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Friday 21 August

Plasma Physics

AXUV synthetic diagnostic for ASDEX Upgrade and its application for SPI simulations

Ferenc Lengyel, Weikang Tang, Matthias Hölzl et al. (+14 more)

We introduce an Absolute eXtended UltraViolet (AXUV) diode-based camera forward-modelling tool to support the validation of mitigated disruption simulations and the interpretation of experimental phenomena, with applications to the ASDEX Upgrade (AUG) tokamak. AXUV diodes measure electromagnetic radiation across a wide spectral range with a significantly higher time resolution (~microseconds) than foil bolometers (~milliseconds), albeit with a non-uniform spectral responsivity. AXUV is suitable for examining fast phenomena, such as shattered pellet injection (SPI), where the radiation localisation and radiated power provide information on the deposition of pellet material. Due to the characteristics and degradation of AXUV diodes, absolute power measurements are subject to large systematic uncertainties, especially when the spectra are time-varying, as in e.g. mixed Ne/D2 SPI experiments. These challenges motivated the development of a synthetic diagnostic within the Cherab-Raysect optical modelling framework, which is applied here to four AXUV cameras in two poloidal cross-sections of AUG. The synthetic diagnostic provides a means to understand how the diodes measure radiation under SPI conditions and to connect first-principles plasma simulations with experimental measurements. The details of the synthetic diagnostic are presented, and the capabilities are illustrated with applications to AUG SPI simulations performed in JOREK. The synthetic signals generated from these simulations are compared with experimental measurements from the 2022 SPI campaign and show qualitatively similar features in many respects. Particularly good agreement was found in the time evolution of the studied high Ne-content (10%) pellet, whereas a different, low Ne-content (0.17%) case exhibited more pronounced differences, likely due to the absence of background impurities in the underlying SPI simulations.
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All authors Ferenc Lengyel, Weikang Tang, Matthias Hölzl, Matthias Bernert, Matěj Tomeš, Peter Halldestam, Paul Heinrich, Gergely Papp, Stefan Jachmich, Umar Sheikh, Mathias Dibon, Pascal de Marné, Jörg Hobirk, Thomas Eberl, Gergő I. Pokol, for the ASDEX Upgrade Team, the EUROfusion Tokamak Exploitation Team

Plasma Physics · Cross-listed from math.NA

A Memory-Efficient Adjoint State Optimization Method Based on Time-Reversible Dynamical Low-Rank Approximation

Lukas Einkemmer and Julian Mangott

The primary challenge of conducting PDE-constrained optimization for high-dimensional problems, such as kinetic equations, is the often prohibitive memory cost. Computing gradients using the adjoint state method would require the storage of the entire time history of the forward solution. For such problems, where the memory cost for storing a single instance of the forward solution can already be a limiting factor, this is clearly not feasible. In this paper, we propose a memory-efficient adjoint state method that compresses the forward and adjoint solution with a dynamical low-rank approximation (a model order reduction technique) and bypasses the need to store the entire forward solution by employing a time-reversible low-rank integrator. The dynamical low-rank approach introduces a number of challenges: reversibility can fail in the rank-deficient case and the low-rank trajectories can show chaotic behavior. In particular, the latter has a number of important consequences for the optimization problem. We address those challenges and show that our method can drastically reduce the memory requirement for gradient-based optimization of kinetic equations. In particular, we consider two examples from kinetic plasma physics: optimizing beam profiles to suppress a bump-on-tail instability and shaping a particle beam using external electric fields.
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Plasma Physics

First step toward multi machine ELM energy scalings and extrapolations to SPARC and ITER

R. Perillo, A. Redl, T. Eich et al. (+14 more)

It is shown that the ELM energy loss normalized by the plasma stored energy ({\Delta}EELM/Wplasma) for high-density small/QCE ELM regimes scales inversely with the separatrix turbulence parameter a_t. In contrast, the neoclassical electron collisionality at the pedestal top, nu*e,neo, expected to regulate {\Delta}EELM/Wplasma according to the Loarte scaling (Plasma Phys. Control. Fusion 2003 45 1549), does not adequately capture {\Delta}EELM/Wplasma data for peeling-ballooning-limited type-I ELMs and ballooning-limited small/QCE ELMs, limiting its applicability for extrapolation to one scenario window. A multi-machine database including seven tokamaks and with {\Delta}EELM/Wplasma ranging from 0.5% to 14%, has been analyzed. A regression analysis on only type-I ELMs yields (({\Delta}E_ELM)/W_plasma )_(Type-I ) [%]=6.8*T_(e,ped)^0.03 n_(e,ped)^(-0.4) ąppa}^(-0.4) R_major^0.4, corresponding to {\Delta}EELM/Wplasma =4.5% for nominal SPARC pedestal parameters and 12% for the ITER D-T Q=10 scenario. For the small/QCE ELM class, however, as a_t increases, the pedestal moves toward a ballooning-limited boundary, the toroidal mode number increases, the ELM frequency rises following the scaling f_ELM=46e^((2.25*a_t)), and {\Delta}EELM/Wplasma decreases via the relation ({\Delta}E_ELM)/W_plasma [%]=1.6e^(-({\alpha}_t/2)). For SPARC QCE-relevant a_t=0.86 and ITER high-fueling scenario a_t = 0.64, the scaling favorably predicts {\Delta}EELM/Wplasma of 1.0% and 1.2%, respectively, with values below 1% if the small/QCE ELM regime is pushed beyond a_t >1. The small/QCE ELM-fitted results represent an initial step toward future analysis on broader datasets, which will be necessary to improve the accuracy of projections for future reactor-relevant scenarios.
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All authors R. Perillo, A. Redl, T. Eich, C. J. Lasnier, A. Nelson, R. Rizkallah, D. Silvagni, A. Stagni, J.A. Boedo, A. McLean, P. Traverso, N. Vianello, the DIII-D team, JET contributors, the ASDEX Upgrade team, the TCV team, the EUROfusion Tokamak Exploitation team

Plasma Physics

Reinforcement learning for vertical position control on the EXL-50U spherical tokamak

Lei Xing, Huicong Ma, Changquan Yu et al. (+14 more)

Vertical position control is essential for sustaining high-performance operation in spherical tokamaks, where increased plasma elongation introduces stringent requirements on fast and robust stabilization. This work presents an experimentally validated reinforcement-learning(RL)-based vertical position control framework for the EXL-50U spherical tokamak. A high-fidelity discharge-reconstructed simulation environment is developed by integrating physics-based plasma-circuit models with experimental equilibrium information, enabling systematic controller synthesis and sim-to-real evaluation. Within this framework, RL is benchmarked in simulation against operational proportional–integral–derivative (PID) and model-based linear quadratic regulator (LQR) controllers under identical plant dynamics, actuator constraints, and measurement imperfections.Simulation results show that RL achieves tracking accuracy comparable to PID with consistently lower vertical-stabilization coil effort, while lightweight integral compensation improves robustness against residual model–plant mismatch. The RL controller is subsequently deployed on EXL-50U for closed-loop experiments. Across more than ten discharges with RL takeover, stable vertical regulation is achieved within the controlled windows. For seven representative discharges, RL maintains millimetre-scale tracking accuracy comparable to the operational PID controller (MAE typically ~ 1-5 mm) while consistently reducing actuator effort. These results demonstrate the feasibility of learning-based plasma control on a real spherical tokamak and establish a practical pathway toward future fusion control systems.
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All authors Lei Xing, Huicong Ma, Changquan Yu, Xuanhe Wang, Jiayi Zhi, Pei Guo, Mengyao Li, Zhengyuan Chen, Yapeng Zhang, Guoyang Shi, Dongkai Qi, Xiang Gu, Siqi Ding, Yong Liu, Jianguo Chen, Tianyuan Liu, the EXL-50U Teama

Plasma Physics

Advantage-level Aggregation Reinforcement Learning for X-point Target Magnetic Configuration Control in an EXL-50U Experiment-Calibrated Simulation Environment

Siqi Ding, Xuanhe Wang, Pei Guo et al. (+10 more)

Managing divertor heat loads is a central challenge for compact, high-power tokamaks. To increase local flux expansion and decouple the dissipation volume from the core, EHL-2 adopts the X-point target (XPT) divertor. This requires the secondary X-point to remain on the divertor leg; displacement degrades the topology and exhaust geometry. Current experiments, including EXL-50U discharges, rely on precomputed feedforward waveforms with PID loops on global quantities. Lacking dedicated closed-loop feedback for the secondary null, XPT operation is repeatable but not routine. We formulate XPT feedback as a multi-objective reinforcement learning (RL) control problem in a free-boundary environment calibrated to EXL-50U discharge #13906. To address strong coupling among plasma current, shape, and null constraints - where reward scalarisation collapses objective-specific temporal credit - we develop Advantage Aggregation (AdvA). AdvA preserves objective-wise temporal credit before worst-objective-aware nonlinear scalarisation and introduces a residual correction to policy updates. AdvA-PPO is evaluated against Reward-PPO and a feedforward-plus-PID baseline under nominal operation, measurement uncertainties, and unseen initial equilibria. On a 500 ms rollout, AdvA-PPO raises the mean worst-channel score from 0.23 to 0.81 over Reward-PPO, reducing X-point flux RMSE by ~20x. Under combined measurement uncertainties, it is the only learned controller completing the horizon while retaining a usable XPT shape. Multi-initialization fine-tuning enables a single AdvA-PPO policy to complete full-horizon operation across divertor and limiter initial equilibria. These results provide a simulation-based foundation for future real-time XPT validation on EXL-50U.
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All authors Siqi Ding, Xuanhe Wang, Pei Guo, Guoyang Shi, Changquan Yu, Yiting Wang, Xianming Song, Xiang Gu, Zhengyuan Chen, Lei Xing, Yapeng Zhang, Jianguo Chen, Tianyuan Liu

Plasma Physics

Two-fluid effects on the nonlinear dynamics of RFP relaxation

Wentan Yan, Ping Zhu, Hong Li et al. (+2 more)

This study investigates the role of two-fluid effects during magnetic relaxation in reversed-field pinch (RFP) plasmas. Within the multiple-helicity (MH) regime, two-fluid simulations produce distinct sawtooth oscillations, in contrast to the sawtooth-free state obtained in single-fluid simulations. Analysis of the magnetic field aligned projection of Faraday's law reveals that, tearing modes collectively generate a dynamo electric field that sustains the magnetic relaxation, a process analogous to the flux-pumping in tokamaks. Despite the reduced linear tearing-mode growth rates, stronger two-fluid effects produce more pronounced sawtooth activity over the parameter range considered. Modal energy analysis shows that Hall-mediated nonlinear energy redistribution disrupts the coherent tearing-mode dynamics required to sustain steady flux-pumping, thereby facilitating intermittent reconnection. This transition is interpreted as a Hall-mediated dynamical bifurcation between steady flux-pumping and quasi-periodic sawtooth relaxation.
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All authors Wentan Yan, Ping Zhu, Hong Li, Wandong Liu, Bing Luo

Plasma Physics

Power-law-anchored residual learning for H-mode energy confinement time in tokamaks: interpolation and parameter-defined extrapolation

Zhaokun Wang, Tianyuan Liu, Jianguo Chen et al. (+4 more)

Reliable prediction of the energy confinement time is essential for magnetic-confinement fusion. Conventional power-law scalings provide constrained extrapolation trends but cannot represent complex nonlinearities, whereas neural networks interpolate accurately but may behave unpredictably outside the training distribution. We propose a unified power-law-anchored residual-learning framework in which a frozen empirical power-law scaling supplies the global trend and a nonlinear model learns only the systematic residual in logarithmic space. PLR-KAN is developed as the primary implementation, while a parameter-matched PLR-MLP serves as a controlled architecture replacement. Using the ITPA DB5.2.3 H-mode confinement database, we evaluate interpolation and parameter-defined held-out cohorts over ten complete training pipelines. PLR-KAN retains near-best interpolation accuracy, achieving R2=0.9671+/-0.0027, while substantially improving the stability of direct KAN under parameter-defined distribution shifts. It outperforms direct KAN across all five non-epsilon single-parameter-defined cohorts and the core-five joint cohort, reaching R2=0.9263+/-0.0157 in the latter. Results from PLR-MLP further demonstrate that the benefit of power-law anchoring is not specific to KAN, although the effectiveness of residual transfer remains architecture and direction dependent. As an exploratory extension, a Mahalanobis-distance-based prediction-time gate improves stability in selected shifted regions but is not universally beneficial and cannot compensate for missing device or physics-regime coverage. Overall, power-law-anchored residual learning provides a practical balance between nonlinear interpolation capability and empirically constrained extrapolation behavior.
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All authors Zhaokun Wang, Tianyuan Liu, Jianguo Chen, Guoyang Shi, Siqi Ding, Yuejiang Shi, Xianmei Zhang

Plasma Physics

Zero-dimensional multi-physics-constrained parameter design and optimization for advanced quasi-isodynamic stellarators

Ziyuan Sun, Zixuan Guo, Xianglin Hao et al. (+3 more)

A zero-dimensional (0D) multi-physics-constrained framework for parameter design and optimization of Stable Quasi-Isodynamic Designs (SQuIDs) is presented. Single- and multi-objective optimizations for three staged devices are carried out using an in-house stellarator 0D systems code: YF-1 for discharge demonstration, YF-2 for scientific break even, and YF-3 for a commercial demonstration plant. Pareto searches map the main design trade-offs across the three generations. The equal weight optima for YF-2 and YF-3 both lie in the electron-root favorable regime of the adopted root proxy: YF-2 recovers $Q_{phys} \sim 1$, while YF-3 reaches an ignited point at reactor scale. Future work will couple engineering feasibility and economic assessment modules for integrated plant evaluation.
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All authors Ziyuan Sun, Zixuan Guo, Xianglin Hao, Longjun Qin, Qian Liu, Xiang Teng

Plasma Physics · Cross-listed from physics.optics

Gas Beam Dump and Power Meter for High Energy Lasers

Harsha Rajesh, Sida Cao, Ke Ou et al. (+3 more)

Gaseous optics exhibit substantially higher laser-induced-damage thresholds than solid materials, enabling high-intensity operation without permanent degradation. In this Letter, we describe a gas-based beam dump and power meter that exploits the strong absorption of ozone in the Hartley band (200-320 nm). We develop and experimentally validate a model for ozone absorption that predicts the propagation length required for complete energy deposition and the resulting gas temperature rise. By calibrating the temperature rise against incident pulse energy, we demonstrate accurate millijoule-scale pulse energy measurements with an all-gas absorber. These results establish gas-phase absorbers as a scalable, debris-free platform for high-damage-threshold beam dumps and energy diagnostics for high-fluence lasers.
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All authors Harsha Rajesh, Sida Cao, Ke Ou, Devdigvijay Singh, Pierre Michel, Matthew R. Edwards

Thursday 20 August

Plasma Physics

Gas release from metals under irradiation with elliptic Gaussian laser beam during LID-QMS analysis

A.A. Stepanenko and Yu.M. Gasparyan

Laser-induced-desorption quadrupole-mass-spectrometry (LID-QMS) diagnostics is considered as one of the candidate methods for the remote control of tritium inventory in the ITER first wall. Studies involving LID-QMS generally assume the circular shape of the laser spot on the analyzed surface. At the same time, the diagnostics laser source cannot be always positioned so as to irradiate tokamak tiles under normal angles, which results in the laser spot shape differing from the circular one. In this contribution, we analyze the tritium removal process under sample irradiation by an elliptic Gaussian laser beam, extending the results of our previous analysis [A.A. Stepanenko, Yu.M. Gasparyan, Physica Scripta 99 (8), 085604 (2025)]. The thermal desorption model governing the heat transport and tritium removal from the solid is formulated. The new analytical expression describing the sample temperature dynamics is derived. The developed model is used to examine the impact of the laser beam/spot ellipticity on the tritium desorption process from a tungsten sample. The conditions, under which the elliptic beam can be approximated with the circular one for the rapid assessment of the amount of desorbed tritium, are assessed.
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Plasma Physics

Excitation of the lower-hybrid drift instability in the outflow of electron-only magnetic reconnection

B. K. Russell, K. Sakai, Y. Zhang et al. (+17 more)

We report experimental evidence for the lower-hybrid drift instability in the current sheet normal direction of electron-only magnetic reconnection. In our laser-driven capacitor-coil experiment, the system size ($\sim$3 ion skin depths) places it in the electron-only regime. Yet, Thomson scattering reveals out-of-plane electron drift oscillations at the local lower-hybrid frequency, with kinetic energy density reaching $\sim$18% of the local magnetic energy density. Linear theory with the measured parameters predicts more than ten e-folding times of growth, indicating that the instability reaches the nonlinear regime within the measurement window. Supported by particle-in-cell simulations, these results demonstrate the importance of ions in the dissipation and energy transfer in electron-only reconnection where their significance has not been previously recognized.
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All authors B. K. Russell, K. Sakai, Y. Zhang, L. Gao, E. G. Blackman, W. Daughton, C. Dong, J. Katz, S. R. Klein, C. C. Kuranz, X. Li, X. M. Li, A. L. Milder, J. Ng, K. Orr, G. Pomraning, J. P. Schell, A. Stanier, J. Yoo, H. Ji

Plasma Physics · Cross-listed from physics.optics

Wavefront shaping of terahertz radiation using two-color flying-focus pulses with time-dependent focal velocities

A.L. Elliott, H. Markland, K.G. Miller et al. (+3 more)

Properly phased two-color laser pulses drive photoionization currents that emit broadband THz radiation. With a conventional two-color pulse, the ionization front travels at a nearly constant superluminal velocity, generating THz radiation with conical wavefronts at a Cherenkov-like angle. Here we show that the dynamic intensity peak of a two-color ultrashort flying-focus pulse can be used to control the shape of the THz wavefronts. Simulations demonstrate that non-uniform motion of the intensity peak and the ionization front it drives result in a time-dependent emission angle that determines the wavefront shape. A decelerating intensity peak, in particular, enables the generation of THz radiation with parabolic wavefronts that are well suited for collection and focusing.
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All authors A.L. Elliott, H. Markland, K.G. Miller, S.W. Jolly, J.J. Pigeon, J.P. Palastro

Plasma Physics

Symmetry-agnostic stellarators for collisionless confinement

W. Sengupta, A. Bhattacharjee and S. Buller

Quasisymmetry, omnigenity and piecewise omnigenity confine trapped particles by making the bounce action independent of the field-line label. Recent optimizations produce mixed-symmetry stellarators that confine alpha particles well without them. We propose a general theory for them. From Whitham modulation theory we define iso-action, which requires only that the drift surface close and allows misalignment with flux surfaces. A solvable model supplies an exact relation between trapped segments while branch actions vary. We develop a proxy $\Gamma_W$ for the reach that misalignment costs.
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Plasma Physics

Effect of ion temperature on lunar photoelectron sheath

Trinesh Sana and Sanjay K. Mishra

Observations suggest that the solar wind ions impinging on the lunar surface possess a finite temperature. The effect of ion temperature on the plasma sheath over the lunar surface is investigated in this paper. To account for thermal effects, a thermal pressure term has been added to the ion-flow force equation. Quantitative estimation of sheath characteristics has been performed by solving the Poisson equation, accounting for photoelectrons, solar wind electrons, and warm ions. Notably, the presence of warm ions within the sheath, i) changes the potential, field, sheath population density structures, ii) changes the sheath thickness, iii) reduces the photoelectron trapping, and iv) reduces solar wind electron reflection, compared to cold ions. The effect is more prominent at high lunar latitudes, which in turn may significantly modulate the dust dynamics.
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Plasma Physics

Simulation of trapped non-neutral plasma dynamics with rotating wall compression

Luisa Riik and Oliver Boine-Frankenheim

A spectral macroparticle simulation scheme is used to model non-neutral plasma dynamics in a Penning-Malmberg trap. This grid-free numerical scheme is well adapted to the specific geometry of trapped plasmas, with varying longitudinal and transverse profiles in a cylindrical conducting pipe. The cutoff spectral harmonics provide control over the trade-off between accuracy and noise smoothing, which is especially important in the three-dimensional case. We demonstrate the ability of this scheme to obtain the dispersion relation of the plasma modes from the inherent simulation noise, providing a valuable tool for understanding the plasma's behavior. In the presence of a rotating wall drive, we show that the numerical scheme can reproduce the time evolution of the driven eigenmodes, identify the onset of mode mixing with increasing drive amplitude and retrieve the compression rate dependency on the drive frequency, known from the weak drive regime of the rotating wall technique. To the authors' knowledge, such simulations have not been reported in the literature so far.
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Plasma Physics

Revisiting the Growth Rate of the Relativistic Tearing Instability: The Role of the Non-ideal MHD Structure

Kaoru Sugimoto, Kunihito Ioka and Masaru Shibata

Magnetic reconnection in magnetically dominated pair plasmas is a key process in high-energy astrophysical systems. We revisit the relativistic tearing instability in a Harris current sheet and derive an improved analytical expression for its linear growth rate and the most unstable wavenumber. The key modification is the treatment of the vector potential perturbation in the non-ideal magnetohydrodynamic (MHD) region. Instead of the conventional constant-A approximation, we use an extrapolated-A approximation, in which the ideal-MHD solution is linearly extrapolated into the non-ideal region. Comparison with two-dimensional particle-in-cell simulations shows that the revised theory improves the prediction of the most unstable wavenumber. The improvement is most pronounced at low particle drift velocities, where the particle gyroradius is smaller than the current-sheet thickness and the fastest-growing mode shifts to longer wavelength. The resulting analytical expressions provide an updated benchmark for magnetically dominated reconnection and its applications to high-energy astrophysical plasmas, including gamma-ray bursts and fast radio bursts.
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Plasma Physics

Collisionless Resonances Set the Size of a Weyl Exceptional Ring

Xianhao Rao, Hong Li and Xuan Sun

Eliminating continuum degrees of freedom converts conservative dynamics into a dispersive, non-Hermitian response whose topology need not represent the causally selected kinetic dynamics. In collisionless magnetized plasma, retarded Landau and cyclotron resonances damp two Weyl modes differently and replace a cold Weyl point by an exceptional ring. Axial roots and root-normalized polarization mixing predict its off-axis radius, $R_{\rm EP}^{\rm pred}=|\Delta\Gamma|/(2|v_{\rm mix}|)$, before any double root is computed. Across five nonrelativistic scans spanning a 310-fold range of differential damping, $R_{\rm EP}^{\rm pred}$ agrees with the nonlinear double-root radius $R_{\rm EP}^{\rm full}$ within $0.12\%$. The selected retarded kinetic-response bundle is a continuous graph over the electric root line bundle and has the same first Chern class under stated conditions. The cold interface mode continues to a localized Keldysh pole with one oriented crossing in the tested line-gap window, consistent with the inherited charge.
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Plasma Physics

Characterizing the temporal evolution of Biermann-battery-driven magnetic reconnection in laser-ablated plasmas

T. Morita, Y. Muramoto, S. Isayama et al. (+30 more)

This paper presents an experimental investigation of magnetic reconnection between two laser-produced expanding plasmas, focusing on the quantitative evaluation of the reconnection rate and energy conversion under varying initial conditions. By changing the separation distance between the drive laser focal spots (1 mm and 2 mm), we systematically controlled the inflow parameters. The reconnection region was probed using a two-directional laser Thomson scattering (LTS) system, which simultaneously measured the local plasma parameters parallel to the outflows and along the current sheet. Based on our established method incorporating macroscopic energy and mass conservation laws to derive the upstream magnetic field directly from LTS spectra, we characterized the temporal evolution of the current sheet and the reconnection rate. The larger spot separation allows the plasma bubbles to expand for a substantially longer time before the formation of a reconnection current sheet, resulting in different upstream conditions. Nevertheless, both configurations yielded comparable upstream magnetic fields and reconnection rates. These results suggest that the reconnection rate is relatively insensitive to the global inflow conditions and is primarily controlled by the local physics of the reconnection layer once a current sheet is formed.
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All authors T. Morita, Y. Muramoto, S. Isayama, M. Edamoto, M. Hanano, R. Ishikawa, Y. Kanesada, K. Koba, H. Kondo, S. Kurimaru, K. Maeda, Y. Maenosono, S. Matsukiyo, A. Morita, Y. Nagamatsu, G. Nakayama, T. Ogawa, K. Oshida, Y. Pan, K. Sakai, T. Sano, Y. Sato, N. Shimoda, J. Shiota, Y. Sudo, Y. Suzuki, T. Takezaki, S. J. Tanaka, K. Tomita, S. Ueno, S. Yakura, R. Yamazaki, Y. Sakawa

Wednesday 19 August

Plasma Physics

Determining neutral fueling response to RMPs in MAST using a multi-reservoir particle balance model and EMC3 EIRENE

Kurt Flesch, James Harrison, Andrew Kirk et al. (+4 more)

The application of certain configurations of resonant magnetic perturbations (RMPs) has been shown to cause a plasma pump-out in both L- and H-mode discharges at MAST. In this paper we discuss the impact of neutral fueling on this density reduction. The neutral fueling and average particle confinement time $\tau_p$ of the main ion species were calculated using a 0-D particle balance analysis. When the RMPs were applied, it was found that there was an increase in ionizations and a 15$\%$ reduction in $\tau_p$ for L-mode discharges and a similar result for inter-ELM (edge localized modes) periods of H-mode discharges. A time-dependent global multi-reservoir particle balance (MRPB) was developed, which included atomic and molecular reservoirs, to further investigate the role neutrals had on the density change. We discuss how this model was able to accurately reproduce the experimentally measured density reduction and ionization increase due to either a reduction in $\tau_p$ or a reduction in particle fueling efficiency. Results from EMC3-EIRENE modeling indicate this change could be attributed to neutral particle fueling occurring in locations with now-opened field lines due to the chaotic edge-region from RMP applications.
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All authors Kurt Flesch, James Harrison, Andrew Kirk, Ian Waters, Heinke Frerichs, Oliver Schmitz, Livia Casali

Plasma Physics

Informing spectral models for dense plasmas with K-edge absorption measurements of warm dense copper

T. Cordova, E. V. Marley, D. A. Chin et al. (+14 more)

Warm dense matter remains a challenging regime to characterize experimentally and to model with predictive accuracy. Recent experimental platforms have been developed to generate, characterize, and diagnose uniform warm dense matter, enabling detailed comparisons with models. Here, we present experiments conducted at the OMEGA laser facility that compress and heat a buried layer target to warm dense matter conditions, where the targets are heated to temperatures of approximately 20 eV and compressed to densities of 25 g/cm^3. We probe the warm dense plasma using x-ray absorption spectroscopy, using the K-edge and bound-bound absorption features to constrain the temperature and charge state distribution of the plasma. We compare these measurements with two types of models: collisional-radiative models with detailed electronic structure and ad-hoc density effects, and a multi-ion model based on density functional theory in combination with excited-state projector augmented-wave potentials. Neither approach fully reproduces the observed data, We show that the broad structure and position of the K-edge region can be modeled using density functional theory in combination with excited-state projector augmented-wave potentials. The density functional theory results are contrasted with a collisional-radiative model approach that incorporates ad-hoc density effects, which show incomplete agreement with the experimental observations, highlighting a need for improved density-dependent atomic modeling in warm dense plasmas.
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All authors T. Cordova, E. V. Marley, D. A. Chin, R. A. London, S. B. Hansen, S. M. Vinko, J. E. Pask, H. A. Scott, H. P. Le, D. Aberg, M. K. G. Kruse, T. Döppner, F. N. Beg, J. Emig, P. M. Nilson, P. Sterne, M. J. MacDonald

Plasma Physics

High-power TCV scenario for conventional and alternative divertor studies

K. Lee, C. Theiler, M. Carpita et al. (+20 more)

Alternative divertor configurations (ADCs) must be evaluated under boundary plasma conditions approaching reactor-level values to be considered a reliable, physics-based solution for tokamak power exhaust. Most ADC experiments performed to date were at relatively low exhaust power. This work presents a high-power scenario on the TCV tokamak enabling the study of a wide variety of divertor magnetic shapes under an expanded SOL and power exhaust parameter space. The scenario is characterized by high power levels of electron cyclotron resonance heating ($2.5\,\text{MW}$ fully absorbed in a $\sim1\,\text{m}^{3}$ plasma) at high plasma current (edge safety factor $q_{95}\approx 2.5$), and low upstream separatrix densities ($n_{e,\text{u}}\approx1\times10^{19}\,\text{m}^{-3}$, Greenwald fraction $f_{\text{G}}\approx 0.1$). Stationary parallel heat fluxes up to $100\,\text{MW m}^{-2}$ are measured at the divertor target, an order of magnitude above previous TCV power exhaust studies. The obtained SOL collisionality and Lengyel detachment scaling metric lie within range of values expected in future reactors (SPARC, ITER, ARC).
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All authors K. Lee, C. Theiler, M. Carpita, M. Zurita, P. Sintre, O. Février, F. Pastore, H. Reimerdes, K. Verhaegh, M. Winkel, D. Brida, B.Y.K. Brown, M.J.H. Cornelissen, R. Ducker, G. Durr-Legoupil-Nicoud, D. Hamm, R.I. Morgan, A. Perek, O. Sauter, E. Tonello, Y. Wang, the TCV Team, the EUROfusion Tokamak Exploitation Team

Plasma Physics

Trapping: The "Waterfall" of Vlasov-Poisson Dynamics and the Post-Transient Emergence of Drift-Independent Hole Structures in Collisionless Plasmas

Hans Schamel

A key aspect of structure formation in collisionless plasmas is particle trapping or, more precisely, the gap this process leaves in the theoretical description. While the dynamics prior to the onset of trapping are governed by Landau physics and phase mixing, the full effects of nonlinearity only emerge during the trapping phase, for which an explicit description is lacking. Consequently, this gap divides the temporal evolution into a linear Vlasov-Poisson phase preceding the transition and a post-transient Vlasov-Poisson phase characterized by a wide spectrum of particle trapping scenarios and associated Schamel equilibria. Metaphorically, this trapping process can thus be compared to a waterfall in fluid dynamics, where, similarly, no explicit dynamic link can be established between the regions above and below the cascade. To resolve this problem of the gap, an analysis using the method of matched asymptotic expansions is proposed. Applied to the singular boundary of the separatrix zone, this approach employs local smoothing techniques, such as those mediated by the two-particle correlation function. It offers a suitable mechanism for selecting hole equilibria in the post-transition phase. Furthermore, we report the existence of a spatially periodic Langmuir hole that vanishes if regularity requirements are too stringent, and we demonstrate that self-acceleration, accompanying the transition from a slow electron acoustic hole to a fast Langmuir hole, is driven by the release of particularly deeply trapped electrons.
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Plasma Physics

Beam-Tracing-Based Quantitative Reconstruction of Density Fluctuations in QUEST Using Doppler Backscattering

T. Kinoshita, T. Tokuzawa, V. H. Hall-Chen et al. (+8 more)

A three-channel X-/Ku-band Doppler backscattering (DBS) system has been developed and installed on QUEST for turbulence and electric-field measurements. In spherical tokamaks, the large magnetic-field pitch angle increases the geometric mismatch between the probing beam wave vector and the local magnetic-field vector, reducing the effective perpendicular projection and resulting in a systematic underestimation of the measured scattering intensity. In addition, in QUEST, where low plasma density requires a low-frequency probe beam, beam propagation effects become increasingly significant, further complicating the interpretation of the measured DBS power in terms of local density fluctuation amplitude. To address these issues, a quantitative correction methodology based on the synthetic DBS code SCOTTY was established. All relevant diagnostic response effects were evaluated using SCOTTY along ray trajectories, yielding a correction factor for reconstructing the local turbulence amplitude from the measured scattering signal. The correction factor exhibits strong spatial and frequency dependence, varying by up to an order of magnitude between the plasma core and edge regions, highlighting the necessity of frequency-dependent corrections. By applying the derived correction factor to experimental measurements, quantitative density fluctuation amplitudes were reconstructed from the detected scattering signals. Evaluation of the fluctuation amplitude indicates enhanced turbulence activity in the plasma edge region, where a finite negative radial electric field is inferred. This work demonstrates the first quantitative turbulence evaluation using low-frequency X-/Ku-band DBS measurements in QUEST and establishes a framework for quantitative DBS analysis in spherical tokamaks.
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All authors T. Kinoshita, T. Tokuzawa, V. H. Hall-Chen, Y. T. Tan, T. Ido, H. Idei, R. Ikezoe, K. Hanada, M. Hasegawa, T. Onchi, Y. Peng

Tuesday 18 August

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

Evolution of lunar wake potentials: structure, energy conversion, and their imprints on velocity distributions

Xin An, Vassilis Angelopoulos, Jasper S. Halekas et al. (+4 more)

We study the evolution of electric potentials in the lunar wake. The wake potential exhibits two distinct spatial scales. The macroscopic scale arises from solar wind expansion into the vacuum, with a potential length-scale growing with distance from the Moon; the microscopic scales arises from ion acoustic shocks near the wake center, with transition layers spanning tens of local Debye lengths. This two-scale potential mediates energy conversion between ions and electrons during wake refilling. The macroscale potential retards electrons and accelerates ions to supersonic velocities, converting electron thermal energy to ion kinetic energy. The microscale potential then decelerates ions to subsonic velocities and heats both species, converting ion kinetic energy back to thermal energy. Together, the two-scale potential imprints distinct signatures on velocity distributions, including ion beams and electron flat-top distributions, consistent with ARTEMIS observations.
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All authors Xin An, Vassilis Angelopoulos, Jasper S. Halekas, Terry Z. Liu, Shaosui Xu, Andrew R. Poppe, Ferdinand Plaschke

Plasma Physics

Plasma dynamics near the magnetic X-point of the two-wire model: Theory and Simulation

Bin Ahn

The two-wire model (TWM) is a magnetic configuration generated by two parallel current-carrying wires, and it contains an X-point at its center and a separatrix. Since the TWM magnetic field is described by a closed-form analytic expression and contains no guide field, it offers a tractable setting for studying how a true magnetic null shapes plasma dynamics. This work investigates two complementary regimes: collisionless charged particle dynamics and collisional low temperature plasma transport. In the collisionless regime, a Lagrangian analysis identifies two particle motion invariants: the total kinetic energy and the base field line value, which is derived from the conserved axial canonical momentum. Collisionless test particle simulations show that the magnetic moment undergoes shifts when the particle traverses the large gradient region near the null. These shifts enable particles to migrate, the phenomenon in which a particle gyrating about one branch of a base field line jumps to the corresponding branch on the other side of the X-point. A threshold energy for migration is derived, and an empirical expression for the migration confinement time is formulated. In the collisional regime, reduced drift-diffusion models for low temperature plasmas are developed in a conformal field-aligned coordinate system, and they predict density plateau formation near the separatrix in the strongly magnetized regime. Self-consistent particle-in-cell simulations are performed to verify the predicted density plateau. The two complementary studies establish a fundamental understanding of plasma dynamics near a true magnetic null for both the collisionless and collisional regimes.
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Plasma Physics

Neural network predictions of plasma confinement loss in Wendelstein 7-X pellet-fueled discharges

K. C. Hammond, J.-P. Bähner, J. Baldzuhn et al. (+17 more)

The energy confinement time is a key parameter of a magnetized fusion plasma, helping to determine whether ignition can occur. Experiments in tokamaks and stellarators have shown that the confinement time can be improved via pellet injection. The state of enhanced confinement brought about by a given pellet typically deteriorates over time unless and until a subsequent pellet is injected. In this work, we develop a data-driven model that predicts, at any moment, the remaining time before a plasma in Wendelstein 7-X (W7-X) will lose its enhanced confinement state. This "remaining time" metric effectively sets a deadline for when the next pellet must be injected in order to steadily maintain a high confinement time. We describe the development and training of the model and compare its predictions to observations from previous experiments. At least 90% of the model predictions are accurate to within 51 ms, which is below the typical W7-X energy confinement time as well as the minimum time separation between subsequent pellet injections. The model can be evaluated rapidly and could be suitable for use in a control system that optimizes the pellet injection rate in real time.
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All authors K. C. Hammond, J.-P. Bähner, J. Baldzuhn, S. Bozhenkov, K. J. Brunner, A. Dinklage, E. Edlund, G. Fuchert, M. Huck, A. I. Mohammed, N. Pablant, M. Porkolab, G. L. Schmidt, J. Smoniewski, T. Stange, N. Tamura, E. Villalobos Granados, A. von Stechow, G. Weir, the W7-X team

Plasma Physics

Numerical investigations of low-latitude ground magnetic fields due to cavity mode oscillations

Khilav Majmudar, Robert L. Lysak and Kazue Takahashi

Pi2 oscillations have been known to be associated with cavity mode oscillations (CMOs) in the inner magnetosphere at low latitudes. CMOs can also be excited by interplanetary shocks on the dayside. We present results from a global linear MHD wave model in spherical coordinates that allows us to analyse these waves at equatorial latitudes. We see that the model reproduces some of the principal CMO harmonics. The standing wave cavity modes are set up due to a dense plasmasphere. The CMOs associated with interplanetary shocks are compared with ground observations from the EMMA array at mid-latitudes. In accordance with observations of Pi2 pulsations, we also see a small phase difference between compressional mode Alfvén waves at midnight at low altitudes and on the noon ground, both at equatorial latitudes. This supports the finding that upon being excited in the nightside inner magnetosphere, these waves travel quickly around to the dayside. We compare our results to observations of the night-to-day time delay of ground Pi2 pulsations.
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Plasma Physics

Control of Magnetic Reconnection in High Energy Density Plasmas

J. L. Latham, B. K. Russell, C. Dong et al. (+6 more)

Magnetic reconnection governs the explosive release of magnetic energy in systems from the solar corona to fusion plasmas, yet controlling it in the laboratory has remained out of reach. Here we demonstrate active control of reconnection in high-power laser-driven plasmas using a third, relativistic-intensity laser pulse that injects filaments of electron current into the reconnecting system. Two moderate-intensity lasers drive colliding magnetized plumes that reconnect, forming plasmoids in the current sheet as seen in proton deflectometry. The relativistic laser generates magnetic fields matching the polarity on either side of the layer, and, depending on its arrival time, either accelerates the breakup of the current sheet or suppresses reconnection. Arriving early, before the plumes strongly interact, it builds a pocket of magnetic pressure that repels them via flux pileup; arriving after the current sheet forms, it accelerates electrons that extend current filamentation instabilities into the upstream, causing rapid dissipation of the reconnecting magnetic field. This approach opens a route to steering magnetic energy flow in fusion plasmas and broadens the range of systems accessible to laboratory astrophysics.
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All authors J. L. Latham, B. K. Russell, C. Dong, C. A. Walsh, K. G. Miller, P. T. Campbell, L. Willingale, P. Nilson, K. Krushelnick

Plasma Physics

Tunable high-charge relativistic electron beams via direct laser acceleration in hohlraum-preheated foam targets

Ziyao Wang, Jieru Ren, Zhigang Deng et al. (+28 more)

Direct laser acceleration (DLA) in near-critical-density (NCD) plasmas can efficiently generate high-charge relativistic electron beams, yet beam parameters depend critically on precise plasma state manipulation. Solid-ablation NCD plasmas evolve rapidly, posing severe controllability challenges. We produce NCD plasma via indirectly heating foam targets with ns laser driven hohlraum soft X-ray. Electrons are generated through irradiating the plasma with another picosecond laser. Tuning the laser pulse delay $\tau$ enables control of plasma profiles and beam parameters. Experiments show that when the foam is heated ($\tau$ = 6 ns, 9 ns), the beam exhibits $T \sim 13$ MeV effective temperature, $E_k \sim 80$ MeV cutoff energy, and hundreds of nC/sr charge for $E_k > 7.5$ MeV. These values are significantly higher than those from solid-foil ($T$ $\sim$ 2.7 MeV, $E_k$ $\sim$ 20 MeV, $Q$ $\sim$ 9 nC/sr) and cold-foam ($T$ $\sim$ 12 MeV, $E_k$ $\sim$ 50 MeV, $Q$ $\sim$ 5 nC/sr) interactions. At a longer delay of $\tau$ = 15 ns, the charge increases further while the temperature decreases, and at a shorter delay of $\tau$ = 3 ns, both temperature and charge are lower. 3D PIC simulations link these observations to the interplay between the microstructure of the cold foam and the evolving plasma density profile at different delay times, which together determine the beam charge, effective temperature, and divergence. The finding provides a routine to generate and tailor the relativistic electron beams, which is essential for designing laser-driven electron sources for high energy density physics and photonuclear reaction applications.
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All authors Ziyao Wang, Jieru Ren, Zhigang Deng, Wenqing Wei, Wei Qi, Olga N. Rosmej, Nikolay E. Andreev, Sergey Yu. Gus'kov, Rafael Yakhin, Yifang Gao, Bubo Ma, Mingzhe Yang, Shizheng Zhang, Xuyang Luo, Dieter H. H. Hoffmann, Peng Zhou, Ke Jiang, Taiwu Huang, Bo Cui, Weiwu Wang, Shaoyi Wang, Quanping Fan, Zhurong Cao, Sixin Wu, Yue Yang, Leifeng Cao, Yuqiu Gu, Yuchi Wu, Weimin Zhou, Zongqing Zhao, Yongtao Zhao

Plasma Physics

Influence of the ectonic emission mechanism on the collisionless expansion of a multicomponent cathode plasma

Vasily Yu. Kozhevnikov

In this paper, the authors delve into the collisionless kinetic theory of planar vacuum diodes to investigate how a pulse-periodic (ectonic) cathode plasma emission mechanism influences the non-thermal expansion of the cathode plasma and "anomalous" ion acceleration. Another aspect explored in this paper is how pulse-periodic emission affects the comparative dynamics of motion of multi-charged ions from the cathode material. Using kinetic theory, we conclude that the ectonic emission mechanism affects both the macroscopic picture of plasma expansion and the intricacies of electric potential redistribution over one complete emission period.
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