-energy tail, thereby reducing the useful peak charge density and ultimately decreasing the efficiency of the laser-plasma accelerator. Our results provide new insight into beam extraction and phase-space evolution in laser plasma accelerators and highlight the importance of controlling these collective effects for future applications.
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All authors Sheroy Tata, Salome Benracassa, Heychal Davidovich, Aaron Liberman, Anton Golovanov, Eitan Levine, Yang Wan, Eyal Kroupp, Victor Malka

Plasma Physics

Two-length spatial correlation function of turbulence in TCV

Olivier Panico, Pascale Hennequin, Sascha Rienäcker et al. (+4 more)

Spatial correlation functions of density fluctuations are measured in the Tokamak à Configuration Variable (TCV) using a dual-channel Doppler backscattering (DBS) diagnostic. In certain cases, the spatial correlation function exhibits two characteristic length scales. By analogy with nonlinear reduced simulations, the presence of two correlation lengths may be indicative of avalanche-like transport. The correlation functions obtained from DBS are compared with those from short-pulse reflectometry measurements and show reasonable agreement. Both short- and long-range correlations are measured in the same plasma geometry for different heating powers. Short-scale correlation lengths are found to be on the order of 3-5 Larmor radii, while large-scale correlations extend over approximately 5-15 Larmor radii. The correlations are found to decrease towards the very edge of electron cyclotron heated discharges, coinciding with a narrow Er well.
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All authors Olivier Panico, Pascale Hennequin, Sascha Rienäcker, Oleg Krutkin, Benoit Labit, Yanick Sarazin, the TCV team

Plasma Physics

Shape effects and Shafranov shift reversal in analytical Grad–Shafranov equilibria with non-convex boundaries

D. Abate

Analytical Solov'ev equilibria with freely prescribed plasma boundaries can be constructed by enforcing the boundary condition in a least-squares sense on a polynomial basis of homogeneous solutions. Within this approach, a systematic scan of boundary shape is carried out well beyond the convex regime: non-convex star polygons of arbitrary symmetry order and concavity are examined alongside conventional convex tokamak cross-sections, using a quantitative boundary-fidelity criterion to delimit the range of shapes the polynomial basis can represent. For standard convex shapes, poloidal beta is insensitive to shaping due to the Solov'ev current profile, while polygon boundaries raise it monotonically with the number of sides, driven by the flat sides compressing flux surfaces toward the axis. For the Shafranov shift, odd-fold boundaries admit a critical concavity below which the geometric centre of the boundary overtakes the magnetic axis, reversing the sign of $\Delta/a$; no such reversal occurs for even-fold boundaries, and the effect has no analogue among convex shapes.
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Plasma Physics

Phasor-particle-in-cell algorithm for bidirectional external-circuit coupling of inductively coupled plasma

Zhaoyu Chen, Zili Chen, Jingwen Xu et al. (+4 more)

We develop a self-consistent phasor-particle-in-cell algorithm to capture bidirectional inductive and capacitive feedback between an inductively coupled plasma and a distributed radio-frequency coil circuit. The method couples a two-dimensional axisymmetric particle-in-cell model with Monte Carlo collisions to a network that resolves the current and potential of each coil turn. Precomputed unit-current Helmholtz solutions provide the vacuum impedance matrix, while projection of the plasma-only field supplies the series back electromotive force. An established charge-based electrostatic coupling is extended to individual turns: surface charges obtained from a discrete Gauss law consistent with the Poisson solver yield shunt displacement currents through their fundamental harmonics. Both responses are accumulated over a complete radio-frequency period to update the relaxed circuit state once per period. Vacuum tests showed differences of -1.58 percent in single-solenoid inductance and -4.38 percent in mutual inductance relative to analytical references. A particle-free five-node manufactured benchmark with both coupling channels active yielded a scaled complex-state error of 1.90 x 10^-5 against the continuous reference and approximately second-order spatial convergence. Application to an unshielded argon reference cell reached statistically stationary states at coil-port powers of 60 and 100 W. Net capacitive heating accounted for 29.35 percent and 20.07 percent, respectively, of the combined inductive and capacitive plasma power. Independently accumulated field-side and circuit-side powers, including conductor loss, agreed within approximately 0.5 percent. These results support turn-resolved bidirectional field-circuit coupling for axisymmetric plasma simulations using a fundamental-frequency circuit representation.
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All authors Zhaoyu Chen, Zili Chen, Jingwen Xu, Yonghua Ding, Wei Jiang, Donghui Xia, Ya Zhang

Sunday 13 September

Plasma Physics

Ion-Acoustic-Like Modes in Ion-Loaded Pulsar-Wind Current Sheets: A Pressure-Balanced Existence Criterion

Manpreet Singh, Ripin Kohli and Siming Liu

Pulsar wind electron–positron plasma lacks the heavy inertial species required for the conventional ion-acoustic-like compressive modes. However, if ions are mixed into the reconnecting striped-wind current sheet, a low-frequency compressive branch can appear. We develop a local, comoving-frame theory for such ion-acoustic-like modes in an ion-loaded, pressure-balanced pulsar-wind current sheet. The background model connects directly to pulsar observables through the light-cylinder magnetic field and Goldreich–Julian density, while the sheet structure is represented by a Harris-like field reversal and species-dependent compression factors. The pair species are treated as inertialess thermodynamic shielding populations, whereas the ions are described by a warm, nonrelativistic, magnetized fluid. Pressure balance fixes the pair and ion temperatures rather than prescribing them freely. This gives a closed expression for the pair-shielded ion-acoustic speed in terms of pulsar spin parameters. We derive the warm-ion electrostatic dispersion relation and discuss the conditions under which the ion-acoustic-like branch can exist in such a current sheet. We find that the ion-acoustic-like mode does not occur for all sheet parameters but is restricted to specific regions of parameter space. Thus, ion loading alone is not sufficient to sustain the mode; the local current-sheet conditions determine where an admissible ion-acoustic-like mode can exist. Consequently, any wave-driven anomalous dissipation or particle heating mediated by this mode must be highly localized rather than distributed uniformly across the striped wind. This framework provide the physical domain where the mode can exist, providing the necessary foundation for future studies of kinetic excitation and damping.
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