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Physics Physics Plasma Physics

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Wednesday 14 October · still arriving

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

Flares and flux ropes: an entangled story

Rony Keppens, Yang Guo and Wenzhi Ruan

Solar flares serve as standard candles for all magnetic-reconnection-based energy releases, and their consequences into the heliosphere can be modeled from first principles. This review presents the latest developments in modeling solar flares and their associated flux rope expulsions with state-of-the-art multidimensional magnetohydrodynamic (MHD) descriptions. Observationally established ingredients beyond a single-fluid, thermal plasma evolution have been incorporated to varying degrees, and we highlight self-consistent efforts realized in "standard flare" setups.
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Plasma Physics · Cross-listed from astro-ph.SR

Stationary Resistive MHD Solutions Near an Isolated Three-Dimensional Magnetic Null

Kuljeet Singh Saddal, Dieter H. Nickeler, Rodrigo Meneses Pacheco et al. (+1 more)

Magnetic null points define the magnetic field topology in stellar coronae, planetary magnetospheres, and astrospheres, and are commonly associated with magnetic reconnection. We investigate the local structure of stationary three-dimensional (3D) resistive, incompressible magnetohydrodynamic (MHD) flows in the vicinity of an isolated magnetic null and an isolated velocity stagnation point, assuming that the two nulls are colocated. The governing MHD equations are solved self-consistently using a first-order local expansion of the magnetic and velocity fields about the null. For any given magnetic-null configuration, the equations admit a two-parameter family of stationary solutions, comprising multiple solution branches with distinct velocity topologies. Along these families, the velocity-null polarity can change at bifurcation points where the flow becomes effectively 2.5D. A single magnetic null can therefore support positive-radial, negative-radial, and spiral stagnation flows, with qualitatively different inflow, outflow, and rotational properties. The associated pressure and resistivity distributions are likewise determined self-consistently and vary across the solution family, demonstrating that they are coupled to the flow topology rather than prescribed independently. The generic solutions do not require the velocity and magnetic separatrices to coincide, allowing plasma to cross the magnetic fan separatrix and providing a local reconnective flow. These results demonstrate that, in the presence of nonuniform resistivity, the magnetic topology alone does not uniquely determine the associated stationary flow, pressure, or resistivity structure around a 3D magnetic null, revealing the nonlinear feedback between all macroscopic variables and resistivity. The constant-resistivity field-aligned and spiral field-crossing solutions identified previously are recovered as special cases.
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All authors Kuljeet Singh Saddal, Dieter H. Nickeler, Rodrigo Meneses Pacheco, Michaela Kraus

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

Magnetic Reconnection Governs the Entire Lifecycle of Solar Eruptive Magnetic Flux Rope

Xinkai Bian, Chaowei Jiang, Qingjun Liu et al. (+3 more)

While magnetic flux ropes (MFRs) are widely accepted to exist prior to many solar eruptions, the physical mechanism responsible for triggering and subsequently driving their eruptions remains debated. Classical models commonly attribute eruption onset to ideal magnetohydrodynamic instabilities, particularly the torus instability, while magnetic reconnection is often regarded as a secondary process. Using high-resolution three-dimensional MHD simulations, we investigate the eruption of an MFR that forms self-consistently through flux cancellation driven by converging motions and photospheric diffusion following an initial shearing phase. The pre-eruption MFR rises quasi-statically and reaches a height where the decay index exceeds the classical torus instability threshold, yet the system continues to evolve quasi-statically without eruption. Instead, eruption begins only when the current sheet beneath the MFR becomes sufficiently thin for magnetic reconnection to set in. Increasing numerical resolution systematically delays the eruption onset, whereas the decay index at eruption onset remains nearly unchanged, demonstrating that the onset is governed by reconnection rather than torus instability. During the eruption, magnetic reconnection continuously converts surrounding sheared arcades into newly reconnected twisted flux that merges with the pre-eruption MFR and continuously to build the erupting MFR. A component-resolved analysis of the Lorentz force work further shows that the newly reconnection flux contributes predominantly to the main acceleration phase. These results demonstrate that magnetic reconnection plays a central role throughout the entire lifecycle of the eruptive MFR: it builds the MFR, directly triggers its eruption, and provides the dominant driving of the eruption.
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All authors Xinkai Bian, Chaowei Jiang, Qingjun Liu, Xueshang Feng, Pingbing Zuo, Yi Wang

Tuesday 13 October · still arriving

Plasma Physics

Coulomb collision operator

R. D. Hazeltine

Properties of the Coulomb collision operator that have been found useful in plasma kinetic theory and plasma transport theory are gathered and reviewed, with derivations. The use of velocity-space coordinates appropriate to a magnetized (drift-kinetic) plasma is emphasized. Little of the material is new, although the derivations may be more detailed than is customary.
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Plasma Physics

Controlling writhe in stellarator optimization

Tobias M. Schuett and Sophia A. Henneberg

The writhe of a closed curve measures its coiling, and for a ribbon along it, writhe is the contribution to the linking number not due to twist. In stellarators, large writhe implies large helical excursion and has previously been either an uncontrolled outcome in optimization, or fixed beforehand by the choice of the coordinate system. We present a simple method to target the underlying writhe of the plasma boundary during optimization. Our method calculates writhe and linking number directly from the underlying elliptical boundary of the stellarator. It therefore does not require any properties of the MHD equilibrium solve, such as the true magnetic axis, making it fast to evaluate and smooth to differentiate. Examples for quasi-helically symmetric stellarators show that this target works well in practice.
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Plasma Physics

Validation of geometric proxies using negative and positive triangularity plasmas with varying elongation on TCV and multi-machine gyrokinetic simulations

Alessandro Balestri, Justin Ball, Stefano Coda et al. (+7 more)

Negative Triangularity (NT) tokamak plasmas can achieve H-mode-like confinement while avoiding Edge Localized Modes, making them an attractive reactor scenario. Predicting in which configuration this benefit occurs requires understanding how magnetic geometry controls turbulent transport. Using a set of geometric proxies based on the poloidally averaged magnetic drift velocity, its poloidal variation, and Finite Larmor Radius (FLR) effects, this work validates their predictive power against gyrokinetic simulations and experiments. First, the proxies are used to predict how the relative benefit of NT over Positive Triangularity (PT) evolves with elongation $\kappa$, forecasting a detrimental effect of NT below $\kappa\sim0.75$–$0.8$. This prediction is confirmed by linear and nonlinear GENE simulations in both ITG- and TEM-dominated regimes. Additionally, a dedicated experimental elongation scan on TCV shows that NT plasmas at $\kappa^{\rm LCFS}=1.5$ have $\beta_N$ and $H_{98y2}$ roughly 1.5 times larger than PT counterparts, while no difference is observed at $\kappa^{\rm LCFS}=0.95$. The analysis is then extended to realistic ITG-dominated scenarios from DIII-D, AUG, and JET, including up-down asymmetric equilibria where conventional triangularity-based descriptions break down. In all three devices, simulations of NT equilibria exhibit substantially reduced nonlinear heat fluxes relative to PT (factors of 1.8, 1.6, and 2.1, respectively). A systematic swap of geometric coefficients between NT and PT equilibria shows that magnetic drifts and FLR effects are the dominant stabilizing mechanisms across all machines, with parallel dynamics playing a secondary role. These results establish the geometric proxies as reliable, computationally inexpensive predictors of NT transport benefits across diverse shapes and turbulence regimes.
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All authors Alessandro Balestri, Justin Ball, Stefano Coda, Olivier Sauter, Paola Mantica, Alberto Mariani, Benoit Labit, Branka Vanovac, Joerg Hobirk, the TCV team

Plasma Physics

Effects of Coulomb Collisions on the Structures of Fast Magnetosonic Shocks

Michael B. Farnell III, Fan Guo, Yi-Hsin Liu et al. (+1 more)

We present fully kinetic particle-in-cell simulations to demonstrate qualitative effects of binary Coulomb collisions on fast magnetosonic shocks. We find that with sufficiently high collisional frequencies, shock rippling and reformation can be inhibited, leading to stationary, laminar shock fronts. We observe that ion-ion collisions disrupt the motion of particles reflected back upstream that is the key for cyclical reformation. Meanwhile, the electron-ion collisions provide a Spitzer resistivity that can dissipate energy from the instability that causes shock rippling. This result helps us understand the role of collisions in laboratory shocks and possibly the outer heliosphere.
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All authors Michael B. Farnell III, Fan Guo, Yi-Hsin Liu, Xiaocan Li

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

A modern particle-in-cell code for beam-beam simulations of future colliders

Arianna Formenti, Remi Lehe, Axel Huebl et al. (+10 more)

A key challenge in the design of next-generation colliders is predicting the effects of beam-beam interactions and using these predictions to optimize machine performance. Beam-beam interactions are characterized by strong focusing forces, radiative effects, and the generation of background particles. While analytical formulas provide insight into the dynamics of beam-beam interactions, these nonlinear, coupled effects are most accurately described with numerical methods. In this paper, we introduce the open-source particle-in-cell code WarpX as a modern tool for modeling the interaction point in future lepton colliders. WarpX provides high-performance execution on distributed GPU architectures within an open-source and actively maintained code base. We present convergence scans and benchmarks against the established beam-beam codes GUINEA-PIG (v1.2.2) and CAIN (v2.42) for luminosity spectra, photon emission, and secondary-particle production under a wide range of beam parameters corresponding to different collider concepts. Our results show that WarpX (v26.09) reproduces the benchmark beam-beam observables while enabling GPU-portable, extensible simulations. These results establish WarpX as a scalable platform for high-fidelity beam-beam modeling in future collider studies.
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All authors Arianna Formenti, Remi Lehe, Axel Huebl, Stepan S. Bulanov, Jens Osterhoff, Andrew Myers, Weiqun Zhang, Spencer Gessner, Peter Kicsiny, Anna Kinderman, Bao Nguyen, Jinseo Park, Jean-Luc Vay

Monday 12 October

Plasma Physics

Dynamic Optimization Workflow for Gyrokinetic Stability and Transport

E.A. Belli, J. Larson, J. Candy et al. (+1 more)

A dynamic optimization workflow for linear gyrokinetic stability that couples the gyrokinetic code CGYRO with the libEnsemble framework for ensemble management has been developed. The workflow is designed to efficiently explore multi-dimensional plasma parameter spaces and to address the practical challenges of simulation-driven optimization in fusion plasma physics, including expensive objective evaluations, unavailable gradient information, non-smoothness arising from transitions between dominant instability branches, and simultaneous multi-core simulations on high-performance computing resources. Rather than optimizing the growth rate at a single wavenumber, the objective maximizes the growth rate-to-wavenumber ratio over the ion-scale spectrum. A derivative-free, model-based optimization algorithm employs manifold sampling to handle the nonsmooth composite structure of the objective function. The approach is applied to the optimization of local magnetic flux-surface shaping parameters, with an equilibrium baseline drawn from DIII-D negative triangularity experimental parameters. The workflow is general and extensible and provides a foundation for future optimization studies involving nonlinear transport objectives and higher-dimensional parameter spaces.
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All authors E.A. Belli, J. Larson, J. Candy, I. Sfiligoi

Plasma Physics

QUARZ: a quasi-static particle-in-cell code with non-uniform radial grids for plasma wakefields

Alexander Pukhov and Thomas C. Wilson

We present a quasi-static particle-in-cell scheme for beam-driven plasma wakefields in cylindrical geometry that works on an arbitrary non-uniform radial grid. The field equations are discretized with hat-function finite elements and conservative finite volumes on any monotone set of radial nodes, and the transverse magnetic field is obtained explicitly, without iterations, following Baxevanis and Stupakov. An optional azimuthal mode $m=1$ describes beam offsets, tilts and hosing. The discretization keeps the discrete Gauss law exact, deposits a uniform plasma exactly on every node including the axis, and remains second-order accurate on smoothly stretched grids. Resolution can therefore be placed where the physics needs it. For a tightly focused witness beam that pulls the plasma ions onto the axis, a grid that is fine only near the axis gives the same near-axis fields as a uniform grid with the same axis resolution, with thirty times fewer cells and in thirty times less computing time. Such a grid also resolves the singular closure of a cold-plasma bubble, which a uniform grid hides by smoothing it over one cell; smoothing the plasma sources over a fixed length, chosen independently of the grid, regularizes the closure and makes the wake behind it converge. The scheme is implemented in the open-source C++/Kokkos code QUARZ (Quasistatic Arbitrary-resolution RZ), which runs on CPUs and GPUs and uses MPI with a pipelined decomposition along the co-moving coordinate. Simulations of the self-modulation of the AWAKE proton bunch agree with the three-dimensional code HiPACE++. QUARZ is, to our knowledge, the first production-grade simulation code in plasma physics that is zero-touch: all of its source code, tests and documentation were written by an AI agent directed by a physicist who did not write a single line of the code.
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Plasma Physics

Relativistic Six-Fluid Rotating-Plasma Force-Balance Equilibria for p-11B Fusion Conditions in Spherical Tori

Yueng_Kay Martin Peng and Akio Ishida

We formulate an axisymmetric spherical torus (ST) six-component magnetofluid equilibrium model for p_11B plasmas composed of thermal and suprathermal proton, boron, and electron fluids. The model is designed for p_11B parameter regimes where ion energies, strong current drive, energetic electron populations, and ion-electron temperature separation can render a single_fluid static_equilibrium description inadequate. It generalizes rotating_plasma equilibrium theory by incorporating relativistic thermal enthalpy corrections to the electron fluids via h(T/m_ec^2) and by constructing modified flux functions Y_{\alpha} associated with canonical angular momentum for each component {\alpha}. The equilibrium is determined by a Grad-Shafranov-like Ampere law for the poloidal magnetic flux Psi, algebraic density relations constrained by quasineutrality, and prescribed profile functions F_{\alpha}(Y_{\alpha}), T_{\alpha}(Y_{\alpha}) and K_{\alpha}(Y_{\alpha}). The manuscript includes the relativistic magnetofluid equations, the axisymmetric equilibrium formulation, the six_fluid computational method, and four numerical equilibria: two reactor_relevant ST p_11B parameter sets and two EHL-2 ST cases. The examples span toroidal beta b_T = 0.058_0.253, plasma currents I_p = 2.697_12.59 MA, and suprathermal electron temperatures T_eh = 287 keV_2.37 MeV, with h up to 19.1. Over these parameter ranges, Ampere's law accuracy, electrostatic potential accuracy, and component_resolved radial force balance can have errors ranging from 3% to below 0.1%. These results indicate that high-temperature p_11B ST equilibria can differ qualitatively from the single_fluid tokamak equilibria and require analysis of equilibrium component_resolved force balance, relativistic electron enthalpy, and explicit reproducibility diagnostics.
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Plasma Physics

Implicit PIC Simulations of Hydrogen Plasmas with Intense Negative Ion Production

Denis Eremin

High-density plasma discharges in hydrogen operating at low pressures are of considerable interest for negative ion production used in many applications. Their weak collisionality requires a kinetic, nonlocal description, and a popular approach is to model such discharges numerically using the particle-in-cell (PIC) method combined with Monte Carlo collisions (MCC). However, the most commonly used explicit momentumconserving variant of this method is highly inefficient because it must resolve both the Debye length and the plasma period. In this context, the implicit energy-conserving PIC/MCC scheme appears to be an attractive alternative, as it can model the plasmas of interest using their actual parameters without resorting to artificial scaling techniques. This work considers simulations of two types of high-density plasma discharges performed using electromagnetic or electrostatic energy-conserving PIC/MCC codes in hydrogen, and discusses the observed negative ion production.
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Sunday 11 October

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

Radial increase of the compensated interplanetary magnetic field magnitude as a result of the growth of spherically polarized Alfvén waves

Stuart D. Bale, Timothy S. Horbury and Lorenzo Matteini

Measurements from the NASA Parker Solar Probe (PSP) mission demonstrate that inside $\sim20$ solar radii ($\Rs$) the magnitude of the interplanetary magnetic field does not fall off simply as $1/R^2$, with $\avg{|B|}\,R^2$ 15–20% below its value at $\sim60\,\Rs$. Here we use PSP magnetic field measurements over $10.6$–$56.8\,\Rs$ to show that the outward rise of $\avg{|B|}\,R^2$ is not due to the global structure of the heliospheric magnetic field or proximity to the heliospheric current sheet but is the direct result of the growth of large-amplitude, spherically polarized Alfvén waves (so-called `switchbacks'). Over the same distance range, $\avg{B_R}\,R^2$ stays constant, so that the sector-signed radial flux is conserved. The wave amplitude $\epsilon = B_{\perp,{\rm rms}}/\avg{B_\parallel}$ grows from $0.26$ to $0.57$ as $R^{1/2}\cos\alpha$, and $\avg{|B|}/\avg{B_R}$ rises by $20\%$. A simple model parametrized by the broadening of the distribution of deflection angles reproduces the effect to high accuracy. These results demonstrate that as the relative wave amplitudes grow with increasing solar distance, a dynamo-like process operates to enhance the field magnitude by up to $20\%$. By enhancing magnetic pressure this has implications for the large scale solar wind structure in the low-beta inner heliosphere, as well as partially resolving the long-standing discrepancy between remote measurements of the solar magnetic field and those made in situ by earlier missions farther from the Sun.
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Plasma Physics

A reduced model for neoclassical transport in non-axisymmetric magnetic configurations

Y. Ban, X. Garbet, R. Varennes et al. (+6 more)

A unified framework is proposed to compute a neoclassical equilibrium for tokamaks with non-axisymmetric magnetic perturbations, based on a Hamiltonian description combined with a variational approach that relies on a principle of minimum entropy production. This approach allows for a transparent identification of the respective contributions from different classes of trapped particles in magnetic well minima. Analytic expressions are derived for specific collisionality regimes, and a fast and efficient numerical procedure is devised to compute the entropy production rate. Finding the extremum of the entropy production leads to tractable expressions of transport coefficients that can be computed quickly and efficiently.
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All authors Y. Ban, X. Garbet, R. Varennes, Z. Qu, K. Lim, Y. Sarazin, A. Ibrahim, Y.C. Tay, B.B. Ang

Saturday 10 October

Plasma Physics

Plasma-neutral collision operators for hydrogen isotopes: kinetic and gyrokinetic descriptions, comparison to fluid

Matthieu Protais, Guilhem Dif-Pradalier, Yanick Sarazin et al. (+4 more)

Plasma-neutral interactions critically affect the edge dynamics of magnetically confined fusion devices. Neutral atoms modify ion and electron velocity distributions, potentially affecting local instabilities Standard fluid models simplify these processes (namely charge exchange, ionization and recombination) using Maxwellian assumptions and linearized source terms. We revisit these assumptions for a hydrogen plasma interacting with hydrogen atoms. We derive plasma-neutral interaction terms in the kinetic and gyrokinetic frameworks. Accounting for velocity-dependent differential cross-sections, the model captures the kinetic impact of inelastic collisions on ions and electrons beyond energy and momentum exchange. We demonstrate that the fluid moments of the kinetic model reduce to standard fluid source terms under the appropriate assumptions of Maxwellianity and linearity. We compare the kinetic model with fluid models through their effects on distribution functions and fluid moments. Substantial discrepancies are observed, even for Maxwellian distributions. For charge exchange, the kinetic model yields momentum and energy exchange rates up to twice those predicted by fluid descriptions. We show that a fluid treatment of ionization greatly overestimates the mean kinetic energy of the produced electrons. These results highlight the limitations of fluid descriptions and the importance of kinetic effects in edge-plasma momentum and energy exchange.
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All authors Matthieu Protais, Guilhem Dif-Pradalier, Yanick Sarazin, Virginie Grandgirard, Yannick Marandet, Srikanth Sureshkumar, Naren Varadarajan

Plasma Physics · Cross-listed from cs.LG

Toward a Multimodal Foundation Model for Plasma State Reconstruction

Bin Xia, Chuanfei Dong, Boting Li et al. (+9 more)

Plasma experiments observe coupled physical states through sparse, heterogeneous, and incomplete diagnostics. We present a proof of concept toward a multimodal foundation model for plasma state reconstruction during magnetic reconnection, motivated by the Facility for Laboratory Reconnection Experiments (FLARE). Using 250 synthetic VPIC simulations at fixed reference electron beta $\beta_{e0}=0.2$, with 225 training runs and 25 disjoint validation runs, we train a 5.5M-parameter residual 3D U-Net to reconstruct three magnetic-field components and electron density from incomplete observations. The model combines spatial-only pooling, which preserves all 24 temporal frames, with global spatiotemporal self-attention and rotary positional encoding. Independently masking magnetic and density modalities across random, grid, block, and temporal patterns enables sparse sensing, cross-modal completion, density super-resolution, and temporal completion with a single set of weights and no task-specific retraining. With density fully hidden, just 0.1% magnetic spatial visibility reduces median density NRMSE from approximately 0.9 to 0.1. Conversely, observed density improves current-density reconstruction when magnetic data are absent or extremely sparse. With no magnetic observations, roughly 1% density visibility approaches the density reconstruction accuracy achieved with fully observed magnetic fields, although current structure remains more dependent on direct magnetic measurements. Full magnetic conditioning also stabilizes completion of missing future density frames. These results demonstrate bidirectional cross-modal predictive coupling within this simulation regime. Generalization across plasma regimes or to experimental measurements remains untested.
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All authors Bin Xia, Chuanfei Dong, Boting Li, Yilan Qin, Zikang Xie, Sung Hyun Son, Adam J. Stanier, Jongsoo Yoo, Hantao Ji, Ahmed Diallo, Han Ding, John Wise

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

Multi-Objective Design of C-Shaped Targets for Energy–Divergence Control in Laser-Driven Proton Acceleration

Mohammad Rezaei-Pandari, Mahdi Shayganmanesh and Mohammad Hossein Mahdieh

We optimize the energy-divergence trade-off of laser-driven protons from a C-shaped target using two-dimensional particle-in-cell simulations and constrained multi-objective Bayesian optimization. Five geometry and focusing parameters are varied to maximize proton cutoff energy and minimize the RMS divergence of forward protons with E_k >= 5 MeV under a proton-only conversion-efficiency constraint of eta_H >= 9%. Of 82 designs, 60 are feasible and 15 Pareto-optimal. The Max-Energy and Min-Divergence endpoints yield 28.64 MeV at 52.51 degrees and 11.47 MeV at 11.13 degrees, respectively; a representative compromise yields 21.44 MeV at 38.44 degrees. Matched-stage diagnostics link these regimes to optical-field buildup, electron energization, charge separation, and outer-sheath organization. Numerical tests support the principal trends. Reflection timing does not support a monotonic trapping-duration law, and the charge-sheath relation remains non-local. These results establish a design-resolved route to controlling proton energy and angular quality.
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Plasma Physics

First achievement of 100 million degree electron temperatures in a spherical torus: the EXL-50U

Guang Yang, Shaodong Song, Songjian Li et al. (+14 more)

Here,we report an ST plasma in which Thomson scattering measures a central electron temperature exceeding 8.6 keV (100 million degrees kelvin) on the EXL-50U spherical torus device.The high-temperature phase is obtained at low density by sequential operation of two 50 GHz ECRH systems: ordinary-mode (O-mode) ECRH3 followed by extraordinary-mode (X-mode) ECRH4. In contrast to neutral-beam-heated high-ion-temperature ST scenarios, the discharge is sustained by ECRH only.
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All authors Guang Yang, Shaodong Song, Songjian Li, Xiaobing Xie, Zhaokun Wang, Xiang Gu, Jia Li, Xianli Huang, Xinchen Jiang, Yupeng Guan, Huicong Ma, Junquan Lin, Dong Guo, Jiaqi Dong, Yunfeng Liang, Yuejiang Shi, the EXL-50U Team

Plasma Physics

SPIRE: Spin-Resolved Particle-in-Cell with Radiative Corrections for Extreme Laser–Plasma Interactions

Xian-Zhang Wu, Huai-Hang Song, Wei-Min Wang et al. (+1 more)

We present SPIRE, a spin- and photon-polarization-resolved particle-in-cell (PIC) code for self-consistent simulations of laser-plasma interactions in the strong-field quantum electrodynamics (QED) regime. The classical PIC framework combines a Yee-grid finite-difference time-domain Maxwell solver, a relativistic Boris particle pusher, shape-function particle-grid interpolation, Esirkepov charge-conserving current deposition, laser boundary injection, and a relativistic moving window in one, two, and three spatial dimensions. A vectorial Laguerre-Gaussian laser injector based on an angular-spectrum formulation supports tightly focused, nonparaxial optical orbital-angular-momentum modes. The strong-field QED module treats nonlinear Compton scattering and nonlinear Breit-Wheeler pair production as stochastic processes using spin- and polarization-resolved probabilities in the local constant-field approximation. Electron and positron spin evolution includes stochastic transitions, no-emission corrections, and Thomas-Bargmann-Michel-Telegdi precession with a field-dependent anomalous magnetic moment. Photon polarization is represented by the Stokes vector and evolves through nonlinear Compton scattering, pair production, vacuum birefringence, and vacuum dichroism. The implementation is verified with one-, two-, and three-dimensional benchmarks. Classical PIC results are cross-checked against SMILEI, and the spin- and polarization-resolved QED modules reproduce published results for polarized electron generation, vacuum birefringence and dichroism, and dense polarized positron production. These benchmarks verify the coupled classical PIC, strong-field QED, spin, and photon-polarization modules implemented in SPIRE.
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All authors Xian-Zhang Wu, Huai-Hang Song, Wei-Min Wang, Yan-Fei Li

Friday 9 October

Plasma Physics

A New Degree of Freedom in Stellarator Coil Design

Sophia A. Henneberg and Annika Zettl

Reversed toroidal field (RTF) coils introduce a new degree of freedom in stellarator coil design. While the total plasma-linked current is fixed by Ampere's law, compensating RTF currents allow the modular-coil current to exceed this value without altering the net linkage. This redistribution decouples key coil constraints and modifies the external coil geometry without significantly compromising the confining magnetic field. As a result, the achievable coil-plasma separation can be increased in the configurations studied, potentially enabling more compact stellarator designs.
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Plasma Physics · Cross-listed from physics.comp-ph

Physical-state supervision for design-only Gaussian process surrogates of implosion simulations

Chang Liu, Hao Shen and Chuanying Li

Physical diagnostics recorded during simulation can organize a sharply varying response, but using them as surrogate inputs normally requires another calculation at prediction. We construct a physical-state-supervised Gaussian process surrogate that learns these diagnostics from design variables. Cross-fitted state estimates augment a retained design representation; a global Gaussian process and an optional soft mixture of response-band experts use identical inputs. On a database of 1,987 paired one-dimensional implosions, learned burn-weighted states from calculations without alpha-particle heating reduce mean squared log-response error by 60.25% and 52.98% for the global predictor on random holdout and design-space extrapolation. Random-projection and shuffled-state supervision do not reproduce these gains. For CH, matched-input comparisons show that experts help at smaller training budgets but contribute little at 1,000 designs with this representation. A thermal-feedback benchmark validates the complete learned-state procedure, while 3,000 newly calculated Be/Cocktail designs expose weaker extrapolation gains and cases where experts degrade accuracy. A conditional state-error bound separates encoding sensitivity from routing effects. An open Python implementation reproduces the archived predictions and is released with the data, per-design results, and measured costs. Physical-state supervision is thus a useful representation for finite-data surrogates, whereas expert specialization should be selected by validation.
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Plasma Physics · Cross-listed from physics.comp-ph

Energy-dependent trapping of backscattered helium during magnetron sputtering growth: A molecular dynamics study

Pascal Brault, Anne-Lise Thomann and Alejandro Borroto

Helium incorporation during magnetron sputtering can strongly affect thin film growth and lead to the formation of gas/solid nanocomposites and nanoporous structures. Here, molecular dynamics (MD) simulations are used to investigate the trapping mechanism of helium atoms (He) and the subsequent formation of He bubbles during the sputter deposition of aluminum (Al) and titanium (Ti) thin films. The simulations account for the kinetic energy distributions of sputtered metal atoms and backscattered gas species. Results indicate that He is efficiently incorporated into the growing films, leading to the formation of high-pressure, dense He bubbles, with a much higher He-to-metal atomic ratio in Ti ($\sim$ 60%) than in Al ($\sim$ 12%). More importantly, the simulations reveal a clear energy dependence of He trapping in Ti: background He atoms at thermal energies are essentially not incorporated, whereas efficient trapping starts at an incident energy of approximately 20 eV and increases up to a saturation regime near 100 eV. Simulated thermal desorption spectroscopy (TDS) further reveals distinct release mechanisms in the two investigated metals: He desorption from Al occurs at higher temperatures and is preceded by the removal of surface Al atoms, whereas He and Ti atom desorption starts continuously around 950 K, leading to a porous structure. Comparative simulations with argon (Ar) demonstrate negligible trapping even at comparable kinetic energy or momentum, showing that neither parameter alone accounts for the strong He incorporation observed in Ti. The above results are in very good agreement with what is reported in literature for low-kinetic energy He ion implantation in metal surfaces, suggesting similar insertion mechanisms. They clarify the atomistic conditions governing noble gas trapping during film growth and provide a basis for controlling He incorporation and porosity in films synthesized by magnetron sputtering.
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Plasma Physics

Implementation and verification of a new free boundary module for Resistive Wall Modes simulations with the nonlinear 3D full MHD code SpeCyl

Francesco Galeazzi, Daniele Bonfiglio, Luca Spinicci et al. (+1 more)

Self-consistent numerical simulations of free boundary instabilities, such as resistive wall modes (RWMs) and external kink modes, require special attention given their relevance on the path towards magnetic fusion based nuclear reactors. In this work, a new implementation and a verification benchmark are presented for the nonlinear three-dimensional (3D) full magneto-hydrodynamics (MHD) code SpeCyl, which self-consistently combines a free flow 3D treatment for the edge plasma velocity with double resistive wall thin shell magnetic boundary conditions, capable of capturing the essential features of common setups in experimental devices. Two coaxial resistive shells surround the plasma. Vacuum conditions are assumed between and outside the resistive shells, whose relative positions and time constants can be varied on purpose. The implementation is extensively verified against Shafranov's analytic theory, valid for a circular cross section large aspect ratio tokamak and assuming a flat current profile. The correctness of newly implemented Boundary Conditions (BCs) is confirmed by the good agreement obtained for the linear growth rates of (m=2,n=1) RWM and external kink, over a wide range of wall positions, resistive shell time constants, and for different initial reference equilibria. The (m=2,n=1) RWM growth rates are also benchmarked against previous different RWMs modelling and successful validation studies for RFX-mod tokamak plasmas. The new BCs make SpeCyl a flexible and robust tool to investigate free boundary MHD modes in large aspect ratio magnetic confinement configurations, such as those achieved with the RFX-mod machine and expected in RFX-mod2.
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All authors Francesco Galeazzi, Daniele Bonfiglio, Luca Spinicci, Susanna Cappello

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

Resistive alignment of a black-hole magnetosphere

Ruben Lier and Oliver Porth

Recent numerical simulations have shown that black-hole magnetospheres evolve rapidly toward alignment between the magnetic axis and spin axis but the physical origin of this process has remained unclear. We develop an analytic theory of this alignment within resistive relativistic magnetohydrodynamics. Taking the slow-spin Blandford-Znajek monopole solution as the exterior magnetic field, we reduce the induction equation to a relaxation equation for the current-sheet inclination angle. Our analysis shows that alignment results from second-order spin effects in the Blandford-Znajek solution combined with explicit resistive dissipation. We test this prediction using resistive-GRMHD simulations at several spins and resistivities and find a regime in which the measured late-time secular decay agrees quantitatively with the analytic prediction.
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Thursday 8 October

Plasma Physics · Cross-listed from math.NA

A neural characteristic mapping method: Lagrangian PINNs based on flow maps for transport-dominated problems

Martin Campos Pinto, Victor Fournet, Emmanuel Franck et al. (+2 more)

We propose a novel numerical method for transport-dominated problems using Physics-Informed Neural Networks (PINNs). In such problems, the solution can develop large gradients and fine structures over time. This is particularly challenging for classical PINNs, which represent the solution itself. In our approach, rather than the solution, we choose to approximate the flow map of the equation. The solution is then recovered by pulling back the exact initial condition, so that its fine structures are produced by the map rather than represented by the network. Our method is constructed by approximating the flow map of the equation with multiple PINNs, one per time subinterval, and composing them using the semigroup property of the flow, so that each network only has to represent a map close to the identity. To highlight the advantages of our method compared to classical PINNs or classical numerical schemes, we present numerical results on the linear advection equation, the incompressible Euler equation in vorticity formulation, the Vlasov-Poisson equation, and finally a drift-kinetic equation.
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All authors Martin Campos Pinto, Victor Fournet, Emmanuel Franck, Philipp Krah, Victor Michel-Dansac

Plasma Physics

Kinetic wave activity and proton heating in 3D hybrid simulations of decaying balanced and imbalanced Alfvénic turbulence

C.A. González, T.A. Bowen, A. Mallet et al. (+1 more)

We investigate turbulent dynamics from large to sub-proton scales, the kinetic wave activity and resulting proton heating in three-dimensional hybrid-kinetic simulations of freely decaying balanced and imbalanced Alfvénic turbulence. We considered initial fluctuations with same amplitude and spectral shape but different degrees of correlation between velocity and magnetic field perturbations (i.e., normalized cross-helicity), which define the corresponding turbulent regimes. We found that our balanced turbulence simulation exhibits stronger energy dissipation and more efficient proton heating compared to the imbalanced case. On the large fluid scales, the two simulations undergo distinct nonlinear dynamic and energy cascades, leading to different spectral properties once turbulence is well developed. At sub-proton scales, fluctuations are predominantly oblique with right-hand polarization, a property common to both regimes. In contrast, only the imbalanced simulation displays substantial wave power parallel to the guide magnetic field, with clear signatures of ion-cyclotron waves. These waves are excited in regions with strong temperature anisotropy that arise during the initial nonlinear steepening of Alfvénic fluctuations, a process that simultaneously generates parallel-propagating proton beams. Furthermore, non-Maxwellian features emerge in velocity space, indicating different heating mechanisms in balanced and imbalanced regimes and providing evidence for the role of kinetic instabilities in regulating the turbulent dynamics. Overall, these results show that cross-helicity is a key parameter controlling the large-scale evolution, kinetic activity, and energy dissipation in collisionless turbulent plasmas.
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All authors C.A. González, T.A. Bowen, A. Mallet, A. Tenerani

Plasma Physics

FHiVI$^{2}$: A Fast High-Voltage approach for Ionisation Improvement in the context of plasma-assisted magnetron sputtering

R. Jean-Marie-Desiree, L. De Poucques and S. Cuynet

Ionisation control remains a key challenge in plasma-assisted processes and more particularly in magnetron sputtering. In this context, the former directly governs species transport and thin-film properties. Although HiPIMS substantially enhances the ionisation rate of sputtered species, its performance remains constrained by the coupling between sputtering and ionisation, which limits discharge tunability. Here, we introduce Fast High Voltage for Ionisation Improvement (FHiVI$^{2}$), a new transient electrical regime designed to improve the ionisation rate without requiring additional elements. By applying an ad hoc very short, high-voltage pulse during plasma sputtering processes, FHiVI$^{2}$ regime is expected to induce a strongly anisotropic electric-field burst intended to enhance sputtered metallic species ionisation while limiting back-attraction losses and sputtering. To assess the ionising potential of this regime, optical emission spectroscopy together with mass spectrometry were carried out. More particularly, mass spectrometry measurements show an improvement of the ionisation rate by a factor of 2.7 compared with the HiPIMS regime, for an applied voltage of 2 kV which correspond to an additional mean power of only 1.2 W, using a 2-inch tungsten target. These findings suggest that FHiVI$^{2}$ provides a flexible route towards improved ionisation control in magnetron sputtering and opens new perspectives for plasma-assisted processes.
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Plasma Physics

Compressional heating above 1 keV on the LM26 magnetized target fusion machine

S. J. Howard, D. Krotez, P. Carle et al. (+43 more)

The LM26 device has achieved a peak electron temperature of $T_e = 1180 \pm 65$ eV as measured by filtered X-ray diodes, supported by adjacent Thomson scattering measurements of $T_e = 1090 \pm 40$ eV, with a deuterium ion temperature of $T_i = 459 \pm 33$ eV as inferred from neutron yield. LM26 compresses a spherical tokamak plasma inside an initially 1.76 m diameter solid lithium liner imploded by theta-pinch coils. Radial compression by a factor of 2.75 increases $T_e$ over fivefold, while $T_i$ increases as much as twofold. The integrated observation of magnetic flux compression, electron heating, ion heating, and neutron production in a magnetized plasma compressed by a large lithium liner is an important milestone for magnetized target fusion
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All authors S. J. Howard, D. Krotez, P. Carle, J. Sanchez Rojo, R. Underwood, A. Froese, M. Reynolds, N. Sirmas, K. Conquergood, K. Epp, P. Forysinski, D. Plant, R. Zindler, E. Love, D. P. Brennan, N. Kumar, C. MacDonald, A. Wong, W. Zawalski, B. Rablah, W. Kozicki, C. Preston, A. Lee, X. Feng, M. Schellenberg-Beaver, R. Tingley, J. Gorenstein, V. Suponitsky, Z. Seifollahi Moghadam, D. Froese, E. Cessford, J. Pratt, J. Crofts, J. Sardari, G. Faust, D. Ross, J. Wilkie, S. Bernard, C. Connor, S. Bolanos, C. Gutjahr, E. Chan, C. Eyrich, A. Mahoney, M. Davidson, M. Laberge

Plasma Physics

WEST advanced wall protection achievements toward long pulse operation

R. Mitteau, M.H. Aumeunier, L. Dubus et al. (+8 more)

Long pulse operation in magnetic fusion devices requires well controlled plasma power exhaust to the divertor & wall, and avoidance of wall hot spots that could evolve in wall damage. At WEST, 10 major plasma facing components are monitored using 10 series of temperature/power indicators, based on multiple diagnostic systems, among which the infrared viewing system is especially relevant. These indicators span from the most basic ones (temperatures, power and energy from deterministic models) to advanced processes using artificial intelligence acquired through machine learning. Some advanced processes do operate in real time, and feedback on power actuators through the plasma control system, providing active control toward remaining within the safe operational domain. Other advanced processes intervene as forensic tools post discharge to identify possible dangerous situations regarding the power loading to the wall, so that the discharge run plan is adjusted to avoid running into aggravating wall events. No critical wall power event happened during the campaigns C9 to C11 (2024-2025), totalling about 13h of plasma, that would have affected the campaign plan. While it cannot be demonstrated that the active & intelligent wall protection enabled the new plasma duration record of 1337 seconds, the wall protection system as a whole plausibly contributed to obtaining these record durations by preventing wall hot spots to become critical during the campaigns.
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All authors R. Mitteau, M.H. Aumeunier, L. Dubus, J. Gerardin, V. Gorse, E. Grelier, V. Moncada, S. Vives, X. Litaudon, M. Jakubowski, J. Fellinger

Plasma Physics

Imaging ablator-fuel mix of hot spot in inertial confinement fusion via resonant X-ray absorption using an X-ray free electron laser

Lingen Huang, Long Yang, Alejandro Laso Garcia et al. (+44 more)

A novel diagnostic is proposed for active probing of doped inertial confinement fusion (ICF) spherical shells, enabling direct measurements of mix and burn dynamics at next-generation implosion facilities via resonant absorption imaging with an X-ray free-electron laser (XFEL). The charge-state-sensitive imaging of embedded dopants provides spatiotemporally resolved constraints on ionization, opacity, and material mix seeded by hydrodynamic instabilities in stagnated plasmas. Proof-of-principle experiments demonstrating resonant XFEL probing of hot spot in coated copper wires driven by an ultra-short relativistic laser pulse, have established the feasibility of this approach. Furthermore, atomic and radiation-hydrodynamic simulations, combined with synthetic resonant X-ray absorption imaging, extend the diagnostic concept to laser-driven direct-drive ICF shells with copper-doped ablators and radiation-driven indirect-drive shells with tungsten-doped ablators. This unique approach, combining a laser-driven implosion facility with a high-brightness XFEL, could enable precise measurements of fusion-relevant hot dense plasmas at multi-keV temperatures, provide stringent benchmarks for radiation-hydrodynamics models, and advance the realization of inertial fusion energy.
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All authors Lingen Huang, Long Yang, Alejandro Laso Garcia, Oliver S. Humphries, Michal Šmíd, Mikhail Mishchenko, Victorien Bouffetier, Carsten Baehtz, Erik Brambrink, Samuele D. Di Dio Cafiso, Gabriel Pérez-Callejo, Yangzhe Cui, Thea Engler, Wen Feng, Sebastian Göde, Joerg Grenzer, Christian Gutt, Johannes Hagemann, Marie-Luise Herbert, Arthur Hirsch-Passicos, Hauke Höppner, Thomas Kluge, Dominik Kraus, Julian Lütgert, Masruri Masruri, Motoaki Nakatsutsumi, Özgül Öztürk, Paweł Ordyna, Xiayun Pan, Franziska Paschke-Bruehl, Alexander Pelka, Thomas R. Preston, Chongbing Qu, S.V. Rahul, Lisa Randolph, Martin Rehwald, Hans-Peter Schlenvoigt, Samuel Schumacher, Ulrich Schramm, Jan-Patrick Schwinkendorf, Georgiy Shoulga, Monika Toncian, Jan Vorberger, Karl Zeil, Ulf Zastrau, Toma Toncian, Thomas E. Cowan