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Tuesday 18 August

Plasma Physics

Bright dual-pulse betatron X-ray generation from a laser wakefield accelerator

Bo Guo, Yang Wan, Shuang Liu et al. (+3 more)

Pump-probe experiments using dual ultrashort X-ray pulses provide unique opportunities for resolving non-equilibrium dynamics initiated by intense X-ray excitation. Betatron radiation from laser wakefield accelerators offers femtosecond duration, micrometer-scale source size, and intrinsic synchronization with the driving laser, making it a promising candidate for compact ultrafast X-ray sources. Here, we experimentally demonstrate a high-flux, dual-pulse betatron X-ray source based on a density-tailored gas-mixture target. Two electron bunches are generated within a single plasma wakefield through ionization-induced and shock-front-triggered injection, subsequently producing twin X-ray pulses. The measured electron spectra and dual-component X-ray angular profiles, together with particle-in-cell simulations, identify the contributions of the two electron populations to the radiation. The total X-ray photon yield reaches the level of 10^{10} photons per shot with a 40-TW laser system. These results establish a compact, single-stage route toward high-flux dual-pulse betatron sources for laboratory-scale ultrafast X-ray spectroscopy.
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All authors Bo Guo, Yang Wan, Shuang Liu, Xiaonan Ning, Jianfei Hua, Wei Lu

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

Field deployment of a laser wakefield accelerator for on-site application

Bo Guo, Xiaonan Ning, Dexiang Liu et al. (+7 more)

Successive innovations in particle accelerators have continually expanded the frontiers of scientific discovery. Laser wakefield accelerators promise to transform science, medicine, and industry, yet moving them from laboratory demonstrations to reliable real-world operation has remained a central, long-standing challenge. Here we report a field-deployable system that produced 100-MeV-class electron beams with 1%-level energy stability during 72 hours of continuous operation and supported routine full-power use throughout a seven-month field trial in an industrial setting. Applied to in situ micro-nondestructive testing, the system generated tens-of-MeV bremsstrahlung X-rays that enabled three-dimensional microtomography of dense materials at sub-50-{\mu}m spatial resolution and revealed 100-{\mu}m-scale internal defects in large composite structures, extending the capabilities beyond those of existing high-energy X-ray sources. These results mark a transition of laser wakefield acceleration from laboratory proof of concept toward practical deployment in scientific and industrial applications.
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All authors Bo Guo, Xiaonan Ning, Dexiang Liu, Yue Ma, Weiwang Zeng, Mingyuan Wei, Shengtai Wei, Jianfei Hua, Yang Wan, Wei Lu

Plasma Physics

Sub-Second Collisionless Gyrokinetic Eigenvalue Solutions via Orbit-Invariant Decomposition

Anrui Luo, Jingyi Yu, Huasheng Xie et al. (+1 more)

Fast analysis of microscopic drift-wave instabilities based on linear gyrokinetic simulations is desirable for modeling anomalous transport in fusion devices. In this work, we present an orbit-invariant decomposition method for solving collisionless gyrokinetic eigenvalue problems. By discretizing velocity space along orbit invariants using particle energy and magnetic moment, the full eigenvalue matrix is separated into independent orbit blocks that couple with each other through the field equation, greatly reducing both matrix dimension and computational cost without sacrificing physics. Based on this method, we extend the MGK code [Phys.\ Plasmas 24, 072106 (2017)] with both CPU and GPU implementations, supporting collisionless electrostatic and electromagnetic linear simulations in $s$–$\alpha$ and Miller equilibrium models. For kinetic ion temperature gradient (ITG) and trapped electron mode (TEM) eigenvalue problems, the solver reduces single-solution times to the 0.01–0.1~s range—more than three orders of magnitude faster than CGYRO on the same hardware—enabling efficient large-scale parameter scans. For fully electromagnetic KBM cases, it also achieves a speedup of three orders of magnitude over CGYRO and HD7. The eigenfrequencies and mode structures are verified by comparing with CGYRO results. The method is generally applicable to all collisionless gyrokinetic eigenvalue formulations and has been extended to fully electromagnetic simulations. [Python code available at: https://github.com/FusionAlpha/mgk]
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All authors Anrui Luo, Jingyi Yu, Huasheng Xie, Jian Bao

Plasma Physics

The FLARE Facility

Hantao Ji, Jongsoo Yoo, Peiyun Shi et al. (+53 more)

The Facility for Laboratory Reconnection Experiments (FLARE) has been constructed to study magnetic reconnection in multiple X-line regimes relevant to space, astrophysical, and fusion plasmas. Building upon the successful design of the Magnetic Reconnection Experiment (MRX), FLARE features a larger physical volume, stronger magnetic fields, and an independent ohmic heating drive to significantly extend the accessible parameter space, targeting Lundquist numbers up to S ~ 10^5 and normalized system sizes up to \lambda ~ 10^3. This paper details the facility's core engineering components, including the primary vacuum vessel, internal flux cores, highly segmented external coil systems, modular capacitor banks, and the safety interlock and control architecture. An initial diagnostic suite is presented, comprising high-resolution 2D magnetic probe arrays, triple Langmuir probes, a fully fiber-coupled interferometer, ion Doppler spectroscopy, and fast camera imaging. Initial operations demonstrate the device's experimental flexibility and reliability, successfully executing symmetric push-pull reconnection, spheromak merging, and asymmetric downstream configurations. Currently operating within "Stage 2.5" with S ~ 2,500 and \lambda ~ 60 for anti-parallel reconnection, FLARE provides immediate access to the multiple X-line regimes. Planned hardware upgrades, advanced diagnostic additions, and integration with fully kinetic simulations will further expand its capabilities as it transitions into a collaborative user facility for the broader plasma science community.
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All authors Hantao Ji, Jongsoo Yoo, Peiyun Shi, Euichan Jung, Kush Maheshwari, Adam Robbins, Sunghyun Son, Adam Stanier, Yang Ren, Sayak Bose, Dylan Corl, Keith Corrigan, Robert Cutler, William Daughton, Robert Ellis, Geoffrey Gettelfinger, Ronald Hatcher, Philip Heitzenroeder, Frank Hoffmann, Jonathan Jara-Almonte, Michael Kalish, Thomas Kozub, Enrique Merino, Weiguo Que, Benjamin Smith, John Wallace, Xin Zhao, Sofia Avrutsky, Neal Crocker, Seth Dorfman, Yuka Doke, Corina Dunn, William Fox, Boting Li, Xiaocan Li, Clayton Myers, Shohgo Okazaki, Joshua Pawlak, Tongnyeol Rhee, Byonghoon Seo, Taiju Suzuki, Yang Zhang, Stuart Bale, Amitava Bhattacharjee, Troy Carter, Chuanfei Dong, James Drake, Jan Egedal, Erik Gilson, Michiaki Inomoto, Jonathan Menard, Yasushi Ono, Stewart Prager, John Sarff, Hiroshi Tanabe, Masaaki Yamada

Monday 17 August

Plasma Physics

Real-time feedback control of ELM frequency using divertor gas puffing and its effects on tungsten-induced radiation and plasma performance in KSTAR

Minseok Kim, Young-Ho Lee, SangKyeun Kim et al. (+10 more)

The edge-localized mode (ELM) frequency ($f_{\mathrm{ELM}}$) was successfully controlled in real time on KSTAR using a proportional-integral (PI) feedback controller, employing a $\mathrm{D}_2$ divertor gas puff as the actuator under tungsten lower-divertor conditions. The controller accurately tracked a two-step target—a 30 Hz increase in $f_{\mathrm{ELM}}$ for 4 s, followed by a 30 Hz decrease for 3 s—yielding mean and median absolute percentage errors of approximately 13% and 12%, respectively. Compared to a reference discharge, the actively controlled shot did not exhibit a significant drop in volume-integrated core radiation, confirming that excessive gas use merely degrades overall plasma performance. However, when contrasted with the exponential increase in core radiation observed in the absence of divertor gas puffing, these results underscore the critical need for real-time optimization. Specifically, divertor gas commands must be actively managed to maintain an $f_{\mathrm{ELM}}$ sufficient for flushing tungsten from the core while maximizing global plasma performance.
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All authors Minseok Kim, Young-Ho Lee, SangKyeun Kim, Minwoo Kim, Sang-hee Hahn, Hiro J. Farre-Kaga, Ricardo Shousha, Juhyeok Jang, SooHyun Son, Yoon Seong Han, Junghoo Hwang, Boseong Kim, Egemen Kolemen

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

Bayesian Optimization of Molybdenum-99 Production by Laser Wakefield Acceleration Using Coupled PIC and Monte Carlo Simulations

Bruno Silveira Nunes, Nilson Dias Vieira Junior, Mirko Salomón Alva Sánchez et al. (+2 more)

This work applies Bayesian optimization to a loop composed of PIC simulations of laser electron acceleration and Monte Carlo (MC) simulations of bremsstrahlung-induced nuclear reactions, to maximize the production of molybdenum-99, the precursor of the most used radiopharmaceutical in nuclear medicine, metastable technetium-99. PIC and MC simulations are computationally intensive, and besides reducing the time spent, the Bayesian optimization coupling both simulations resulted in an improvement of an order of magnitude in the $^\text{99}$Mo yield over a previous work, in which the output of an optimization loop based solely on PIC simulations was used a posteriori to estimate $^{99}\mathrm{Mo}$ production through a MC simulation.
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All authors Bruno Silveira Nunes, Nilson Dias Vieira Junior, Mirko Salomón Alva Sánchez, Alexandre Bonatto, Ricardo Elgul Samad

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

In-situ measurements of space plasma: recent progress and future challenges

Daniel Verscharen

Space plasmas like the solar wind or the Earth's space environment offer unique opportunities to observe fundamental plasma processes and their impact in situ. With modern space instrumentation, we measure the velocity distribution function of the plasma particles as well as the electromagnetic fields at high resolution and with minimal perturbation of the observed plasma systems. Plasma measurements like this are often not possible in laboratory settings on Earth. This review article focuses on modern diagnostic methods for the in-situ detection of plasma particles in space. It presents the detection principle of top-hat electrostatic analysers and highlights recent examples of scientific discoveries based on data from the heliospheric space missions Parker Solar Probe and Solar Orbiter. These examples demonstrate the capabilities of modern space plasma instrumentation. The article then discusses future directions in space plasma physics as well as the involved challenges in terms of the required plasma diagnostics. These new developments include, for example, upcoming and proposed space missions such as the operational space-weather mission Vigil, the multi-spacecraft mission HelioSwarm, the Mars mission M-MATISSE, and the electron-astrophysics mission Debye.
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Plasma Physics · Cross-listed from astro-ph.HE

Collisionless Shock Driven by a Supersonic Velocity Shear

Kazuki Kamiido and Yutaka Ohira

The long-term evolution of a relativistic collisionless velocity shear in an unmagnetized electron-positron plasma is investigated using a first-principle particle-in-cell simulation. The Alves instability converts the shear kinetic energy into thermal and magnetic field energy. The resulting pressures push the plasma, leading to the formation of collisionless shocks. The generated collisionless shocks would accelerate high energy particles, which is a possible solution to the injection problem of shear acceleration. In addition, the collisionless shocks generate a magnetic field turbulence that is required for the shear acceleration to work.
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Plasma Physics

New Formalism for Modelling Flowing Plasmas in a Magnetic Field

Subhasish Bag, Ashish Ganguli, Vikrant Saxena et al. (+2 more)

A new formalism for modelling a flowing plasma in a magnetic field, having potential for application in plasma thrusters, materials processing, space plasmas, etc., is developed. In this framework, the plasma expands downstream from the source region into an expansion chamber along an axisymmetric magnetic field. For plasma flowing along a magnetic field, the ion velocity parallel to the magnetic field lines is much greater than the perpendicular velocity. This characteristic permits a unique ordering of the relevant flow variables when the flow equations are transformed into a magnetic coordinate system (MCS), in which the coordinate axes are parallel and perpendicular to the field lines. The ordering of the flow variables in the MCS further simplifies the flow equations by allowing them to be split into sets of reduced equations. As a specific implementation, the proposed formalism was validated by using data (for boundary conditions) from a small volume plasma system experiment (Ganguli et al 2016 Plasma Sources Sci. Technol. 25 025026). The model shows favourable agreement with the experiments and is used to predict various physical quantities relevant for applications. The corresponding physical implications are discussed in detail.
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All authors Subhasish Bag, Ashish Ganguli, Vikrant Saxena, Ramesh Narayanan, Debaprasad Sahu

Plasma Physics

Data-Driven Reconstruction of Spatially Resolved Electron and Ion Energy Distributions from Macroscopic Plasma Quantities with Deep Neural Networks

Libin Varghese, Kaushik Prajapati and Bhaskar Chaudhury

Spatially resolved EEDFs/IEDFs provide essential kinetic information about low-temperature plasmas (LTPs) and play a central role in determining transport, chemical reaction rates, and plasma surface interactions. While kinetic simulations directly resolve these distributions, experimental measurements remain challenging and are often invasive, spatially limited, or require assumptions regarding the distribution shape such as a Maxwellian. However, several macroscopic plasma observables can be measured non-invasively using advanced diagnostic techniques, providing spatially resolved information about the plasma state. An important inverse problem is therefore whether readily measurable macroscopic plasma quantities contain sufficient information to reconstruct the underlying kinetic state. In this work, we investigate this problem by learning a nonlinear mapping from spatially resolved macroscopic plasma observables to the corresponding spatially resolved EEDFs/IEDFs using a deep learning framework. Paired datasets comprising 2D macroscopic observables and spatially resolved EDFs are generated using 2D-3V PIC-MCC simulations. Three representative learning paradigms, a U-Net, a FNO, and a MeshGraphNet, are employed in this study to learn this inverse mapping. The predicted EDFs reproduce both bulk plasma and sheath characteristics with good agreement to the PIC-MCC reference data, with the FNO providing the best overall performance. Beyond conventional metrics, physics-based validation demonstrates that the reconstructed EDFs accurately recover the corresponding density and temperature, and rate coefficients. These results demonstrate that macroscopic plasma observables encode sufficient information to infer important kinetic properties in LTPs, providing a potential foundation for surrogate kinetic modeling and next-generation plasma diagnostics.
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Plasma Physics

Microcoulomb-level electron beam and multi-Joule hard X-rays driven by a high-efficiency laser-plasma accelerator

B. Mahieu, L. Ribotte, W. Cayzac et al. (+38 more)

We report on the production of ultrahigh-charge relativistic electron beams and the development of a laser-wakefield acceleration platform at the LMJ facility. Making use of the kilojoule-class, sub-picosecond PETAL laser pulse focused onto a supersonic helium gas jet, electron beams carrying a total charge beyond 1 $\mu$C were generated, with energies up to $\sim$500 MeV. Given the ps-scale laser pulse duration, an on-target intensity approaching $10^{19}~\mathrm{W/cm^2}$, and a plasma density reaching 2% of the critical density, electron energisation arises from a combination of self-modulated laser wakefield acceleration (SMLWFA) and direct laser acceleration (DLA). The resulting electron spectrum exhibits a Maxwellian-like distribution, characteristic of this mixed SMLWFA/DLA regime. The total energy carried by the electron beam is estimated to be up to 17 J, within a sub-ps duration. A broadband Joule-level photon beam was also produced by Bremsstrahlung, demonstrating the potential for future applications. Experimental results are supported by start-to-end numerical simulations, including 3-D particle-in-cell and Monte-Carlo particle transport calculations. These findings pave the way for applications requiring high-charge electron beams, including the generation of high-power secondary radiation or particle sources. The use of these beams to probe matter in high-energy density states driven by the nanosecond-duration LMJ beams represents another promising avenue.
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All authors B. Mahieu, L. Ribotte, W. Cayzac, G. Boutoux, R. Parreault, J. Gastineau, E. Lamoine, F. Audo, R. Babjak, D. Batani, N. Blanchot, J. L. Bourgade, M. Brochier, T. Caillaud, P. Canel, S. Cavaro, C. Chappuis, S. Debesset, R. Diaz, E. D Humieres, W. Duchastenier, R. du Jeu, A. Duval, B. Etchessahar, M. Ferri, M. Garandeau, L. Gremillet, V. Henot, E. Journot, J. C. Kieffer, I. Lantuejoul, L. Le Deroff, N. Lemos, C. Rousseaux, F. Scol, K. Ta Phuoc, W. Vaillant, B. Vauzour, M. Vranic, X. Davoine, F. Albert

Plasma Physics

Proton Radiography with Isotrajectory Optics

Dmitry Varentsov, Maksym Miski-Oglu, Zs. Major et al. (+3 more)

We consider a compact isotrajectory-optics scheme for proton radiography. The calculation uses a 30-60 MeV proton interval as a reference example, typical of high-energy short-pulse laser acceleration in the TNSA regime. The idea is to use synchronized time-dependent fields such that, for one selected energy-arrival-time branch, the proton energy changes mainly the arrival time and not the transverse image position. The reference lattice consists of four pulsed electric quadrupoles with horizontal sign pattern + - + -, magnification Mx = My = -3.0 at a detector plane of 2.57 m, a common Fourier plane at zF = 0.962 m, and equal outer and equal inner lens lengths. The ideal closure residual is calculated from second central moments. Fourth central moments are kept separately as a diagnostic of non-Gaussian tails. The quoted residual widths are numerical closure tests of the ideal map, not experimental resolution predictions. Real electrode fields, waveform errors, scattering, detector blur, and space charge define the next level of the problem.
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All authors Dmitry Varentsov, Maksym Miski-Oglu, Zs. Major, Paul Neumayer, Martin Schanz, John Schmidt

Plasma Physics

Assessing the Projector Augmented-Wave Method for Stopping Power Calculations

Bryn Lloyd, Dirk O. Gericke, Gilles Rodway-Gant et al. (+1 more)

The stopping power of charged particles is investigated using time-dependent density functional theory (TDDFT). Such simulations are made possible by recent advances in computational resources and numerical implementations of this first-principles method. In practice, DFT simulations widely employ the projector augmented-wave (PAW) method to approximate all-electron behaviour, but the implications of the PAW approximation for non-adiabatic stopping simulations remain insufficiently explored. Here, the suitability of the PAW method for stopping power simulations is evaluated. A workflow for generating and selecting PAW datasets tailored to these simulations is developed, enabling systematic optimisation of augmentation radii and projector constructions. The approach is applied to proton stopping in FCC aluminium, demonstrating how dataset design influences stopping predictions, and enabling an investigation of crystal channelling effects on charged-particle transport.
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All authors Bryn Lloyd, Dirk O. Gericke, Gilles Rodway-Gant, Gianluca Gregori

Sunday 16 August

Plasma Physics

Hybrid kinetic-MHD simulation on the formation of runaway electron current plateau during a current quench process

Weimin Fu and Ping Zhu

Runaway electron (RE) generation is of great concern for high-current tokamak operations. A full-f particle-in-cell (PIC) model for RE dynamics has been developed and coupled with the 3D nonlinear extended magnetohydrodynamic (MHD) model implemented in the NIMROD code. Our model accounts for RE generation using analytical source terms and advances the RE motion along guiding-center (GC) orbits. The model was employed to simulate the formation of RE current plateau during a disruption, in which the plasma current becomes dominated by the RE current. The model agrees well with several RE codes based on fluid models when the RE GC drifts are ignored. For highly relativistic REs, the grad-$B$ and curvature drifts can play a significant role in the RE generation and motion due to the increase in the safety factor profile during the current quench process.
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Plasma Physics

Minimum-q induced alternation between infernal modes and EP-driven modes in advanced tokamak configurations

Shiwei Xue, Ping Zhu and Haolong Li

For an advanced tokamak configuration in the presence of energetic particles (EPs), the dominant instability is found to alternate between infernal modes and Alfvén eigenmodes with the variation of the minimum safety factor $q_{\min}$. For relatively high $q_{\min}$, the mode is identified as a reversed-shear Alfvén eigenmode (RSAE), characterized by its finite Alfvénic frequency and radial localization near the minimum of safety-factor profile. As $q_{\min}$ is further reduced, the dominant branch sequentially transitions through a low-frequency infernal-mode interval, then an energetic-particle-mode (EPM) regime, and finally another low-frequency infernal-mode interval. Increasing the EP beta fraction $\beta_h$ tends to destabilize the RSAE and EPM branches but to stabilize the infernal modes. Phase-space diagnostics further indicate that the destabilizing effects of EPs on the RSAE and EPM branches are mainly associated with trapped-particle drive, whereas the stabilizing effects of EPs on the infernal modes is dominated by passing particles.
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Saturday 15 August

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

Relative and absolute dosimetric commissioning of the ELIMAIA–ELIMED laser-driven proton beamline at 23.4 MeV

G.A.P. Cirrone, G. Petringa, A. Kurmanova et al. (+12 more)

The development of laser-driven proton beamlines for biomedical and radiobiological applications requires traceable dosimetry and reliable online monitoring at the irradiation point. In this work, we report the relative and absolute dosimetric commissioning of the ELIMAIA–ELIMED laser-driven proton beamline at ELI Beamlines using an energy-selected proton beam with an average energy of about 24~MeV. Radiochromic-film measurements were used to characterize the transverse dose distribution, the depth–dose profile, and the proton energy spectrum at the irradiation point. The reconstructed spectrum was centred at $23.45~\mathrm{MeV}$ with a FWHM of $2.60~\mathrm{MeV}$, while the transverse dose distribution showed an approximately $5.5~\mathrm{mm}$ field size with a millimetre-scale homogeneous region. The Faraday Cup was used as the absolute reference detector for dose to water determination and for cross-calibrating the Dual-Gap Ionization Chamber, which was operated as the primary online dose monitor. The Integrating Current Transformer and Secondary Electron Monitor were evaluated as upstream relative beam monitors. An independent RCF–FC dose comparison performed over fifty consecutive shots yielded $30.45 \pm 3.5~\mathrm{cGy}$ from the FC and $36.46 \pm 1.8~\mathrm{cGy}$ from the EBT3 film. Dedicated G4ELIMED Monte Carlo simulations showed that the RCF positioned upstream of the FC perturbs the FC measurement through proton losses outside the FC acceptance and through an increase in the effective beam area at the FC entrance. Applying the resulting correction factor, $C_{\mathrm{MC}}=1.109$, reduced the residual difference between the FC- and RCF-derived doses to about $7\%$. These results establish the dosimetric chain of the ELIMAIA–ELIMED beamline under the present low-fluence commissioning conditions and identify the operational limits of the online monitoring system.
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All authors G.A.P. Cirrone, G. Petringa, A. Kurmanova, R. Catalano, A. Amato, A. Pappalardo, M.G. Guarrera, F. Schillaci, P. Blaha, F. Grepl, M. Tryus, A. Velyhan, L. Giuffrida, D. Margarone, G. Cuttone

Plasma Physics

AlgoPlasma: Open Algorithms for Plasma Modeling

Yinjian Zhao, Zhongping Zhao, Zhe Liu et al. (+9 more)

AlgoPlasma is an open-source library in which core numerical algorithms for plasma modeling are implemented as modular, well-documented, and independently testable components. Rather than offering a complete simulation code, it allows researchers to select, adapt, and assemble the required components into application-specific workflows. The current release is centered on particle-based simulation, while AlgoPlasma is designed to encompass a broader range of approaches to plasma modeling. It provides components for particle initialization and advancement, particle-grid coupling, field solution, collision modeling, neutral transport, selected continuity updates, probe signal analysis, and input/output, together with Message Passing Interface (MPI) exchange routines and selected Open Multi-Processing (OpenMP) kernels. Documentation links mathematical formulations to source implementations, interfaces, and usage, while verification and validation cases evaluate numerical accuracy and physical behavior. AlgoPlasma thus establishes a shared algorithmic foundation for plasma modeling, transforming repeatedly reimplemented numerical methods into open, reusable, tested, and explainable components for research, verification, education, and collaborative development.
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All authors Yinjian Zhao, Zhongping Zhao, Zhe Liu, Baisheng Wang, Zilong Peng, Xin Luo, Lihuan Xie, Xi Chen, Zhijun Zhou, Kunpeng Zhong, Yingjie Chen, Changzheng Hu

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

A self-consistent solar coronal heating model by Alfvenic waves

Hidetaka Kuniyoshi and Richard J. Morton

Alfvenic waves are prevalent throughout the solar atmosphere and are believed to play an essential role in coronal heating, classified as alternating current (AC) heating in contrast to direct current (DC) heating associated with quasi-static magnetic field line braiding. The relative importance of AC versus DC heating depends on the details of the photospheric driver and on the configuration of the magnetic field. Moreover, even if AC heating prevails, several wave dissipation mechanisms have been proposed, and which of them dominates remains unclear, as its efficiency depends on plasma compressibility and density inhomogeneity. We address these issues by performing three-dimensional radiative magnetohydrodynamic (MHD) simulations of a coronal loop spanning from the upper convection zone to the corona, which self-consistently capture many relevant physical processes. We find that the corona is predominantly heated by AC heating, with Alfven wave turbulence providing the primary contribution, accounting for at least 80% of the entire coronal heating in the present simulation. Our results strongly support the use of Alfven wave turbulence-based models employed in space weather and stellar activity research, such as the Alfven Wave Solar Model (AWSoM) and the Magnetohydrodynamic Algorithm outside a Sphere (MAS).
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Plasma Physics

Electron Dynamics in a Wakefield Accelerator Driven by Laser Pulses Carrying Orbital Angular Momentum

Q. Qian, M. Li, Y. Ma et al. (+21 more)

Laser pulses carrying orbital angular momentum (OAM) provide a new degree of freedom for controlling plasma-based accelerators. Here, we experimentally demonstrate OAM-driven laser wakefield acceleration, producing electron beams with a two-beamlet structure that indicates unique azimuthal dynamics inside the plasma wake. Particle-in-cell simulations reproduced the observed spectral features and revealed helical electron trajectories driven by OAM-pulse-driven wakefields. These results show that the laser driver's phase structure can shape electron acceleration dynamics, opening a route to optimal control of beam structure in compact laser-driven accelerators.
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All authors Q. Qian, M. Li, Y. Ma, P. T. Campbell, C-H. Chang, E. Denis, N. Ernst, R. Fedosejevs, B. Hou, A. James, M. W. von der Leyen, K. Krushelnick, C. Kuranz, A. Longman, A. Maksimchuk, J. Nees, P. Norreys, T. Nutting, J. B. Ohland, J. Palastro, A. G. R. Thomas, R. Timmis, L. Willingale, M. Burger

Friday 14 August

Plasma Physics

Bifurcation and Hysteresis in Vacuum Diodes

Ashmita Panda and Allen L. Garner

For electrons with nonzero initial velocity $u_0\ne0$ in vacuum, the space-charge-limited current density (SCLCD) is given by $J_{Jaff\acute{e}}$. For current densities $J>J_{Jaff\acute{e}}$, electron reflections occur along with virtual cathode (VC) oscillations. Past studies have exhibited hysteresis by sustaining these oscillations until $J=J_{hys}$, often claiming $J_{hys}\approx J_{LD}<J_{Jaff\acute{e}}$, where $J_{LD}$, also called the bifurcation solution, represents the steady-state current density where the electron velocity is zero at the VC. In this study, we demonstrate that $J_{LD}$ is not a valid steady-state solution since it represents a higher charge density than $J_{Jaff\acute{e}}$, the true SCLCD. We further demonstrate that $J_{Jaff\acute{e}}$ also corresponds to the true, mathematical bifurcation solution. Using particle-in-cell simulations across various gap distances and voltages, we demonstrate that $J_{hys}$ consistently differs from $J_{LD}$ and can be represented by a simple semi-empirical relationship as a function of $J_{Jaff\acute{e}}/J_{CL}$, where $J_{CL}$ is the SCLCD in vacuum for $u_0=0$.
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Plasma Physics

Limitations of post accelerating ion beams using the snowplow field in a near-critical density target

Davide Terzani, Stepan S. Bulanov, Lieselotte Obst-Huebl et al. (+11 more)

Laser-matter interaction at relativistic intensities is central to almost every scientific case for multi-PW laser facilities. In particular, the progress in laser-driven ion acceleration brings this interaction closer to multiple applications, ranging from material science to biomedical research. One of the most important questions regarding ion acceleration is how to obtain high charge ion beams with relativistic energies. Increasing laser energy and intensity usually leads to a number of limitations on maximum achievable ion energies rooted in the fundamental properties of charged particle interaction with strong electromagnetic fields. Following advances in staged laser-plasma electron acceleration, staged ion acceleration could offer a path to relativistic energies. Here, such scheme is explored and the possibility of scaling the acceleration of ion beams to relativistic energies is discussed.
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All authors Davide Terzani, Stepan S. Bulanov, Lieselotte Obst-Huebl, Carlo Benedetti, Franklin Dollar, Eric Esarey, Axel Huebl, Aodhan McIlvenny, John Palastro, Jessica Shaw, Carl B. Schroeder, Douglass Schumacher, Mingsheng Wei, Louise Willingale

Plasma Physics

Physics of Circular Polarized Ion-Scale Waves in Hybrid Simulations of Alfvénic Fluctuations

Hai Yang Harry Qian, Trevor A. Bowen, Carlos A. Gonzalez et al. (+5 more)

Ion cyclotron waves (ICW) and fast magnetosonic/whistler waves (FMW) are fundamental electromagnetic modes at ion kinetic scales, yet their generation mechanisms and roles in plasma evolution remain poorly understood. We analyze a 2.5D hybrid simulation of broadband Alfvénic fluctuations, where the proton velocity distribution is modeled as a sum of two bi-Maxwellian components: a thermal core and a drifting beam. Using wavelet-based wave identification, bi-Maxwellian VDF fitting, and the PLUME linear dispersion solver, we find that ICW behave as linear modes. Growth is intermittent, occurring when core temperature anisotropy builds up, and is driven mainly by the core (the beam contributes negligibly). Poynting flux analysis shows that ICW are predominantly forward-propagating, with a net energy flux ratio of $+1$ across all frequencies, consistent with the initial condition. FMW present a stark contrast: PLUME solutions often yield very small (near-zero) linear growth/damping rates. The species decomposition breaks down when $|\gamma/\omega_r| \gtrsim 0.368$, indicating that linear theory predicts these waves to be strongly damped and not describable by linear eigenmodes. Nevertheless, FMW are clearly observed in the wavelet helicity spectrogram, indicating that they are generated by nonlinear processes (e.g., parametric decay or phase steepening) and persist despite linear damping. The net energy flux ratio for FMW is close to $+1$ at low frequencies but decreases at higher frequencies, yet never reaches zero (net energy flow remains forward). These results demonstrate that ICW are linear, core-driven waves that transfer energy to the plasma, while FMW are heavily damped, nonlinearly generated waves.
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All authors Hai Yang Harry Qian, Trevor A. Bowen, Carlos A. Gonzalez, Nikos Sioulas, Alfred Mallet, Kristopher G. Klein, Daniel Verscharen, Stuart D. Bale

Plasma Physics

Enhancement of axial magnetic field generation during relativistic self-channeling of laser radiation propagating along thin films

Vitalia A. Kuleshova and Artem V. Korzhimanov

Within the framework of a stationary model of relativistic self-focusing, it is shown that the addition of a thin layer of denser plasma on the propagation axis of a circularly polarized beam increases the axial magnetic field generated by it via the inverse Faraday effect by more than a factor of 10 compared to the case of a homogeneous plasma. The magnitude of the axial field in this case can exceed the magnetic field of the laser wave, which makes it possible, at radiation intensities $\sim 10^{26}$~W/cm$^2$ potentially achievable in the near future, to approach the level of teragauss quasi-stationary fields.
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Thursday 13 August

Plasma Physics

Macroscopic Stability of a Rapidly Rotating Theta Pinch

Richard Fitzpatrick

The macroscopic ideal-MHD stability of an axisymmetric mirror device with sonic levels of plasma rotation is analyzed by approximating the plasma equilibrium as a rotating theta pinch possessing an artificial gravity. An eigenmode equation is derived that governs the stability of the equilibrium to small perturbations in the case of an arbitrary plasma angular velocity profile. The stability of the m=1 and m=2 modes is investigated. The plasma is found to be stable to these two modes provided that it is sufficiently short in the axial direction. The critical axial length of the device below which the modes are stabilized first decreases with increasing plasma rotation, attains a minimum value when the rotation is roughly sonic, and then increases with increasing plasma rotation. The value of the plasma rotation off the magnetic axis is found to have a significantly stronger effect on the stability of the modes than the value on the magnetic axis.
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Plasma Physics · Cross-listed from astro-ph.SR

The Production of Electron-Capture Elements in Thermonuclear Supernovae: Theory vs. Observations

S. Shiber, P. Hoeflich, T. Mera et al. (+9 more)

Type Ia supernovae (SNe Ia) explosively destroy carbon-oxygen white dwarfs (WDs) in multiple stellar systems. They produce approximately 50% of the iron-group elements in the Universe, synthesize electron-capture (EC) elements, drive nuclear physics experiments, and underpin high-precision cosmology. To first order, the outcome is governed by nuclear physics, a property often described as stellar amnesia. Recently, this stellar amnesia has begun to be broken by the nearly universal detection of EC elements with JWST. These elements trace high-density burning, largely ruling out the currently popular helium-triggered, sub-Mch detonation models as the dominant channel. Instead, the ubiquitous presence of EC is shifting back the focus to dynamical and secular mergers, and near-Mch explosions similar to the deflagration model W7, but in which the nuclear flame undergoes a deflagration-to-detonation transition. The early deflagration phase is especially important because spherical simulations identify the central WD density, and thus the WD mass, as a key parameter governing the explosion. Here, we present detailed magneto-hydrodynamical simulations. We find that small-scale, pre-existing turbulence expected from the pre-explosion smoldering phase is essential for overcoming the fundamental challenges imposed by the intrinsic 3D physics. This turbulence systematically reduces the production of EC elements by about a factor of two, implying the need for WD central densities closer to those associated with accretion-induced collapse to a neutron star. We also demonstrate the effect of magnetic fields near the saturation field strength and highlight the need for higher-precision EC rates at low Ye.
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All authors S. Shiber, P. Hoeflich, T. Mera, E. Fereidouni, Z. Levy, D. Maci, C. Ashall, K. Medler, J. M. DerKacy, E. Baron, M. Shahbandeh, C. M. Pfeffer

Plasma Physics

Flow-thermochemistry coupling governs pressure-dependent CO$_2$ conversion in vortex-stabilized microwave plasma reactors: Insights from three-dimensional CFD modeling

Qinghao Shen, Cas van Deursen, Pieter Willem Groen et al. (+2 more)

In this work, a three-dimensional computational fluid dynamics model is developed for a vortex-stabilized microwave CO$_2$ plasma reactor operating over the pressure range of 100-400 mbar. The model combines experimentally constrained, emission-based plasma sizes and volumetric heat-source distributions with thermally dominated finite-rate heavy-particle chemistry for a multi-component mixture. Turbulent flow and transport are described using the SST k-omega model. The model reproduces the measured radial gas-temperature profiles in the plasma core and the non-monotonic pressure dependence of CO$_2$ conversion, including a maximum at 150 mbar and a pronounced decrease at 400 mbar. A vortex-driven recirculation region redistributes gas upstream. Turbulent mixing and cooling are strongest near the upper reactor boundary, but their contribution decreases as pressure increases. The pressure dependence of conversion is determined by the competition between CO$_2$ dissociation and CO recombination. CO$_2$ direct dissociation reaction dominates in the high-temperature plasma core, whereas O-assisted conversion reaction contributes near the plasma edges and in the surrounding hot region. At 150 mbar, enhanced CO$_2$ dissociation is accompanied by limited CO loss, resulting in the highest conversion. With pressure increasing to 400 mbar, slower cooling and more frequent three-body collisions promote CO recombination in the afterglow, causing more than 60% of the CO formed near the plasma to be lost downstream. Moreover, additional CO loss occurs in the upper region of the reactor at higher pressures because of the reduced cooling rate.
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All authors Qinghao Shen, Cas van Deursen, Pieter Willem Groen, Lex Kuijpers, Mauritius C.M. van de Sanden

Plasma Physics

Time-Domain Benchmark Solutions for Bernstein Waves

M. Pelkner and K. Hallatschek

In previous work, we introduced a semi-analytical method for computing time-domain solutions of linearized Vlasov problems. Rather than representing the plasma response as a sum of residues associated with Landau poles, the method constructs a regularized frequency-domain response spectrum that is subsequently inverted numerically. Explicit applications have so far been limited to unmagnetized plasmas. In this work, we extend the construction to plasmas with a uniform background magnetic field and a Maxwellian equilibrium distribution. We apply the resulting formulation to the electrostatic ion-Bernstein density response of a plasma with kinetic ions and adiabatic electrons, and provide error estimates for truncating both the spectral integration domain and the cyclotron-harmonic expansion. The solution serves as a time-domain reference for damped finite-$k_\parallel$ regimes, in which dispersion-relation roots alone are insufficient for pointwise-in-time verification of simulation codes. Finally, we indicate how the framework can be extended to fully electromagnetic problems.
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Plasma Physics

Scalable production of nuclear battery alpha emitters using fusion neutrons

J. F. Parisi

Nuclear batteries powered by alpha decay have been deployed successfully for over 60 years, on a worldwide $^{238}$Pu supply of kilograms per year. We show that the 14 MeV neutrons of a single deuterium-tritium fusion plant can produce alpha emitter battery fuels up to tons per year, in three classes: fuels with completely new production pathways ($^{236}$Pu, $^{227}$Ac, $^{210}$Pb), fuels previously proposed whose scarce feedstock the same pathways now breed at scale ($^{232}$U, $^{228}$Th), and the established $^{238}$Pu. OpenMC simulations of actinide channels in a tokamak blanket give, per GW yr of fusion: 11 to 57 kg of $^{236}$Pu, whose chain releases 18 GJ per gram over a century, ending at stable $^{208}$Pb, plus up to 5.2 t of co-product $^{238}$Pu; up to 1.4 t of $^{231}$Pa from thorium, and, from channel with $^{231}$Pa feedstock, up to $\sim$15 t of $^{232}$U or $\sim$122 kg of $^{210}$Pb, with $^{227}$Ac produced at 21 g/yr per tonne of $^{231}$Pa. Neutron capture also upgrades $^{241}$Am to a $^{242}$Cm/$^{242m}$Am/$^{241}$Am/$^{238}$Pu blend with up to 10 times higher power density. The same $^{236}$Pu and $^{232}$U also serve as proliferation safeguards: the $^{237}$Np, $^{232}$Th, and $^{231}$Pa channel products are self-protecting, the plutonium by $^{236}$Pu and $^{238}$Pu decay heat and the 2.6 MeV gammas from $^{208}$Tl content, and similarly the uranium from its $^{232}$U. Many of these fuels ($^{236}$Pu, $^{232}$U, $^{228}$Th, $^{227}$Ac) have an order of magnitude higher power and energy density than current alpha emitters, and at human spaceflight-relevant doses the $^{227}$Ac and $^{210}$Pb chains need less shield mass than $^{238}$Pu or $^{241}$Am above a few hundred watts. Fusion neutrons could therefore enable nuclear batteries at the kilowatt to megawatt scale and unlock new possibilities for power sources requiring exceptionally high energy density.
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Wednesday 12 August

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

The Wave-Regulated Precursor of a Near-Parallel Interplanetary Shock Observed by Parker Solar Probe

Immanuel Christopher Jebaraj, Lucas Colomban, Oleksiy Agapitov et al. (+7 more)

Diffusive shock acceleration, at shocks from coronal mass ejections to supernova-remnant blast waves, presupposes a scattering wave field that the accelerated particles themselves maintain. This self-regulation has not been resolved in situ. We report Parker Solar Probe observations of a fast (~2800 km/s), near-parallel interplanetary shock at 0.24 AU on 2023 March 13 and separate its upstream wave field into four families, a classification not made before at a fast shock near the Sun. Right-hand and left-hand circularly polarized families over a common wavenumber band, with a field-aligned linearly polarized family, are cyclotron-resonant with the suprathermal-to-MeV protons streaming from the shock: the beam drives the field that scatters it, and the measured mean free path, half the precursor scale, leaves the beam anisotropic enough to sustain the drive. Outside this loop lies a weak, oblique, linearly polarized component, a few per cent of the wave power, resolved here for the first time at an in situ foreshock. Its in-phase density and field-magnitude fluctuations identify the compressive part as fast magnetosonic and shift the cyclotron-resonance energies of the resonant families by up to 13 % along the precursor. Acceleration at shocks inside 0.3 AU is governed upstream, in a foreshock the shock builds for itself.
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All authors Immanuel Christopher Jebaraj, Lucas Colomban, Oleksiy Agapitov, Michael Gedalin, Mikhail Malkov, Athanasios Kouloumvakos, Edin Husidic, Shiladittya Mondal, Sunil Yadav, Nicolas Wijsen

Plasma Physics

Monoenergetic acceleration of charge-neutralized ion bunches to GeV-scale energies by the combination of a high-current electron beam and an ionization front

Jiyuan Chen, Jihoon Kim, Roopendra Singh Rajawat et al. (+1 more)

Compact heavy ion accelerators have numerous applications, ranging from heavy ion fusion to carbon ion radiotherapy, and testing radiation-hardened electronics. The demand could be met by developing high-gradient traveling wave plasma accelerators of high-charge ($\sim\mu\mathrm{C}$) relativistic ion beams. We will discuss a novel ion acceleration regime – Counter-propagating ionization Front Acceleration (CFA) – utilizing counter-propagating Ionization Front (IF) and high-current Relativistic Electron Beam (REB). Theoretical modeling and 3D PIC simulations demonstrate the possibility of using typical REBs produced by induction voltage adders propagating through a gas-filled tube undergoing laser ionization to achieve acceleration gradients in excess of $\sim 250 {\rm MeV/m}$ while accelerating micro-Coulombs of ions over meters distance. A unique energy conversion mechanism – from the REB to electromagnetic fields to the ions is discussed, as well as the limits on the accelerated ions charge and the degree of its neutralization, acceleration gradient, and ion energy spread.
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All authors Jiyuan Chen, Jihoon Kim, Roopendra Singh Rajawat, Gennady Shvets