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Tuesday 29 September

Plasma Physics

Global Phase Locking in Superradiant Chorus Emissions

Xin Tao, Fulvio Zonca and Liu Chen

Coherent wave amplification in inhomogeneous systems is usually described in terms of resonance and nonlinear phase locking, but these local phase conditions do not determine how the wave amplitude evolves when the resonance condition changes during wave propagation. Here we identify a global phase-locking principle, in which the stable resonant phase remains nearly unchanged as the resonance detuning evolves. This condition requires the wave amplitude to track the evolving detuning and leads to linear amplitude growth with distance, producing the quadratic intensity scaling characteristic of superradiant emission. We demonstrate this mechanism using self-consistent kinetic simulations of whistler-mode chorus waves, one of the most important coherent electromagnetic emissions in Earth's magnetosphere. The simulated chorus amplitude and effective growth rate follow the predicted superradiant scalings, rather than exponential growth. Direct phase-space analysis further shows that the resonant electron trapping island remains centred at an approximately fixed phase during amplification. These results identify global phase locking as the dynamical origin of superradiant chorus growth and reveal a general phase-organization principle for coherent radiation in evolving resonant systems.
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Plasma Physics

Rectification of plasma-powered active matter into macroscopic work

Ting-yu Yao, Shuo Wang, Shao-peng Li et al. (+3 more)

Plasmas sustain strong energy and momentum flux, yet this activity is typically treated as a dissipative loss channel rather than a usable resource, and its conversion into macroscopic work remains challenging. Here we demonstrate a plasma-powered active engine by coupling self-propelled micromotors to ratchet rectification. Dielectric microspheres in the plasma sheath spontaneously develop asymmetric surface charging and undergo a Quincke rotational instability, forming fast micromotors that extract energy directly from the plasma. Their stochastic impacts are rectified by a sawtooth rotor into a directed angular-momentum flux that drives steady rotation, while an outer asymmetric gear organizes the active bath into coherent circulation to amplify torque transfer. Operating in an inertia-relevant regime, the engine achieves orders-of-magnitude enhancements in both power output and end-to-end efficiency compared with liquid-phase active engines, and remains functional down to the single-micromotor limit. More broadly, our results establish a general route to rectify nonequilibrium plasma activity for macroscopic work.
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All authors Ting-yu Yao, Shuo Wang, Shao-peng Li, Ming-hang Zhao, Bao-quan Ai, Ya-feng He

Plasma Physics

Laser-Polarization Dependence of Betatron X-Ray Production and Angular Collection in Laser-Wakefield Acceleration

Andrei Ciprian Berceanu, Vojtěch Horný, Dan Stutman et al. (+1 more)

Laser polarization affects both the production of betatron x rays and their angular distribution, which together determine the photon yield collected within a fixed angular acceptance. We investigate these effects using particle-in-cell simulations of laser-wakefield acceleration with linearly polarized (LP) and circularly polarized (CP) pulses of equal energy in helium–nitrogen targets of two different lengths. In the longer target, CP produces more photons early in propagation, whereas LP produces more later. For 20–60-keV photons transmitted through 2 mm of aluminum, the LP/CP yield ratio before angular selection changes from 0.88 early on to 1.91 in a late stage. Late-emitting LP electrons develop pronounced transverse oscillations along the laser polarization direction, accompanied by angular broadening of the radiation. This broadening largely removes the late LP production advantage within a central $\pm5$-mrad acceptance, where the two yields are nearly equal. Integrated over the full interaction, however, LP supplies more central photons, while CP provides more uniform illumination. In the shorter target, the extracted charges above 100 MeV are nearly equal, yet transport through a quadrupole triplet delivers 750 pC for LP and 500 pC for CP in the 350–500-MeV energy band within a radius of 5 mm. Collected photon and electron yields are thus shaped by propagation length and collection acceptance.
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All authors Andrei Ciprian Berceanu, Vojtěch Horný, Dan Stutman, Paolo Tomassini

Plasma Physics

Cold atmospheric plasma decontaminates Arabidopsis thaliana seeds and remodels seedling fungal microbiota

Lena Taras, Nicole Chaumont, Caroline Kunz et al. (+2 more)

Seed-associated microorganisms influence seed quality, seedling establishment, and plant health and also constitute a major source of seed-borne pathogens. Cold atmospheric plasma (CAP) has emerged as a promising alternative to chemical seed treatments because of its antimicrobial activity, although its effects on fungal communities associated with developing seedlings remain poorly understood. Here, Arabidopsis thaliana seeds from two ecotypes (Columbia and Landsberg erecta) were exposed to CAP for 5 or 15 min. Seed decontamination efficiency was assessed by culturing on malt extract agar and nephelometric analyses, while fungal communities associated with seedlings derived from treated and untreated seeds were characterized by ITS1 amplicon sequencing. CAP efficiently reduced fungal contamination without affecting seed germination. CAP altered the composition of fungal communities associated with developing seedlings, but the magnitude of these changes depended on seed batch and ecotype. Dominant taxa markedly declined after treatment, whereas several low-abundance taxa increased in relative abundance. These findings demonstrate that CAP is an effective pesticide-free technology for seed decontamination and can also reshape fungal communities associated with developing seedlings, highlighting broader ecological consequences of plasma-based seed treatments.
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All authors Lena Taras, Nicole Chaumont, Caroline Kunz, Thierry Dufour, Christophe Bailly

Plasma Physics

Electron Rephasing in a Truncated Tunable Plasma Channel

Arujash Mohanty, Santhosh Krishnamurthy, Yinren Shou et al. (+6 more)

We report the acceleration of stable electron beams with energies up to 1.5 GeV using a 50 TW laser pulse guided in a preformed plasma channel generated by an axiparabola-focused heater pulse. Varying the channel length changes the plasma density beyond the channel end and produces distinct electron spectra. A short channel causes a large decrease in bubble size after the truncated channel, leaving only part of the electron bunch in the accelerating phase and resulting in a quasi-monoenergetic peak. For a longer channel ending near the target density down-ramp, the smaller density increase rephases and further accelerates a larger fraction of the bunch, producing a continuous high-charge spectrum. Hydrodynamic and particle-in-cell simulations reproduce the observed spectral evolution. Together, the measurements and simulations identify channel length as a key control parameter in guided laser-wakefield acceleration.
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All authors Arujash Mohanty, Santhosh Krishnamurthy, Yinren Shou, Sheroy Tata, Anton Golovanov, Anda-Maria Talposi, Aaron Liberman, Eyal Kroupp, Victor Malka

Plasma Physics

Diffusion prior for KSTAR equilibrium reconstruction under sensor dropout

Hyungkeun Nam and Jaemin Seo

We study equilibrium reconstruction for the Korea Superconducting Tokamak Advanced Research (KSTAR) device under magnetic sensor dropout, with a diffusion model as the prior. Magnetic measurements leave most components of the toroidal current density $J_\phi$ undetermined, and sensor loss leaves more of them to the prior. The diffusion model learns only $J_\phi$, conditioned on the coil currents, the plasma current and the product of major radius and toroidal field, which do not depend on the dropout. The physics enters through a linear forward operator built from sensor response matrices of the LIUQE code. Because the observations are linear, a variant of decoupled annealing posterior sampling fits them with a closed-form linear correction. On measured signals of 37 KSTAR shots, we switch off a random fraction (0.0 to 0.9, ten settings) of the 124 magnetic channels in use and compare with LIUQE under the same masks, taking the full-sensor LIUQE reconstruction as the label. Both methods fit the remaining channels to a similar level. The median relative distance of $J_\phi$ from the label stays at 3.22–4.07% for the diffusion reconstruction in all settings, whereas that of LIUQE reaches 11.89% at dropout 0.9. At dropout 0.5–0.9, the diffusion reconstruction is closer to the label on 27–37 of the 37 shots. At low dropout, LIUQE is closer on most shots. For the derived flux, the diffusion reconstruction is closer on 21–36 shots at dropout 0.5–0.9, and its error at high dropout comes from the vessel current estimate, not from the prior.
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Plasma Physics

Experimental demonstration of broadband-laser suppression of cross-beam energy transfer

Guoxiao Xu, Xiaoran Li, Ning Kang et al. (+22 more)

In direct-drive inertial confinement fusion (ICF), cross-beam energy transfer (CBET) redirects a significant fraction of the incident laser energy out of the plasma. We report the experimental demonstration that broadband lasers reduce CBET-enhanced reflected-light return, performed at the low-coherence Kunwu laser facility with two crossed beams of 0.6% bandwidth and up to 550J at $\sim$2.6$\times$10$^{14}$ W/cm$^{2}$. A coupled ray-tracing model shows that the strong CBET amplification of narrowband reflected light is much weaker under broadband illumination. In symmetric incidence condition of two orthogonal beams, the total fractional scattered energy decreases from 7.57% to 4.64%; in asymmetric incidence condition, which separates stimulated Brillouin scattering (SBS) from specular reflection, the SBS fraction decreases from 3.75% to 0.46% and the specular-reflection fraction decreases from approximately 2.2% to 1.3%. These results establish broadband lasers as an effective, experimentally validated approach to reducing energy escape through CBET-enhanced reflection in direct-drive ICF.
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All authors Guoxiao Xu, Xiaoran Li, Ning Kang, Huiya Liu, Liang Hao, Zhiyuan Li, Guowei Yang, Honghai An, Xichen Zhou, Jian Wang, Lin Yi, Jun Xiong, Zhiyong Xie, Junjian Ye, Yuchun Tu, Zhiheng Fang, Guo Jia, Wei Wang, Lan Xia, Wei Feng, Lailin Ji, Xiaohui Zhao, Anle Lei, Lifeng Wang, Yanqi Gao

Monday 28 September

Plasma Physics

Resonant and Ponderomotive Pumping of Zonal Flows by Turbulence, Alfvén Eigenmodes, and Radiofrequency Waves

C. M. Gallaro and I. Y. Dodin

Analytical studies of zonal-flow (ZF) excitation by fluctuating fields, such as drift-wave turbulence and Alfvén eigenmodes (AEs) driven by energetic particles (EPs) in tokamaks, are usually carried out via intricate gyrokinetic calculations and simulations whose results are notoriously difficult to interpret. Here, we report a transparent result that is also not limited to any particular modes or frequency range. Using oscillation-center theory that captures both ponderomotive forces and quasilinear diffusion, we derive a compact formula for the ZF drive produced by any fluctuating field in terms of the field's canonical momentum and the dissipation power density. For ZFs generated by drift waves, our model subsumes the local relation between the zonal velocity and the drift-wave energy density that was previously derived ad hoc. The simplicity and generality of our result also opens a path toward optimization of ZF excitation with external waves and bridges the physics of AE-EP interactions with that of poloidal flows driven by radiofrequency waves in fusion plasmas.
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Plasma Physics

Plasma Channel Formation Through Laser Resonance Saturation

Mark A. Cappelli, S.K. Wong, R.S. Kissack et al. (+1 more)

We have developed a computer code that models the three dimensional nature of a sodium plasma created by laser resonance saturation. This was accomplished by taking account of the temporal and spatial distortion suffered by the laser pulse as it propagates through a nonuniform sodium atom distribution. We have also undertaken the first measurements of the radial and axial electron density and temperature profiles arising in such a plasma and have been able to demonstrate that our computer code predictions are in reasonable agreement with these results. Absorption of the laser pulse is found to be quite significant for even a few centimeters of $10^{16}$~cm$^{-3}$ sodium atom density and that as a result quite low temperature plasmas can be produced for modest energy laser pulses.
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All authors Mark A. Cappelli, S.K. Wong, R.S. Kissack, R.M. Measures

Plasma Physics

Applications of Poincaré Boundary Condition for 3D Ideal MHD Equilibrium and Optimization

Yigit Gunsur Elmacioglu, Dario Panici, Rory Conlin et al. (+2 more)

Stellarator ideal magnetohydrodynamic (MHD) codes that assume nested flux surfaces such as VMEC and DESC solve the equilibrium problem by prescribing the total toroidal magnetic flux, plasma profiles and the last closed flux surface (LCFS), which analytically determines the unique field in vacuum, whereas at finite $\beta$ bifurcations and distinct equilibria sharing the same boundary have been reported in the literature. In this paper, we propose prescribing the Poincaré cross-section of the field at a single toroidal plane, and implement it in DESC, where the new condition enters only through the linear constraints and therefore costs no more than a fixed-LCFS solve. Fixing the geometry on one plane rather than on a full toroidal surface generally leaves more of the spectral coefficients free, and the ones it frees are those carrying the toroidal variation of the boundary flux surface, which a prescribed LCFS holds fixed at every toroidal angle; together these allow better-converged numerical solutions. We solve equilibria with the cross-section held fixed, starting either from the axisymmetric shape obtained by revolving that cross-section toroidally, or from an existing fixed-LCFS solution. In the latter case, the volume-averaged normalized force error falls by an order of magnitude while the configuration stays close to the original one, and re-solving the resulting boundary with the conventional fixed-LCFS solver recovers the same equilibrium. We further show that the Poincaré coefficients can be used directly as design variables by optimizing a quasi-helical configuration that maintains high-fidelity force balance throughout the process.
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All authors Yigit Gunsur Elmacioglu, Dario Panici, Rory Conlin, Daniel Dudt, Egemen Kolemen

Plasma Physics

Bayesian Active Learning of Ion Loss-Cone Boundaries in Tokamaks

Omar E. Lopez, Minglei Yang, Mark Cianciosa et al. (+1 more)

Loss-cone boundary determination in tokamaks is cast as an active learning problem in which a Bayesian logistic regression model with radial basis function features acts as the learner and the guiding-center orbit integrator acts as the labeling method. The approach is applied to an analytic tokamak equilibrium across a mesh of launch positions, producing a database of loss-probability models with approximate posterior uncertainty, each generated within a prescribed trajectory budget. Once generated, the database supports downstream applications, illustrated here with three examples. First, integrating the probability models against a Maxwellian distribution yields the lost-ion fraction and the energy and parallel momentum carried by lost ions, with posterior uncertainty propagated analytically to each integral. Second, a steady balance between the resulting orbit-loss torque and radial angular-momentum diffusion gives co-current edge rotation whose amplitude is set mainly by the prescribed momentum diffusivity and loss-region renewal rate. Third, a neural network surrogate trained on the database learns the in-domain mapping from launch position to loss-probability image, and transport integrals computed from its predictions closely reproduce the Bayesian reference values on held-out samples from the same mesh.
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All authors Omar E. Lopez, Minglei Yang, Mark Cianciosa, Gary M. Staebler

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

Fast Radio Bursts from Induced Upscattering. I. Magnetar Polar Fireballs

Rei Nishiura, Kunihito Ioka and Bing Zhang

The extreme brightness temperatures of fast radio bursts (FRBs) demand a coherent process that converts magnetar burst energy into millisecond radio emission. We propose that FRBs can be produced within magnetar magnetospheres by induced (stimulated) upscattering in a relativistic fireball outflow. In this paper, we focus on induced Compton upscattering in an electron–positron fireball launched from the polar region of a magnetar. Radiation from the X-ray burst accelerates the polar fireball outflow to Lorentz factors of $\Gamma\sim10$–$10^3$. Low-frequency seed waves, such as fast magnetosonic or Alfvén waves, entering the outflow from the side or against its motion undergo induced scattering and are Lorentz-boosted by the relativistic bulk motion from $\nu_0\sim10\mathrm{kHz}$ to $\nu_{\rm obs}\sim\Gamma^2\nu_0\sim\mathrm{GHz}$. Combining the polar fireball dynamics with the linear growth and nonlinear saturation of the induced scattering instability, we derive the FRB luminosity $L_{\rm FRB}^{\rm iso}$ and fractional spectral bandwidth $\Delta\nu/\nu_{\rm c}$. By considering the possible range of pair densities and temperatures, we find that this polar channel can account for the luminosity of Galactic FRB~20200428 and reach luminosities comparable to those of the brightest extragalactic FRBs. If the pair-loaded region is narrower than the relativistic beaming angle $1/\Gamma$, the model also produces a narrow spectrum and predicts $L_{\rm FRB}^{\rm iso}\propto(\Delta\nu/\nu_{\rm c})^2$, a trend consistent with observations of repeating FRBs. The induced upscattering model thus connects magnetar X-ray bursts to FRBs through a coherent emission process grounded in plasma physics and makes observationally testable predictions.
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Plasma Physics

Electron Density Measurements from Stark Broadened Emission in a Sodium Plasma Produced by Laser Resonance Saturation

Mark A. Cappelli

Electron Stark broadening of the $4^2D$–$3^2P$ multiplet transition in a sodium plasma produced by laser resonance saturation has provided a means of undertaking the first spatial measurements of the free electron density across and along the plasma channel created in sodium vapor of density $10^{15}$–$5\times10^{16}$~cm$^{-3}$. From these measurements and measurements of the neutral sodium density within a heat sandwich oven, we are able to deduce the corresponding electron temperature. These temperatures compare favourably with the electron temperature estimated from a Boltzmann analysis of line intensities and suggest that a highly ionized ($>$10%) plasma of electron temperature $<$6000K can be produced within 100~ns of laser excitation from a laser of modest irradiance ($10^6$–$10^7$~Wcm$^{-2}$). The experimental results are in reasonable agreement with a recently developed 3-dimensional model which predicts a decrease in the degree of ionization along the path of the laser beam as a result of significant depletion of laser energy. These experimental results demonstrate that this LIBORS code is capable of predicting the 3-dimensional nature of this new mode of laser ionization with reasonable accuracy, and may also explain the low electron temperatures and free electron densities observed by other research teams.
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Plasma Physics · Cross-listed from physics.space-ph

Spectral Properties of Solar Wind Charge Exchange from Earth's Magnetospheric Boundaries: MHD and Test Particle Simulations

A. Ramachandran, R. T. Desai, H. L. Norman et al. (+6 more)

The Solar Wind Charge Exchange (SWCX) mechanism, where X-rays can be generated through interactions between heavy, highly charged solar wind ions and neutral atoms, offers an opportunity to observe the large-scale transfers of energy and momentum through the Sun-Earth system. In this study, we simulate SWCX signatures from Earth's outer boundaries using Gorgon global magnetohydrodynamic (MHD) simulations combined with kinetic particle-based simulations which we call the X-ray Imaging of the Magnetosphere (XIM) framework. The XIM-predicted emissivities agree with the predictions of the MHD model, producing enhanced emissions from the magnetosheath and cusp regions, but several differences are apparent. Compared with MHD, XIM shows enhanced structuring in the magnetosheath and notable enhanced emissions along the magnetopause associated with betatron acceleration. The results also evidence species-specific spatial variations, which are shown to vary compared to anticipated relative abundances in the solar wind and ion-specific properties. These simulations provide new insights into the underlying dynamics of SWCX at the Earth's outer boundaries, which can be used to understand what might be observed using X-ray observatories in space, such as the recently launched SMILE mission.
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All authors A. Ramachandran, R. T. Desai, H. L. Norman, Y. Tong, D. Bewicke, S. Nitti, J. A. Carter, J. P. Eastwood, J. P. Chittenden

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

Jet Forecast: Timing of Discrete Jet Ejections of Stellar-Mass Black Holes Anticipated via X-ray Properties

Toshihiro Kawaguchi and Kazutaka Yamaoka

The radio – soft $\gamma$-ray data of the Galactic black hole binary XTE J1859+226 showed that discrete jets are launched when the inner radius of the accretion disk ($R_{\rm in}$) rapidly approaches the innermost stable circular orbit (ISCO). Given the correlations among the X-ray fractional variability amplitude (rms), $R_{\rm in}$, and the X-ray hardness, previous phenomenological findings related to jet ejections (“jet line” and sharp drops of rms) also indicate that the rapid shrinkage of $R_{\rm in}$ down to the ISCO turns on the jet formation. In order to make clear how these properties appear in other objects, we investigated the X-ray and radio data of XTE J1550-564, GX 339-4, and MAXI J1820+070. We found that all the four sources show drops of $R_{\rm in}$ and rms 1-10 days ahead of a jet activity, and exhibit no huge jumps of the accretion rates prior to jet ejections. XTE J1859 and MAXI J1820 show the jet activities under sub-Eddington accretion. Models for the jet production that require an enhancement of $\dot{M}$ or a super-Eddington accretion rate are unlikely. Another behavior common to multiple sources is the curved shape of the rms–$R_{\rm in}$ correlation. Therefore, monitoring of $R_{\rm in}$ seems to be suitable to predict the timing of jet ejections. When both $R_{\rm in}$ and rms continuously decrease and when $R_{\rm in}$ is about to reach the ISCO, alerts can be circulated as a “jet forecast” for triggering Target-of-Opportunity observations.
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Plasma Physics

Electrons with Anomalous Energy Generated in Vacuum Diodes with Different Pulse Duration

Vasily Kozhevnikov, Victor Tarasenko, Maxim Vorobyov et al. (+2 more)

This paper presents results from an experimental study in which anomalous energy electrons (AEE) appear in vacuum diodes under varying amplitude and pulse-duration conditions. AEE refer to high-energy discharge electrons with kinetic energy $T$ exceeding $eU$ values (where $e$ is the electron charge and $U$ is the amplitude gap voltage). Here, two experimental setups were used to generate electron beams with different pulse durations. The electron energy spectra were reconstructed from the beam attenuation curves using an AI-driven methodology for solving an ill-posed inverse problem for the Fredholm equation. This confirms that vacuum diodes generate electron beams with a significant AEE fraction (up to 25~%) only when operating with nanosecond-long voltage pulses. It has been confirmed that AEE in vacuum is generated using nanosecond- or sub-nanosecond voltage pulses. Experiments also indicate that photoelectric absorption of bremsstrahlung cannot produce significant AEE in vacuum discharges.
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All authors Vasily Kozhevnikov, Victor Tarasenko, Maxim Vorobyov, Evgenii Baksht, Andrey Kozyrev

Plasma Physics

Observer-Based Model Predictive Control for Isoflux Regulation in the EXL-50U Spherical Tokamak

Chang Lu, Jiayi Zhi, Xian Guo et al. (+1 more)

Isoflux control in spherical tokamaks requires coordinated plasma-current and boundary regulation under coil-voltage and computational constraints. This study develops observer-based constrained model predictive control (MPC) for EXL-50U. Vacuum-vessel spatial coarsening reduces a 538-state linearized model to 40 states while retaining control-relevant responses. A Kalman observer provides full-state estimates, and MPC jointly optimizes current and boundary-flux tracking with explicit voltage bounds. Matrix precomputation, specialized solver code generation, and structured linear-system solution reduce online computation. Nonlinear free-boundary Grad–Shafranov evolutive simulations evaluate nominal operation, measurement noise, configuration mismatch, a one-step delay, and combined noise and delay against proportional–integral–derivative (PID) control and a linear–quadratic regulator (LQR). Nominal MPC root-mean-square errors are 1.126 kA for plasma current, 0.0164 m for last-closed-flux-surface geometry, and 2.463 mWb for the worst flux channel, all below both baselines. With noise, the boundary error is 0.0199 m over the common evaluation window, and regulation is sustained throughout the test. MPC completes a limiter-to-divertor transition without relinearization or retuning and maintains regulation under delay and combined disturbances. Across all five cases, mean MPC computation times are 0.413–0.441 ms, with all 99th percentiles below 1 ms. These results support joint current and isoflux regulation and millisecond-scale computational feasibility, providing a basis for future EXL-50U experiments.
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All authors Chang Lu, Jiayi Zhi, Xian Guo, Tianyuan Liu

Sunday 27 September

Plasma Physics

A packet-level digital hardware twin for commissioning megahertz diagnostic edge AI and plasma control system integration in tokamaks

Semin Joung, Abhilasha Dave, Luca Scomparin et al. (+6 more)

High-bandwidth plasma diagnostics increasingly provide inputs to machine learning and signal-processing algorithms intended for real-time tokamak control, but the complete path from diagnostic sampling to control-system handoff is difficult to commission because of their sampling rates. We develop a packet-level digital hardware twin for the megahertz diagnostic edge-AI architecture. The simulator represents a 64-channel, 1 MHz beam emission spectroscopy (BES) diagnostic embedded in a 96-channel dual-carrier acquisition system, two 48-channel streams with SPAD0 sample counters, 10 GbE Hardware UDP transport, packet loss and network jitter, FPGA parsing and dual-carrier alignment, causal preprocessing and edge inference, a compact Ethernet result packet, receiver-side shared state, and a 1 kHz PCS-like control cycle. Binary UDP payloads and PCAP files are generated rather than emulating transport only at the array level. With a baseline of 20 samples per packet, each carrier generates 50,000 packets s$^{-1}$ and 100 MB s$^{-1}$ of user payload. A 110 ms reference run produces 11,000 HUDP packets; an intentionally dropped 20-sample packet is detected by the sample-counter continuity logic and invalidates the two overlapping 128-sample inference windows without silent interpolation. For valid windows, the configured engineering latency model gives a median last-input-to-shared-memory latency of 91.6 us and a 99th percentile of 108.1 us. A separate operating-system loopback test sends binary FPGA-result datagrams through a UDP receiver into POSIX shared memory and preserves packet sequence and CRC for 20/20 packets. Interactive GUI interfaces expose timing, packetization, network faults, inference thresholds, and control-state inspection. The framework provides a reproducible environment for testing diagnostic-to-accelerator interfaces and fail-safe behavior for deployment on fusion devices.
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All authors Semin Joung, Abhilasha Dave, Luca Scomparin, Filipp Khabanov, Zheng Yan, Benedikt Geiger, George McKee, Ryan N. Coffee, David R. Smith

Plasma Physics

Implicit-Explicit time integration scheme with Physics-based preconditioning for two-fluid tokamak boundary simulations

Micol Bassanini, Simone Deparis, Paolo Ricci et al. (+4 more)

In this work, a globally stiffly accurate Implicit-Explicit (IMEX) Runge-Kutta scheme is developed and implemented in the GBS code [Ricci et al., Plasma Phys. Control. Fusion, 2012], for two-fluid plasma turbulence simulations. The stiffest phenomena, governed by shear Alfvén waves and parallel diffusion, are treated implicitly, while the remaining non-stiff terms are advanced explicitly. This splitting enables time steps well beyond the Courant-Friedrichs-Lewy limit, without incurring the full computational cost of a globally implicit formulation. To efficiently solve the implicit subsystem at each time step, a three-dimensional physics-based preconditioner, inspired by techniques developed in the magnetohydrodynamic (MHD) context, is used. The resulting framework is verified through the method of manufactured solutions and exhibits both algorithmic and parallel scalability. Significant advantages in numerical stability and computational efficiency are demonstrated with respect to an adaptive explicit Runge-Kutta scheme.
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All authors Micol Bassanini, Simone Deparis, Paolo Ricci, Brenno De Lucca, Davide Mancini, Louis Stenger, Sergio García Herreros

Plasma Physics

A Rapid Integrated Tokamak Modelling Approach for Coupled Profile and Equilibrium Evolution

Ruohan Zhang, Huasheng Xie, Feng Wang et al. (+1 more)

During plasma heating and current ramps the thermal and magnetic states of a tokamak evolve on different timescales. We present a rapid integrated tokamak modelling approach, which couples a variational fixed-boundary Grad–Shafranov equilibrium to sparse particle and energy flux matching and to weak-form current diffusion. An ITER-like pure-deuterium scenario is examined: a current ramp from 10 to 15~MA over 20~s, a 20~s flat-top and a 20~s ramp-down to 14~MA, under 80~MW of electron-cyclotron heating applied at the start. The toroidal current density at the pedestal top more than doubles during the ramp-up and then decreases during the flat-top as current redistributes inward. Thermal stored energy rises from 185.52 to 252.70~MJ and approaches its final value within the first few seconds. An independent calculation with the Fusion Synthesis Engine (FUSE) gives a stored-energy difference of 0.02% at 60~s, whereas the largest profile differences occur in the core current density. Halving the coupling interval changes stored energy by at most 0.006% at the selected radial resolution. The 60~s pulse, advanced through 600 coupling intervals of 0.1~s, takes 8.62~s on an Apple M4 Pro, which is at least two orders of magnitude faster than conventional integrated tokamak modelling solvers.
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All authors Ruohan Zhang, Huasheng Xie, Feng Wang, Zheng-Xiong Wang

Plasma Physics

Continuous Laser-Driven {\alpha} Source

Noam Popper, Griffin D. Glenn, Girik Jain et al. (+7 more)

Laser-driven proton-boron fusion offers a path for generation of $\alpha$ particles on a compact scale. Conventional solid-target experiments, however, are fundamentally limited by target degradation and low shot-repetition capacity. Here, we demonstrate a continuous laser-driven $\alpha$ particle source utilizing target-normal sheath acceleration from a continuously replenishable ultrathin water-leaf jet target. The proton beam interacts with a downstream boron nitride converter, driving the $^{11}\text{B}(p,\alpha)^{8}\text{Be}$ reaction which yields three $\alpha$ particles. The measurement was repeated across $18{,}000$ consecutive shots at a rate of $10\,\text{Hz}$. The outgoing $\alpha$-particle yield and spectral fluence were measured and benchmarked against comprehensive particle-transport simulations. The numerical and experimental results reveal a pronounced flattening in the energy-differential $\alpha$-particle spectrum escaping from the converter, driven by depth-dependent reaction yield and stopping power effects. The sustained stable $\alpha$-particle production at high cumulative yields paves the way for applications in targeted particle therapy and laser-driven plasma-fusion research.
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All authors Noam Popper, Griffin D. Glenn, Girik Jain, Raz Halifa Levi, Tomer Catabi, Michal Elkind, Aviv Levinson, Assaf Levanon, Siegfried H. Glenzer, Ishay Pomerantz

Plasma Physics

Ion-temperature-gradient turbulence from finite to weak magnetic shear regime

Zihao Wang, Tiannan Wu and Shaojie Wang

The tokamak, a toroidal magnetic device confining a hot plasma, is one of the most advanced approaches to fusion energy. A central obstacle is turbulent transport driven by the ion-temperature-gradient (ITG) mode. Operation scenarios that rely on a weak or zero magnetic shear core, such as the ITER hybrid scenario, have achieved markedly improved confinement, yet the underlying mechanism has remained unclear. Here we show that the radial width of ITG poloidal harmonics is constrained not only by the familiar parallel Landau damping, but also by the isotropy of micro-turbulence in the plane perpendicular to the magnetic field. In the weak-shear limit, micro-isotropy dominates and constrains the radial width at one poloidal wavelength, yielding a critical magnetic shear $s_{\text{crit}} \approx 1/(2\pi)$. Above this threshold, the extended Type II ballooning modes prevail; below it, the localized Type I modes emerge, comprising only two or three harmonics. Global gyrokinetic simulations spanning DIII-D, JET~and ITER parameters confirm this geometric criterion. Nonlinear simulations with sustained heating show that weak-shear plasmas spontaneously form internal transport barriers with the turbulence suppressed in the radial region determined by $|s| < s_{\text{crit}}$. This geometric criterion defines the weak-magnetic-shear regime and offers a fresh perspective on turbulent transport in fusion plasmas.
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Plasma Physics

In-situ Generation of Polarized Attosecond Positron Bunches via Radiation-Wakefield Induced Two-Photon Pairs

Zhen-Ke Dou, Qian Zhao, Hao-Tian Wang et al. (+2 more)

High-energy, dense, spin-polarized positron beams with attosecond duration are highly desirable for advanced accelerator physics, laboratory astrophysics, and ultrafast matter-antimatter studies, yet remain beyond the capability of conventional source and injection-based plasma-acceleration schemes. We propose an integrated in-situ method for production, trapping, and accelerating polarized attosecond positron bunches in a single plasma stage. An ultraintense hollow laser propagating in a plasma channel drives a radiative wakefield, where longitudinally injected electrons emit a dense MeV gamma-ray bath through nonlinear Compton scattering. Subsequent photon-photon collisions induce linear Breit-Wheeler pair production, while linear and nonlinear radiative processes mediate polarization transfer to the produced positrons. Spin-resolved QED particle-in-cell simulations indicate that the laser-modulated wakefield naturally captures positrons born in the frontier bubble wake, forming collimated bunch trains with hundred-attosecond duration, GeV-level energies, and densities up to 10^17 cm^-3. Parameter scans demonstrate robustness against variations of laser intensity and plasma-channel density. This single-shot, source-free approach simultaneously addresses positron production, polarization, temporal compression, and plasma injection, offering a compact route toward polarized attosecond antimatter beams.
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All authors Zhen-Ke Dou, Qian Zhao, Hao-Tian Wang, Feng Wan, Jian-Xing Li

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

Magnetizing nonlinear plasma wakefields for positron acceleration: mechanism and operating limits

Yung-Kun Liu, Pisin Chen, Ching-En Lin et al. (+2 more)

Positron acceleration is possible in linear plasma wakes, but the nonlinear electron-driven blowout regime confines simultaneous acceleration and focusing to narrow regions of returning plasma electrons. Here we investigate how an axial magnetic field reorganizes this nonlinear return to open an operating regime for sustained positron acceleration in a uniform plasma. Using quasi-3D and full-3D particle-in-cell simulations, we connect the formation of finite-radius electron columns, or gyro images, to witness transport and its operating limits. The image spacing is governed by the quasi-static invariant, while canonical angular momentum limits the return radius. Modified by the orbit-sampled wake potential, this mechanism yields a positioning law that accurately predicts the density peaks across a wide range of magnetic field strengths. At a plasma density of $10^{16} cm^-3$ and a 35 T axial field, the reorganized structure expands the usable accelerating and focusing interval by more than a factor of four compared to the unmagnetized case. A positron witness bunch placed at the settled phase of this band achieves 92% capture over a 60 mm stage, with the normalized emittance saturating at 83 mm mrad. Independent codes confirm sustained transport, yielding energy gains between 99 and 150 MeV, though numerical convergence on the exact energy gain remains an open challenge. Together with beam loading, alignment, and driver-energy scaling tests, these results establish magnetic control of the nonlinear electron return as a route to single-stage positron acceleration. Beam quality preservation, formation-transient control, and solenoid integration define the necessary next development steps.
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All authors Yung-Kun Liu, Pisin Chen, Ching-En Lin, Spencer Gessner, Bernhard Hidding

Saturday 26 September

Plasma Physics

Review of LHD-ECRH activities and beyond

Masaki Nishiura, Hiroe Igami, Hiromi Takahashi et al. (+11 more)

The electron cyclotron resonance heating (ECRH) system on the Large Helical Device (LHD) played a key role in the progress of helical fusion research from the first plasma in 1998 to the final discharge on December 25, 2025. This paper presents a historical review of ECRH activities on LHD, highlighting the stepwise advances in hardware, including gyrotrons, transmission lines, and launcher systems, together with the exploration of advanced plasma regimes such as overdense heating and long-pulse steady-state operation. Beyond plasma heating, the ECRH system was also used as a high-power probe for advanced diagnostics, including collective Thomson scattering (CTS) as well as correlation electron cyclotron emission (CECE), and as a flexible actuator for transport studies and real-time control experiments. Finally, we discuss how the technological assets and operational expertise accumulated over the nearly three decades of the LHD program are being further developed and applied to next-generation helical devices, particularly CHD and CHD-U, as well as to broader gyrotron and millimeter-wave applications.
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All authors Masaki Nishiura, Hiroe Igami, Hiromi Takahashi, Naoki Kenmochi, Ryoma Yanai, Kenji Ueda, Yoshinori Mizuno, Toshiki Takeuchi, Yasuo Yoshimura, Takashi Shimozuma, Shin Kubo, Toru Ii Tsujimura, Sakuji Kobayashi, Tsuyoshi Kariya

Plasma Physics · Cross-listed from math.NA

Positivity-preserving semi-Lagrangian discontinuous Galerkin methods for multi-species Vlasov-Ampère models of plasma

Dauda Gambo and James A. Rossmanith

We develop a high-order and positivity-preserving kinetic solver for the multi-species Vlasov-Ampère system that is built within a semi-Lagrangian discontinuous Galerkin (SLDG) framework. The method is designed for efficiency and accuracy across a broad range of conditions, including stiff, multi-scale regimes with strong ion-electron scale separation. Phase space is discretized via high-order discontinuous Galerkin finite elements; time stepping is handled via high-order operator splitting and a series of one-dimensional, unconditionally stable, semi-Lagrangian updates. To improve both accuracy and efficiency, each plasma species is represented on a different phase-space mesh. On every element and for each species, local positivity-preserving limiters are applied to prevent spurious undershoots and rigorously ensure positivity of the cell averages at the next time step. The accuracy and robustness of the resulting method are verified on several standard single- and two-species test cases, including manufactured solutions, two-stream instability, weak and strong Landau damping, ion-acoustic wave propagation, ion-acoustic shock formation, and KEEN wave dynamics.
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Friday 25 September

Plasma Physics

Controlling $\gamma$-photon production with additively manufactured microstructured targets

Joshua Luoma, Andreas Kemp, Thomas G. White et al. (+3 more)

Nonlinear inverse Compton scattering (NICS) in high-density targets offers a promising route for generating high flux, broadband sources of MeV $\gamma$-photons which are important for applications like radiography, photonuclear physics, and pair production. Microstructured targets enable control over the NICS process to help shape the photon energy, angular distribution, and total yield. This work presents particle-in-cell (PIC) simulations exploring target microstructures compatible with two-photon polymerization (TPP) additive manufacturing, which is a promising technique that allows high-volume and reproducible printing of sub-wavelength features as small as 200 nm. We investigate how microstructures influence NICS radiation and merge simulation analysis with simple analytical models that guide target design. These results establish pathways to optimize targets for low-divergence, high-energy photon beams with 2.0$\cdot$10$^{8}$ photons/J exceeding 100 MeV and targets for high yields delivering upwards of 2.0$\cdot$10$^{11}$ photons/J above 1 MeV in the multi-PW regime.
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All authors Joshua Luoma, Andreas Kemp, Thomas G. White, Dean Rusby, Hui Chen, Gennady Shvets

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

Transport of Magnetic Fields by Thermohaline Convection in Crystallizing White Dwarfs

Nicolas Frazao Fernandes, J. R. Fuentes and Jim Fuller

The origins of strong magnetic fields observed in many white dwarfs are uncertain. Recent observations show that such fields are more common for white dwarfs with crystallized cores. Crystallization creates a destabilizing composition gradient that drives thermohaline convection in the star's fluid envelope, which may play an important role in magnetic field emergence. We perform 3D magnetohydrodynamical simulations of thermohaline convection with magnetic fields of various strengths and orientations, using the Boussinesq approximation in periodic Cartesian boxes. Our simulations suggest that thermohaline convection can generate magnetic fields when the magnetic Prandtl number is large (i.e., low magnetic diffusivity), but they are likely too weak to account for those observed in magnetic white dwarfs. However, the simulations indicate that strong magnetic fields can be advected outwards by the thermohaline convection without being destroyed, depending on the net magnetic flux. Hence, crystallization-induced thermohaline mixing may transport magnetic fields initially trapped in white dwarf interiors to their outer layers, allowing them to be observed. However, additional simulations using more realistic geometry and magnetic field configurations will be needed to confirm this possibility.
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Plasma Physics

Joint Amplitude-Phase Optimization of Broadband Lasers Raises Absolute Two-Plasmon-Decay Thresholds Above Coherence-Time Predictions

A. S. Joglekar, R. K. Follett, D. H. Froula et al. (+1 more)

Broadband lasers suppress the absolute two-plasmon-decay instability that preheats inertial-fusion targets. The usual scaling relates this suppression to the drive's coherence time, a power-spectrum statistic insensitive to spectral phase. By gradient descent through a differentiable enveloped wave solver, we show that at fixed power spectrum phase optimization raises the threshold from the random-phase median of $2.8\,I_\mathrm{mono}$ to $4.1\,I_\mathrm{mono}$, but produces a transform-limited (TL) pulse train with peak-to-average ratio $R=32$. Joint amplitude–phase optimization reaches $5.6\,I_\mathrm{mono}$ at $R=3.6$, above the best tested TL line-count result of $4.5\,I_\mathrm{mono}$ at $R=16$; relaxing the peak constraint raises the joint threshold to $6.4\,I_\mathrm{mono}$. An exact growth-rate budget evaluated inside the simulation separates the two mechanisms. TL suppresses coupling mainly by concentrating the pump field into short bursts, thereby lowering its time-averaged magnitude at fixed average intensity. The joint optima retain near-random values of this amplitude measure while suppressing both the available coupling and the fraction realized through pump–daughter alignment. The advantage of joint optimization persists from $1\%$ to $4\%$ bandwidth and from OMEGA-scale to ignition-scale conditions.
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All authors A. S. Joglekar, R. K. Follett, D. H. Froula, J. P. Palastro

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

Negative and Positive Cascade Rates of Slow Alfvénic Turbulence in Switchback and Non-switchback Intervals: Wind Observations

Sofiane Bourouaine

We estimate the turbulent cascade rate in imbalanced Alfvénic turbulence in the slow solar wind using a Politano–Pouquet-based approach that does not require an isotropic assumption. We analyze selected 8-hour intervals provided by Wind spacecraft. These intervals are characterized by near-homogeneity, weak velocity shear, and imbalanced Alfvénic fluctuations, and are classified as switchback or non-switchback depending on the angular spread of the magnetic field relative to the mean field. Switchback intervals are identified by large magnetic-field rotations, whereas non-switchback intervals exhibit only small angular deviations. Our results show that non-switchback intervals exhibit a positive total cascade rate and power spectra consistent with a $-3/2$ scaling, indicative of a forward energy cascade. In contrast, switchback intervals are characterized by a negative total cascade rate (inverse cascade), and by steeper power spectra with a spectral index of approximately $-1.6$. This indicates that large magnetic-field rotations can substantially modify the scale-to-scale energy transfer in Alfvénic slow solar wind. The results suggest that switchbacks may be associated with additional physical processes, such as local instabilities, or reconnection-related structures, that alter the direction and magnitude of the turbulent cascade. These findings may help explain previous reports of negative cascade rates in imbalanced Alfvénic solar-wind turbulence.
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