physics.plasm-ph

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physics.plasm-ph 2026-05-13 Recognition

Finite-difference exponential solver matches spectral accuracy in cylindrical PIC

High-order exponential solver method for particle-in-cell simulations in cylindrical geometry

Real-space high-order method reproduces 3D and FBPIC wakefield results without basis transformations while keeping cylindrical cost savings.

abstract click to expand

Recent developments in high peak-power table-top laser systems reaching highly relativistic light intensities have led to significant advances in laser-driven particle acceleration schemes (mainly the laser wakefield acceleration, LWFA) that heavily rely on particle-in-cell (PIC) simulations for the microscopic understanding of the acceleration process. Efficient algorithms have been developed by taking advantage of the cylindrical geometry of the laser-plasma acceleration interaction, which reduces the computational and memory costs of these simulations, but with the trade-off of reduced accuracy compared to the 3D simulations. The most successful solution solves the Maxwell equations on a Fourier-Bessel spectral basis in this geometry, as used by the well-known FBPIC code. In this work, we present a solution that is a real-space equivalent of the latter using the finite difference exponential time-domain method. Spatially, we represent the derivatives with high-order staggered finite differences locally and address issues of the near-axis particle representation. Additionally, we also develop an exponential solution to propagate the laser envelope potential with high accuracy in the cylindrically symmetric PIC model. We show that this method provides a very high accuracy without relying on a transformation to special basis functions. We verified the accuracy and the convergence of these methods in various benchmarks involving laser propagation in vacuum and in underdense plasma. Electron injection in the non-linear laser wakefield regime has also been simulated and the results are compared with 3D simulations, and to the cylindrical spectral solution of FBPIC. We found good agreement between these methods; however, the spectral solution resulted in less energetic electrons and a smoother spatial distribution near the cylindrical axis.

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physics.plasm-ph 2026-05-13 2 theorems

MPEX AI twins reach two phase I milestones on schedule

MPEX AI Digital Twins Milestone Report

Progress report confirms helicon hot-spot controller and beam damage prototype are ready for June demonstration with Galaxy automation in

abstract click to expand

This is the six month progress report to Fusion Energy Science (FES) and the American Science Cloud (AmSC) on the MPEX AI Digtial Twins project that was started in October 2025. There are two milestones to demonstrate the Artificial Intelligence (AI) advantage for MPEX operations and scientific discovery, that will be completed by June 2026. The first is a Helicon AI Hot-Spot Controller (Sec. 3.1), which is the helicon heating component of the more comprehensive planned MPEX AI Hot Spot Digital Twin (Sec. 3). The second is an E-beam Damage Assessment Digital Twin (Sec. 4.1), which is a reduced electron beam damage modality prototype for the MPEX AI Damage Assessment Digital Twin (Sec. 4). These two phase I milestones are on track for the June demonstration. In addition to these two milestones, progress on configuring the Galaxy software interface for automation, validation and data analysis is reported (Sec. 5). This interface now connects a subset of the main physics simulation codes to DOE HPC resources and will connect to the MPEX data acquisition system so that analysis of data, validation and execution of simulations can be performed by the scientist or by AI-Agents. When AmSC is ready to accept connections and data, Galaxy will be the MPEX interface to AmSC

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physics.plasm-ph 2026-05-13 1 theorem

Probabilistic metric isolates model error from fusion test uncertainties

Towards Virtual Qualification in Nuclear Fusion: Demonstrating Probabilistic Model Validation on a High Heat Flux Component

Novel implementation of the area validation metric on a high heat flux heat sink separates simulation discrepancies from experimental data.

abstract click to expand

Qualification of components operating in future fusion power plants will be heavily reliant on simulations of component behaviour. The lack of representative test environments for many aspects of the expected operating environment will necessitate full or partial virtual qualification of components. The cornerstone of virtual qualification is credible validation of the simulation models on which it relies. In this work, we present a probabilistic model validation framework that forms the basis for implementation of virtual qualification in fusion. We demonstrate our framework on a representative component; a high heat flux heat sink subject to a tightly coupled multi-physics loading. We perform data-rich, optimised experiments, in which we implement high fidelity diagnostics and rigorously quantify aleatoric and epistemic uncertainty on all measurements. Our simulation approach efficiently samples input uncertainty distributions to predict probability boxes describing component response, using a statistical surrogate to replicate behaviour of the finite element model we wish to validate. We then used a novel implementation of the modified area validation metric to quantify the model form error of the finite element model, isolating it from the aleatoric and epistemic experimental uncertainty. We discuss the contribution of our validation approach towards virtual qualification, and the benefits of the risk-based decision-making it facilitates. The experimental, simulation, and validation datasets are published as a benchmark of a probabilistic validation approach for fusion, and for use in development of new model validation methodologies.

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physics.plasm-ph 2026-05-12 2 theorems

GW method cuts DC conductivity in warm dense beryllium

Capturing many-body effects in electrical conductivity of warm dense matter

Improved transition energies lower low-T values while electron-electron scattering lowers high-T values, refining planetary and fusion model

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Conductivity models for warm dense matter inform simulations of planetary structure and fusion experiments. State-of-the-art conductivity calculations based on density functional theory approximate many-body physics and neglect electron-electron scattering lifetimes. We introduce a many-body framework for electrical conductivity using the GW approximation of the electronic self-energy. For beryllium, improved transition energies yield a surprisingly large reduction in low-temperature DC conductivity, while electron-electron scattering primarily reduces high-temperature DC conductivity.

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physics.plasm-ph 2026-05-12 2 theorems

MHD alignment reflects survival of intense small-angle events

Dynamic Alignment: A Fragile Survival Effect

Typical fluctuations lack strong alignment while intense ones last longer and bias the diagnostics

abstract click to expand

Dynamic alignment in magnetohydrodynamic (MHD) turbulence is usually interpreted as a cascade-wide tendency of Elsasser increments to become increasingly collinear at smaller scales. We argue instead that the standard measurements mainly detect a conditional survival effect of intense events. In high-resolution Johns Hopkins MHD simulations, the typical folded Elsasser-increment angle remains only modestly below the random-orientation baseline and shows no evidence for a rigid, monotone, volume-filling ordering of the cascade. Much smaller angles appear primarily in the strongest Elsasser-amplitude events, while conditioning on current density leaves the angle close to its unweighted behavior. Shuffled-null tests show that this reduction is caused by a genuine negative covariance between event amplitude and angular misalignment, not by weighting alone. Cross-scale angular correlations are measurable but decaying, indicating partial and non-rigid persistence of the local alignment field. A finite-time state-retention test directly supports the proposed mechanism: high-amplitude large-angle events leave their amplitude--angle sector faster than high-amplitude small-angle events, while incoming transitions continually replenish the large-angle sector. NASA Wind solar-wind observations show the same angle--amplitude hierarchy and negative covariance in Taylor-sampled Elsasser increments. These results indicate that dynamic alignment, as measured by conventional weighted diagnostics, is best understood as selective sampling of longer-lived intense small-angle events, not as a cascade-wide alignment of typical MHD fluctuations.

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physics.plasm-ph 2026-05-12 Recognition

OMEGA sustains collisionless plasma flows for 11 ns

Sustained interpenetrating plasma flows for the investigation of late time kinetic instability evolution

The duration lets researchers watch Weibel instability grow into nonlinear saturation and measure late-time magnetic filaments.

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Sustained collisionless interpenetrating plasma flows have been generated on the OMEGA laser facility to enable direct investigation of nonlinear evolution of fields generated by electromagnetic kinetic instabilities. FLASH simulations and Thomson scattering measurements are used to determine the plasma conditions achieved. Interpenetrating flows are observed to remain collisionless for at least 11 ns, longer than any prior OMEGA experiment, supporting the growth and nonlinear saturation of the Weibel instability. Resulting magnetic fields are measured using proton radiography. This work establishes a unique platform for late-time filament evolution measurements.

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physics.plasm-ph 2026-05-12 2 theorems

Electron inertia removes singularities from incompressible XMHD wave dispersion

Incompressible Extended Magnetohydrodynamics Waves: Implications of Electron Inertia

Ion cyclotron and whistler branches saturate at gyrofrequencies and match two-fluid behavior at high wave numbers.

abstract click to expand

This paper explores plasma wave modes using the extended magnetohydrodynamics (XMHD) model, incorporating Hall drift and electron inertia effects. We utilize the geometric optics ansatz to study perturbed quantities, with a focus on incompressible systems. Our research concludes with the derivation of the dispersion relation for incompressible XMHD and the associated eigenvector solutions, offering new perspectives on plasma wave behavior under these extended scenarios. The dispersion relation shows distinct ion cyclotron and whistler wave branches, with characteristic saturation at the ion and electron gyrofrequencies, respectively. Comparisons between Hall MHD and XMHD demonstrate that XMHD provides a more accurate representation of plasma dynamics, especially at higher wave numbers, bridging the gap between simplified models and comprehensive two-fluid descriptions and smoothing out singularities present in Hall MHD solutions and capturing more physics of the full two-fluid model.

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physics.plasm-ph 2026-05-12 2 theorems

Alpha particles boost burning plasma confinement via zonal flows

How Fusion-Born Alpha Particles Suppress Microturbulence in Burning Plasmas

Alphas excite TAEs that enhance zonal flows, suppressing microturbulence and raising alpha heating by 25% through profile peaking.

abstract click to expand

A central unresolved question in fusion energy research is whether energetic alpha particles, the primary products of deuterium-tritium fusion reactions, enhance or degrade plasma confinement. In burning plasmas, the operating regime of future devices such as ITER and SPARC, alpha particles become the dominant heating source, yet their impact on confinement has remained uncertain. Here, we present self-consistent simulations of burning plasmas that simultaneously evolve microturbulence, alpha-particle heating, and macroscopic plasma profiles to steady state, and find that alpha particles can substantially improve confinement. Fusion-born alpha particles weakly destabilize toroidal Alfven eigenmodes (TAEs), which nonlinearly enhance zonal flows that shear apart and suppress ion-scale turbulence. The resulting reduction in turbulent heat transport drives stronger core profile peaking, increasing alpha heating by up to 25% and establishing a self-reinforcing feedback loop. This mechanism has no direct analogue in present-day experiments, where external heating dominates, and reveals an intrinsic pathway toward improved confinement in burning plasmas.

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physics.plasm-ph 2026-05-12 1 theorem

Self-consistent plasmoid step upgrade makes HPI2 predictive for pellet fueling

Integrated full pulse modeling for pellet injection in tokamaks: HPI2 model improvement and validation in WEST

WEST simulations recover density rise, relaxation and temperature transient while including tungsten radiation losses

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Reliable modeling and control of core density is essential for reactor-relevant magnetic confinement fusion operation, motivating cryogenic pellet injection as a primary fueling actuator and the need for predictive pellet source models in integrated modeling. Here we present an upgrade of the physics-based pellet code HPI2 in which the plasmoid release spatial step is determined self-consistently from ablation physics, $dx_{var}=v_{\mathrm{pel}}\,t_{\mathrm{exit}}$ (optionally rescaled to trade accuracy for computational cost), removing an ad-hoc discretization parameter and improving numerical robustness across injection conditions. The upgraded model is first validated in stand-alone against a high-field-side pellet-fueled, ohmic, WEST discharge (#58656) by comparing synthetic and measured interferometry line-integrated density increments, obtaining a mean error of $\sim 10\%$. We then perform full-radius, time-dependent integrated modeling validation by coupling the new HPI2 within the High Fidelity Pulse Simulator (HFPS) workflow (JINTRAC/IMAS), combining JETTO with SANCO for the impurity/radiation evolution and TGLF-SAT2 for the turbulent transport. The coupled simulations reproduce the main density rise and relaxation after pellet injection and the associated electron-temperature transient, while taking into account the strong influence of tungsten radiation in WEST, supporting the consistency of HPI2 as a predictive pellet particle source in integrated modeling frameworks. Ultimately, this validation study supports the use of pellet modeling tools in integrated modeling studies for larger devices such as ITER.

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physics.plasm-ph 2026-05-12 Recognition

Expansion cooling triggers Weibel filaments in laser plasma

Weibel-mediated filamentary structures observed in the ICF context

Transverse ballistic cooling creates electron pressure anisotropy whose instability produces magnetic structures matching OMEGA and LMJ data

abstract click to expand

In light of novel and past experimental results, we demonstrate how Weibel-mediated filamentary structures can develop in the expanding plasma plume of a laser-irradiated foil. The transverse ballistic cooling that occurs during the quasi-spherical plasma expansion naturally drives an electron pressure anisotropy, resulting in the growth of electron current filaments. This effect competes with electron-ion Coulomb collisions which tend to isotropize the electron distribution function. Based on theoretical and particle-in-cell modeling, we provide estimates of the dominant wavelength and amplitude of the self-generated magnetic fluctuations, which are found to explain experimental data obtained at the OMEGA and Laser Megajoule facilities.

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physics.plasm-ph 2026-05-12 1 theorem

Jenga framework matches MAST-U equilibria and neutron fluxes from shared data

Magnetohydrodynamic equilibrium and neutronics study on MAST-U using Jenga framework

Reproduces full-pulse Grad-Shafranov solutions from EFIT++ inputs and computes neutron spectra on coils and limiter for DT plasma

abstract click to expand

Tokamak design is inherently challenging due to several cross-competing effects which require a careful and calibrated treatment to obtain an optimal operational envelope. Incorporating physics across varied fidelities is crucial in this exercise. Jenga is developed as a unified design and modeling framework for tokamaks, seamlessly coupling systems-level studies to high-fidelity models based on first principles. In this work, static Grad-Shafranov (GS) equilibrium for an entire pulse and the neutronics study of the Mega Ampere Spherical Tokamak Upgrade (MAST-U) tokamak are carried out in Jenga. Coil currents and plasma profiles from the EFIT++ reconstruction of MAST-U shots are used to reproduce the plasma poloidal flux and shape targets at different time slices. The results from Jenga are also in good agreement with FreeGSNKE and Fiesta codes. Neutronics analysis is performed for a hypothetical 50-50 mixture of deuterium-tritium (DT) fuel, using the same data structure as the systems and equilibrium studies. A distributed neutron source is initialized within the last closed flux surface (LCFS) of the plasma, with their strength being functions of the density and temperature of the ions. The distribution of the neutron flux across the energy spectrum is computed for the active coils and the first wall (limiter) independently over multiple scenarios. We demonstrate the capabilities of Jenga with a comprehensive analysis that takes inputs about the plasma geometry, tokamak design and plasma profiles and performs 0D, 2D and 3D numerics for the systems study, equilibrium and neutron transport respectively.

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physics.plasm-ph 2026-05-11 2 theorems

Plasma-pretrained model hits F1 0.837 on grid event detection

TokaMind for Power Grid: Cross-Domain Transfer from Fusion Plasma

Grid provider topology sets detection difficulty more than model capacity; single-window early alerts beat CNN baseline.

abstract click to expand

TokaMind is a multi-modal transformer (MMT) foundation model pre-trained on tokamak plasma diagnostics data from MAST, where it was shown to outperform CNN-based approaches on fusion benchmarks. We investigate whether its learned representations generalize to physically distinct but structurally analogous domains. Through systematic experimentation across four domains-industrial bearing degradation, NASA CMAPSS turbofan degradation, and two independent power grid PMU datasets-we identify four transfer-favoring characteristics that help explain where TokaMind's pretrained representations are most effective. Power grid synchrophasor data matches this target-domain profile most directly, while industrial degradation datasets demonstrate that TokaMind can still yield useful performance under partial alignment, especially when task design and feature construction expose physically meaningful degradation structure. On the GESL/PNNL 500-event benchmark with provider-aware evaluation, TokaMind achieves test $\text{F1} = 0.837 \pm 0.040$ (3~seeds) for severe event classification. Our central finding, however, is not the aggregate score: classification difficulty is structurally determined by provider-level grid topology, not model capacity. In the single-window early-warning regime, TokaMind outperforms a CNN baseline (F1~0.889 vs.~0.878)--a reversal that disappears as more event windows are provided. Furthermore, Critical Slowing Down (CSD) indicators, used as a confidence gate rather than a classification label, improve F1 from 0.696 to 0.750 at 63% coverage-outperforming the CNN baseline (0.636) at any coverage level. These results establish the first cross-domain validation of TokaMind outside nuclear fusion and propose a transferability framework and revised evaluation protocol for multi-source PMU datasets.

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physics.plasm-ph 2026-05-11 2 theorems

TRANSP solver enables reliable tokamak plasma predictions

Predictive capabilities of the integrated modeling TRANSP code for tokamak plasmas

PT_SOLVER and T3D/GX integration provide verified time-dependent simulations for density, energy, and momentum profiles.

abstract click to expand

This paper expands on the TRANSP description given in Computer Physics Communications 312 (2025) 109611 by describing recent progress in TRANSP's predictive functionality and emphasizing the development of the PT_SOLVER module and integration of the high-fidelity T3D/GX framework for plasma profile prediction using a high-fidelity gyrokinetic model for turbulent transport. PT_SOLVER is a modular, multi-region, parallel solver for coupled transport equations of particle density, electron and ion energy, and toroidal angular momentum that uses an implicit Newton method to advance the solution of these equations. The numerical formulation includes source coupling, moving-geometry terms, and nonlinear stabilization based on modified Peclet numbers, thereby enabling the PT_SOLVER to handle the stiffness associated with gradient-dependent transport models. Stabilization occurs via a nonlinear function controlling discretization in zones of steep gradients or rapidly changing transport coefficients. Source terms that account for heating, current drive, alpha-particle effects, and collisional energy exchange are handled thoroughly, and both residual norms and profile-change measures are used to assess convergence. Verification is carried out using analytical benchmark solutions, manufactured solution benchmarks, convergence studies of stiff gradient-dependent diffusivities, and code-to-code comparisons of TGYRO using the TGLF/NEO models for anomalous and neoclassical transport. This paper also describes the TRANSP Interface to the modular T3D/GX workflow and presents verification examples related to the interface for coupled prediction simulations. The results in this paper confirm that the predictive TRANSP framework has a robust numerical implementation for time-dependent predictive transport simulations, and it provides a basis for future hybrid reduced and high-fidelity workflows.

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physics.plasm-ph 2026-05-11 Recognition

MPEX data trains AI digital twins for material metrics

MPEX AI Digital Twins

Experimental and simulation results feed models that predict performance for tested and synthetic materials in plasma environments.

abstract click to expand

Our vision for the MPEX AI Digital Twins project is to supply experimental and physics model simulation data to train Artificial Intelligence (AI) models for data processing, analysis, operational control, PMI and materials simulation to maximize the scientific output of the MPEX device. Ultimately, an AI digital twin of MPEX material assessment metrics for tested and synthetic material types with simulated PMI will be trained by the AI Modeling Teams on the experimental and physics simulation data submitted to the American Science Cloud by this project

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physics.plasm-ph 2026-05-11 2 theorems

Activation products reveal proton energies inside laser fusion targets

Probing In-Solid Proton Energy Distributions in Laser-Driven Fusion via Nuclear Activation Diagnostics

Yields of carbon-11 and beryllium-7 reconstruct the in-solid spectrum that drives fusion reactions.

abstract click to expand

The energy distribution of energetic protons inside a solid target is a key quantity governing nuclear reaction yields and energy deposition in high-intensity laser-driven fusion, including nonthermal proton--boron (p--B) schemes and proton fast ignition. Yet it has remained inaccessible to conventional particle diagnostics, which detect only ions escaping the target and are perturbed by intense plasma electromagnetic fields. Here we establish a quantitative diagnostic that uses nuclear activation reactions occurring within the target itself as an internal probe of the in-solid proton energy distribution. Applied to laser-driven p--B fusion experiments on the kJ-class laser, the method reconstructs an exponential-equivalent in-solid proton energy distribution from the absolute yields of $^{11}\mathrm{C}$ and $^{7}\mathrm{Be}$ produced via $\mathrm{^{11}B(p,n)^{11}C}$ and $\mathrm{^{10}B(p,\alpha)^{7}Be}$, and yields the absolute number of $\mathrm{^{11}B(p,2\alpha)^{4}He}$ reactions through a side-channel analysis with propagated cross-section uncertainties. This work opens a quantitative window onto the in-solid proton dynamics that drive nuclear reactions in laser-driven fusion experiments.

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physics.plasm-ph 2026-05-11 2 theorems

Energy model trained on random conditions reconstructs real plasma diagnostics

Energy-based models for diagnostic reconstruction and analysis in a laboratory plasma device

One network handles reconstruction, infers probe positions, samples trends, and reproduces data modes on LAPD measurements.

abstract click to expand

Energy-based models (EBMs) provide a powerful and flexible way of learning a joint probability distribution over data by constructing an energy surface. This energy surface enables insight extraction and conditional sampling. We apply EBMs to laboratory plasma physics, a domain characterized by highly nonlinear phenomena. These phenomena are studied using plasma diagnostics, which are often difficult to analyze and subject to hardware degradation. In addition, the possible configuration space of a plasma device is sufficiently large that it cannot be efficiently searched using conventional analysis techniques. EBMs address these issues. At the Large Plasma Device (LAPD), a CNN- and attention-based EBM is trained on a set of randomly generated machine conditions and their corresponding diagnostic time series. We demonstrate diagnostic reconstruction using this EBM on real data and show that additional diagnostics improves reconstruction error and generation quality. The energy surface is directly evaluated for an ill-posed inverse problem: inferring probe position from a time-series measurement. This inference illuminates symmetries in the data, potentially leading to a method of inquiry to supplement conventional data analysis. Trends in diagnostic signals are inferred via conditional sampling over machine inputs. In addition, this multimodal EBM is able to unconditionally reproduce all distributional modes, suggesting future potential in anomaly detection on the LAPD. Fundamentally, this work demonstrates the flexibility and efficacy of EBM-based generative modeling of laboratory plasma data, and showcases multiple practical uses of just a single trained EBM in the physical sciences.

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physics.plasm-ph 2026-05-11 3 theorems

Warm magnetized plasmas gain a new topological edge wave

Warm Topological Langmuir Cyclotron Wave

A Weyl point of charge 1 appears only with finite temperature and forces a gap-crossing mode absent from cold theory.

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Finite-temperature effects in magnetized electron plasmas create a new Weyl-point degeneracy between the warm Langmuir and right-circularly polarized waves. The associated topological charge at this warm Weyl point is found to be 1, which, by the index theorem, predicts a gap-traversing topological edge mode. Solving the full warm-fluid eigenmode problem In a 1D inhomogeneous equilibrium, we numerically identify this anticipated mode as the warm topological Langmuir-cyclotron wave, which is absent in the cold limit and occurs in a parameter regime relevant to the LArge Plasma Device (LAPD) at UCLA.

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physics.plasm-ph 2026-05-11 Recognition

Backflow sharply limits net thermionic electron transport

Full-gap kinetic limitation of thermionic-electron transport for electron transpiration cooling

Simulations show net efficiency drops to 46 percent after transition at 7.25e19 m-2 s-1 as backflow exceeds 50 percent

abstract click to expand

Electron transpiration cooling (ETC) can reduce aerothermal loads on sharp hypersonic leading edges, but its performance is governed by whether thermionically emitted electrons escape the hot surface or return as cathode-directed backflow. Here, a one-dimensional-in-space, three-dimensional-in-velocity electrostatic particle-in-cell/Monte Carlo collision model is developed for a full cathode--anode plasma diode, resolving thermionic emission, collisional plasma transport, emitted-electron backflow, and downstream collection. A helium benchmark is used to examine emitted-electron transport and backflow-limited current flow. With increasing imposed emission, the diode first remains in a weak-backflow regime, where net emitted-electron transport and downstream collection both increase with emission. Further increasing the emission produces a sharp transition to backflow-limited transport between $7.0\times10^{19}$ and $7.5\times10^{19},\mathrm{m^{-2},s^{-1}}$. At $7.25\times10^{19},\mathrm{m^{-2},s^{-1}}$, the backflow ratio reaches $54.03%$, while the net transport and downstream collection efficiencies fall to about $46%$. Above this transition, added backflow overcompensates the imposed emission increase, reducing useful emitted-electron transport rather than causing saturation. Boundary energy diagnostics show that stronger emission may still increase the nominal cathode-side cooling metric, but after transition this metric no longer indicates improved emitted-electron escape or full-gap transport. These results show that the present PIC-MCC framework captures the key kinetic processes governing ETC-relevant emitted-electron escape and backflow limitation.

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physics.plasm-ph 2026-05-08 Recognition

Workflow speeds steady-state plasma predictions for fusion

Accelerating integrated modeling with surrogate-based optimization: the MAESTRO workflow

MAESTRO couples PORTALS surrogates with equilibrium and heating solvers to enable efficient full-physics reactor modeling

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This paper introduces the MAESTRO workflow, that enables the coupling of the PORTALS framework [P. Rodriguez-Fernandez et al, Nucl. Fusion 2024] with external solvers for the plasma equilibrium, pedestal physics, divertor constraints and heating. The surrogate-based optimization nature of the transport solver is ideally suited for external coupling, allowing efficient steady-state predictions of plasma profiles with full physics models. Improvements in the surrogate modeling of quasilinear transport models with PORTALS are presented, which enable the efficient handling of discontinuities in the transport fluxes that can arise from numerical issues or physical instabilities with extreme stiffness. The combination of physics-informed methods and advanced numerical techniques allows the MAESTRO workflow to provide accurate and efficient predictions of steady-state plasma profiles, which are critical for fusion reactor design and optimization.

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physics.plasm-ph 2026-05-08

Two admissible Hugoniot relations describe shocks in multi-temperature plasma flows

Hugoniot Relation for Multi-Temperature Euler Equations of Compressible Plasma Flows

Non-conservative terms and loss of microscopic detail leave the jump conditions ambiguous, so external data from experiments or simulations

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Shock solutions for multi-temperature Euler equations are inherently ambiguous due to the loss of microscopic physical detail during model reduction and occurrence of non-conservative terms. This paper presents a detailed analytical study of shock structures in such models. We derive two distinct Hugoniot relations, each corresponding to a physically admissible shock solution: one for the general multi-temperature case and one for two-temperature plasma flows. Through classical analysis \`a la Courant--Friedrichs, we demonstrate that both satisfy admissibility conditions, revealing a fundamental non-uniqueness in shock structures. By relating these solutions to existing numerical schemes, the structure preserving and vanishing viscosity approaches, we provide physically justified references for constructing and evaluating discontinuous numerical approximations. In particular, we emphasize that the Hugoniot relation is not uniquely determined by the macroscopic PDEs alone, but must be supplied from external sources such as experiments or first-principles simulations. This insight demonstrates the essential role of microscopic physics in resolving shock ambiguity and contributes to the theoretical foundation for modeling discontinuous plasma flows.

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physics.plasm-ph 2026-05-08

Weibel instability drives filamentary fields in laser ablation

Proton probing measurements of filamentary electromagnetic structure in laser ablation of solids

Proton probe data shows field growth scales directly with laser energy and target atomic number Z.

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Proton radiography of laser direct-drive spherical implosions has shown anomalous structures that correspond to strong electric or magnetic fields extending throughout the corona. These fields have the ability to affect laser-target interactions and act as an energy sink. To better understand the these fields, simplified experiments were conducted in planar geometry on the OMEGA EP laser at the Laboratory for Laser Energetics. Varying target material, target size, pulse shape, and intensity, and measured the field structure using dual-axis proton radiography and a 4w probe. Proton radiographs were analyzed and quantitatively demonstrate that the growth of these features is dominated by laser energy and target Z. The data strongly supports that a secondary instability as a consequence of the expansion driven Weibel instability in these interactions is the primary driver for these fields.

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physics.plasm-ph 2026-05-08 Recognition

Weibel secondary instability produces filamentary fields in laser ablation

Proton probing measurements of filamentary electromagnetic structure in laser ablation of solids

Planar experiments show growth dominated by laser energy and target Z, supporting this mechanism as the main driver.

abstract click to expand

Proton radiography of laser direct-drive spherical implosions has shown anomalous structures that correspond to strong electric or magnetic fields extending throughout the corona. These fields have the ability to affect laser-target interactions and act as an energy sink. To better understand the these fields, simplified experiments were conducted in planar geometry on the OMEGA EP laser at the Laboratory for Laser Energetics. Varying target material, target size, pulse shape, and intensity, and measured the field structure using dual-axis proton radiography and a 4w probe. Proton radiographs were analyzed and quantitatively demonstrate that the growth of these features is dominated by laser energy and target Z. The data strongly supports that a secondary instability as a consequence of the expansion driven Weibel instability in these interactions is the primary driver for these fields.

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physics.plasm-ph 2026-05-08

Mesh-free method models relativistic electron bunch transport

An Electromagnetic Particle-Particle Method for Relativistic Electron Bunch Dynamics from Early Expansion to Long-Range Transport

It retains exact short-range interactions during early expansion and couples them to long-range geomagnetic control.

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Particle-mesh methods, such as the particle-in-cell (PIC) method, cannot retain exact pairwise interaction at sub-cell scales. For dense nonneutral relativistic electron bunches, this makes it difficult to accurately capture the inter-particle electromagnetic interaction and the associated bunch divergence. In this work, the previously developed electromagnetic particle-particle (EM-PP) model for relativistic two-particle interaction is extended to many-particle electron bunch transport in the Earth's magnetosphere. The method combines the Li\'enard--Wiechert fields, an improved retarded-time evaluation procedure, and a relativistic particle pusher, and adopts a two-stage strategy to couple the dense early self-field-dominated evolution to the later long-range geomagnetic-field-controlled transport. The method provides a practical mesh-free approach for accurately simulating long-range transport of relativistic electron bunches when short-range electromagnetic interaction is important.

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physics.plasm-ph 2026-05-06

PINN solves 1D electron Boltzmann equation for plasmas

A physics-informed neural network approach to solve the spatially inhomogeneous electron Boltzmann equation

Novel architecture with Fourier features and adaptive gates matches benchmarks and generalizes across gases without retuning.

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The accurate determination of electron properties is fundamental to low-temperature plasma simulations, necessitating precise solutions to the spatially inhomogeneous electron Boltzmann equation (EBE). This work explores the use of physics-informed neural networks (PINNs) for obtaining solutions to the spatially one-dimensional (1D) EBE subject to a uniform electric field in atomic gases. Employing the two-term approximation, the resulting equation for the isotropic distribution is solved directly in kinetic energy space without the conventional transformation to total energy. This approach demonstrates the flexibility of the PINN framework in handling diverse equation formulations. To address the convergence difficulties associated with this class of kinetic equations, a new neural network architecture is introduced. It features a Fourier-feature input layer, adaptive activation functions, and a scaled multiplicative gating mechanism. It is demonstrated that this formulation preserves robust gradient flow throughout the network, which is critical for learning physically correct solutions. Benchmarking against reference data reveals that the present architecture achieves excellent agreement across both microscopic and macroscopic properties of the electrons. Furthermore, the architecture exhibits strong generalization across different gas types and a defined range of electric field strengths without requiring case-specific hyperparameter tuning. Ultimately, the excellent accuracy achieved here validates the applicability of the present PINN method.

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physics.plasm-ph 2026-05-06

3D constant-magnitude magnetic rotations require discontinuities

Three dimensional, spherically polarized magnetic fields

A numerical construction shows smooth field rotations keeping |B| fixed can exist only inside bounded regions separated by jumps.

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Turbulence in the solar wind is characterized by Alfv\'enic fluctuations that exhibit spherical polarization, a geometric condition resulting in the nearly constant magnitude of the magnetic field. This property persists even during the largest field fluctuations, sometimes leading to local polarity reversals known as switchbacks. A longstanding question is whether three-dimensional smooth magnetic fields can simultaneously satisfy the constant-$|{\bf B}|$ constraint, and how such fields can be constructed analytically or numerically. Here we propose a new numerical method that allows to construct a magnetic field that is exactly spherically polarized, reproducing key features of solar wind fluctuations. Using this framework, we show that discontinuities are generically unavoidable in three-dimensional configurations. Fundamentally, this implies that field rotations cannot maintain exactly constant $|{\bf B}|$ in an arbitrarily large spatial domain. Rather, field rotations with constant magnitude can exist in limited regions of space separated by discontinuities where magnetic compressibility cannot be neglected. These results provide insights into the structure of solar wind turbulence and more generally into the nature of nonlinear magnetic fluctuations in plasmas.

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physics.plasm-ph 2026-05-06

LaBr3 detectors can reconstruct SPARC fusion power via gamma rays

Synthetic model of gamma-ray emission during DT experiments on the SPARC tokamak

Simulations show a polyethylene attenuator raises gamma-to-neutron ratio enough for 140 MW DT power measurement.

abstract click to expand

In thermonuclear plasmas, plasma ions undergoing nuclear reactions emit gamma-rays with energies in the MeV range. Their spectroscopy can convey much plasma information, such as the DT fusion power, the spatial and velocity distributions of the fast ions, and the plasma heating performance. In the present work, we simulate the gamma-ray emission expected in the SPARC tokamak during a primary reference discharge, when the tokamak is expected to generate $140$ MW of fusion power and reach an energy gain factor of $Q\approx11$. We focus particularly T(D, $\gamma$)He-5, B-10(He-4, p $\gamma$)C-13 and D(He-3, $\gamma$)Li-5 reactions. We use realistic plasma profiles calculated with the TRANSP code and simulate radiofrequency heating of the plasma with CQL3D and TORIC. Possible locations for gamma spectrometers based on lanthanum bromide inorganic scintillators are suggested. For each, the signal-to-noise ratio of gamma-rays over neutrons is evaluated using the ray-tracing code ToFu and high fidelity Monte Carlo models (MCNP and OpenMC) to solve radiation transport in SPARC. A dedicated neutron attenuator made of high density polyethylene is scoped to allow gamma-spectroscopy during high neutron yield experiments. And finally, the performance of LaBr$_3$ detectors in reconstructing the fusion power generated by SPARC is discussed.

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physics.plasm-ph 2026-05-06

Collisions switch plasma turbulence from zonal flows to streamers

Transition from Zonal Flows to Streamer like structures and associated edge Fluctuations

Experiments show ion-neutral collision frequency selects between coherent zonal flows and intermittent streamers, altering edge transport.

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We report experimental observations of a controlled transition from a zonal-flow (ZF) dominated regime to a coexistence regime of ZFs and streamers, and finally to a streamer-dominated state in a linear magnetized plasma column. The controlling parameter is the ion-neutral collision frequency. At low collisionality (2 x 10^-5 mbar), the plasma turbulence is dominated by coherent ZFs (600-700 Hz) that are nonlinearly driven by drift-wave fluctuations. With increasing collisionality (5 x 10^-4 mbar), the ZF growth is reduced and streamers emerge through nonlinear coupling of neighboring drift modes mediated by a mediator mode. At high collisionality (2 x 10^-3 mbar), ZFs are strongly damped and the turbulence becomes streamer-dominated. For each of these turbulent states, the corresponding edge fluctuations transition from coherent, symmetric to intermittent, asymmetric fluctuations with enhanced low-frequency content and larger spatial scales that can result in convective transport. Our results demonstrate the possibility of selective excitation of ZFs and streamers by regulating their collisional damping and establish the ion-neutral collision frequency as an effective control knob for regulating turbulent structures and edge transport in magnetized plasmas.

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physics.plasm-ph 2026-05-06 3 theorems

Low-density HTS tokamak yields 25 MWe for marine and remote use

Yinsen: A low power density HTS tokamak fusion reactor for marine and off-grid applications

Materials-limited design at 0.7 MW per square meter achieves net power and self-sustaining fuel cycle with current high-temperature supercon

abstract click to expand

Yinsen is a high-temperature-superconducting (HTS) tokamak reactor concept for off-grid applications such as maritime propulsion, remote power, and industrial energy. Rather than pursuing grid-scale power density, the design is anchored to a materials-limited fusion power density of $P_f/S_b=0.7~\mathrm{MW/m^2}$, obtained from a 35 DPA structural limit, a 20-year plant lifetime, 40% utilization, and a geometric damage-peaking correction. The resulting device has a V-4Cr-4Ti vacuum-vessel lifetime of $1040~\mathrm{MW\cdot yr}$, pointing to a minimum useful fusion power of $130~\mathrm{MW}$ and more than $25~\mathrm{MWe}$ net output. Integrated FUSE modeling refines the design into a self-consistent high-field baseline with a shaped 9.29 T, 9.67 MA plasma, while ASTRA transport analysis corroborates a broader operating window above the minimum design point. Divertor power handling is addressed with UEDGE modeling, showing that impurity-seeded detached operation is attainable with neon seeding, reducing peak heat fluxes well below $10~\mathrm{MW/m^2}$. OpenMC neutronics calculations with a double-layered WC/W$_2$B$_5$ shield show that the vacuum vessel is the lifetime-limiting solid structure, while the HTS magnets remain lifetime components: at the 130 MW baseline, total TF nuclear heating is 7.4 kW at 20 K, and the TF fast-neutron limit corresponds to roughly sixteen vacuum-vessel lifetimes. The same neutronics analysis gives $TBR\approx1.1$ with 30% $^6\mathrm{Li}$ enrichment and no dedicated neutron multiplier. Plant-level studies detail a supercritical CO$_2$ balance of plant and pulsed-power operation using a 34 kV medium-voltage backbone and local energy storage. Taken together, these results suggest that a low-power-density HTS tokamak offers a near-term path for relevant FOAK fusion reactors where many remaining challenges between $Q>1$ and economic grid operation are alleviated.

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physics.plasm-ph 2026-05-06

Anomalous plasma conductivity varies with turbulence spectrum

Anomalous Conductivity and Anisotropic Transport of Nonrelativistic Electrons in Plasma with Magnetostatic Weibel-Generated Turbulence

Simulations show electron diffusion and mobility factors shift strongly with temperature, field strength, and turbulence details, dominating

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The anisotropic diffusion of electrons with various rigidity and the anomalous conductivity of a collisionless plasma in the presence of Weibel-generated quasi-static turbulent and uniform external magnetic fields are examined. Using an original code based on the Boris algorithm, the electron diffusion coefficients and the longitudinal, transverse, and Hall mobility factors are determined for a representative set of plasma parameters. It is shown that these values and their anisotropy depend strongly on the electron temperature, external magnetic field, average level of magnetic turbulence, and its spectrum. The physical origin and expected limits of such dependencies are indicated. Applications of the results are discussed in the case of coronal plasma, where the anomalous resistivity prevails over the collisional one and can be responsible for the redistribution of large-scale currents in magnetic loops and small-scale currents in the regions of reconnection of magnetic-field lines.

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physics.plasm-ph 2026-05-06

Oblique ECE masks non-thermal signals above critical angle in ITER

Assessing the role of ITER ECE oblique view in resolving non-thermal emissions

Higher harmonic channels in any polarization stay reliable for reconstructing accurate electron temperature profiles.

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Systematic discrepancies between electron temperature (Te) measurements from radially viewing electron cyclotron emission (ECE) and Thomson scattering (TS) diagnostics have been observed in multiple tokamaks and are widely attributed to non-Maxwellian features in the electron velocity distribution function (EVDF). As the International Thermonuclear Experimental Reactor (ITER) is expected to operate at much higher core temperatures than present devices, accurate Te measurements from ECE become increasingly critical, particularly in the presence of non-Maxwellian EVDFs. This work presents ECE spectra simulations performed to assess the diagnostic capability of the ITER oblique view under ITER conditions where non-Maxwellian distributions are present. The results show that above a critical oblique angle, Doppler broadening becomes the dominant effect, masking fine-scale spectral signatures of non-thermal distortions across a wide range of conditions. Furthermore, at higher EC harmonics in either polarization, the spectra remain largely unaffected by non-thermal emissions, enabling reliable reconstruction of Te-profiles. These findings demonstrate that the ITER oblique ECE view retains sufficient sensitivity to non-thermal electrons while providing robust and accurate Te-profile measurements under reactor-relevant conditions. The results presented here also have direct implications for ITER ECE system operation and channel selection, particularly in the presence of non-Maxwellian electron populations.

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physics.plasm-ph 2026-05-06 3 theorems

Fixed coils yield 1.66 million fusion configurations

A programmable stellarator-tokamak hybrid for million-scale magnetic-configuration discovery

Programming currents in a tokamak-like set plus 288 dipoles spans stellarator symmetries and tokamak perturbations with good confinement.

abstract click to expand

Tokamaks and stellarators are the leading magnetic-confinement concepts for fusion, but they rely on complementary design principles. Tokamaks use simple axisymmetric coils and plasma current, whereas stellarators use externally generated three-dimensional fields for steady-state operation. Here, we propose a programmable stellarator--tokamak hybrid that uses a fixed set of simple planar coils to access a broad magnetic-configuration space. The device adds 288 dipole-field coils to a tokamak-like coil set, with only six independent coil geometries required by symmetry. By programming coil currents, the same hardware generates more than 1.66 million optimized stellarator configurations spanning quasi-axisymmetry, quasi-helical symmetry, and quasi-isodynamicity, as well as tokamak-relevant three-dimensional perturbations. Representative configurations exhibit nested magnetic surfaces, low neoclassical transport, and favorable energetic-particle confinement. This approach enables rapid magnetic-configuration discovery without hardware redesign.

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physics.plasm-ph 2026-05-06

Fixed coils generate 1.66 million stellarator configurations

A programmable stellarator-tokamak hybrid for million-scale magnetic-configuration discovery

Current programming on one hybrid hardware set spans many quasi-symmetric fields and 3D perturbations without redesign.

abstract click to expand

Tokamaks and stellarators are the leading magnetic-confinement concepts for fusion, but they rely on complementary design principles. Tokamaks use simple axisymmetric coils and plasma current, whereas stellarators use externally generated three-dimensional fields for steady-state operation. Here, we propose a programmable stellarator--tokamak hybrid that uses a fixed set of simple planar coils to access a broad magnetic-configuration space. The device adds 288 dipole-field coils to a tokamak-like coil set, with only six independent coil geometries required by symmetry. By programming coil currents, the same hardware generates more than 1.66 million optimized stellarator configurations spanning quasi-axisymmetry, quasi-helical symmetry, and quasi-isodynamicity, as well as tokamak-relevant three-dimensional perturbations. Representative configurations exhibit nested magnetic surfaces, low neoclassical transport, and favorable energetic-particle confinement. This approach enables rapid magnetic-configuration discovery without hardware redesign.

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physics.plasm-ph 2026-05-06

Voltage harmonics cut energy cost for oxygen atoms in Ar/O2 plasma

Hybrid simulation of the energy cost of O(¹D) and O(³P) generation in a capacitive Ar/O₂ discharge driven by sawtooth-type voltage waveforms

At 10% oxygen, using two harmonics shifts the discharge to a hybrid mode that increases electron density and lowers generation costs.

abstract click to expand

Low-pressure radio-frequency capacitively coupled plasmas operated in Ar/O$_2$ gas mixtures are widely adopted in critical semiconductor manufacturing processes. O($^3$P) and O($^1$D) are key highly reactive species for oxidation or as oxygen sources for deposited thin films. Optimizing external parameters to realize efficient generation of these species under limited energy deposition is essential for improving process yield.Based on a one-dimensional (1D) fluid/electron Monte Carlo (EMC) hybrid model, this study investigates the energy cost of O($^1$D) and O($^3$P) generation driven by sawtooth up-type voltage waveforms at a fixed peak-to-peak voltage, focusing on the effects of the harmonic number ($N$) and the O$_2$ ratio. The results show that O($^3$P) generation is consistently more efficient than that of O($^1$D). The generation energy cost decreases with increasing O$_2$ ratio, yet increases as $N$ increases. However, in the specific scenario of 10% O$_2$, an inflection point can be observed at $N = 2$. As $N$ increases from 1 to 2, the discharge mode shifts from the DA mode to the $\alpha$-DA hybrid mode, expanding the effective spatio-temporal range of the ionization rate and enhancing its peak, which increases electron density. Consequently, the generation rates are significantly enhanced, leading to a reduction in the generation energy cost.Moreover, as discussed above, monotonically increasing the harmonic number $N$ does not reduce the generation energy cost of O($^1$D) and O($^3$P) associated with medium-energy (8-20 eV) electrons. Only by selecting the appropriate $N$ to sustain the discharge in the hybrid $\alpha$-DA mode, thereby increasing the electron density and promoting the generation of these species, can the generation energy cost be reduced.

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physics.plasm-ph 2026-05-06

DA and LPDE differ in spacecraft-geometry sensitivity for plasma turbulence

Inertial-Range Energy Transfer Free from Isotropic Assumption in Turbulent Space Plasma1

Direction averaging tracks angles independently of arrangement; polyhedral derivatives track separation and shape, informing 3D cascade data

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The idea of an energy cascade in the inertial range is often invoked in turbulent space plasmas to estimate the energy dissipation rate. Laws governing the behavior of third-order structure functions in the inertial range, so-called third-order laws, are among the few rigorous theoretical results quantifying cross-scale energy transfer. The widely used third-order-law derived rate assumes isotropy, which fundamentally conflicts with the anisotropic nature of space plasmas. Elementary questions persist regarding how such anisotropic energy cascades can be quantified using multi-spacecraft constellations. As the heliospheric community increasingly progresses towards multi-spacecraft, multi-scale constellations, such as Plasma Observatory and HelioSwarm, we revisit these crucial issues pertinent to accurately measuring the inertial-range energy transfer. Here we make a systematic comparison between two methods: direction-averaging (DA) and lag polyhedral derivative ensemble (LPDE) to determine the full three-dimensional (3D) dependence of cross-scale energy transfer. We find that DA exhibits both polar and azimuthal dependence, but is insensitive to spacecraft configuration. By contrast, LPDE is strongly affected by spacecraft separation and tetrahedral shape, while being comparatively insensitive to the sampling trajectory. Our findings have direct implications for current and future multi-spacecraft missions. Both DA and LPDE will provide crucial information on the nature of turbulence in space and astrophysics.

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physics.plasm-ph 2026-05-05

TensorFlow gyrokinetic code uses noisy gradients for predictions

iGENE: A Differentiable Flux-Tube Gyrokinetic Code in TensorFlow

The differentiable implementation shows stochastic turbulence gradients still support outer-loop tasks like profile prediction.

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We present iGENE, a fully-differentiable TensorFlow implementation of the electromagnetic local nonlinear gyrokinetic model, which allows us to compute gradients of any simulation output with respect to any input via automatic differentiation. We show that even if the stochastic nature of turbulence prevents the exact evaluation of gradients of nonlinear quantities of interest, they can still be successfully used to perform outer-loop tasks, such as profile predictions. This work enables the integration of gyrokinetics into automated parameter optimization, uncertainty quantification, sensitivity analysis, and AI workflows.

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physics.plasm-ph 2026-05-05

Scaling law sets nonthermal tails in magnetized turbulence

On The Nonthermal Power Laws In Magnetized Turbulent Plasmas

Simulations confirm the relation and tie it to proton acceleration near supermassive black holes plus neutrino output.

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Building on recent progress in the understanding of particle transport in magnetized plasmas, we derive a scaling law for the formation of nonthermal spectral tails in mildly and strongly magnetized turbulent environments. We validate this scaling using driven-turbulence particle-in-cell simulations that incorporate particle escape, allowing the system to reach a steady state. The simulation results show good agreement with our theoretical predictions. We then discuss the astrophysical implications of these findings, focusing on proton acceleration in the coronae of supermassive black holes and the resulting high-energy neutrino emission.

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physics.plasm-ph 2026-05-05

Standardized probe lines needed for MPW interferometry

Scaling Interferometry to the Multi-Petawatt Regime

Without fixed architecture and off-frequency methods, steep gradients and pulses will block reliable pre-plasma density maps.

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Pre-plasma conditions strongly influence laser-plasma interactions in the multi-petawatt (MPW) regime, increasing the need for reliable early-time plasma evolution diagnostics. Among available pre-plasma diagnostics, interferometry remains the most direct method for measuring the spatially resolved electron density of pre-formed plasmas. However, its implementation becomes increasingly challenging at MPW scale dueto steep density gradients, phase-recovery difficulties, strong electromagnetic pulses (EMP), debris accumulation, and high-repetition-rate operation. Compounding these technical challenges, many large-scale facilities lack permanent probe-line architecture and trained diagnostic support, reducing experimental reproducibility and consuming limited beamtime. Future MPW facilities should standardize probe-line architecture, adopt off frequency probing strategies, improve phase-recovery methods for non-symmetric plasmas, integrate emerging real-time analysis capabilities, and engineer diagnostic systems resilient to EMP and high-repetition-rate environments. These advances will enable the user community to reliably characterize pre-plasma formation and laser-plasma dynamics at next-generation MPW facilities.

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physics.plasm-ph 2026-05-05 2 theorems

Electron suppression yields breakeven conditions for beam-target fusion

Breakeven Conditions for Beam-Target Fusion with Electron-Suppressed Targets

Self-consistent stopping power analysis produces quantitative thresholds where fusion output tops beam input.

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This manuscript provides a detailed and extended analysis of the breakeven conditions for nuclear fusion based on beam-target interactions, distinct from conventional plasma-based approaches. Building on the energy-based criterion introduced in the accompanying Letter~\cite{TK_arXiv}, we formulate a self-consistent description of stopping power in electron-suppressed targets and derive quantitative, implementation-agnostic conditions under which fusion energy generation can exceed beam energy loss.

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physics.plasm-ph 2026-05-05

Alfvénic turbulence saturates to yield particle power laws at index -3

Distributions of particles accelerated by strong Alfv\'enic turbulence

Diminishing energy-transfer efficiency as particle energy density grows produces the same scaling in both non-relativistic and relativistic

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This work presents a model for generating nonthermal power-law tails of particles' energy probability density functions in turbulent collisionless plasmas, applicable to both non-relativistic and relativistic scenarios. We propose that strong Alfv\'enic turbulence energizes plasma particles through curvature acceleration, particularly for particles with Larmor radii comparable to the scales of turbulence. When the energy density of the energized particles increases, the efficiency of the energy exchange process diminishes. As a result, the acceleration process saturates, leading to power-law distributions of particle momentum and energy. In the non-relativistic case, the momentum probability density function scales as $f(p) dp \propto p^{-3} dp $, while in the ultrarelativistic case, the energy probability density function scales as $ f(\gamma) d\gamma \propto \gamma^{-3} d\gamma $. This model provides a unified framework for understanding particle acceleration in both energy regimes, complementing existing analytical approaches. Its predictions are consistent with available observations of energetic ion distributions in the heliosphere and with the findings from numerical simulations of ultrarelativistic particle acceleration in magnetically dominated plasma turbulence.

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physics.plasm-ph 2026-05-05

Four-coil setup produces quasi-axisymmetric stellarator equilibria

Equilibrium of a simplified coil quasi-axisymmetric stellarator: Free boundary approach

Single-stage DESC optimization with a triple product metric yields low-transport configurations, sometimes inverting rotational transform.

abstract click to expand

Inspired by the design for the simple Columbia Non-neutral Torus (CNT) proposed by Pedersen and Boozer (Phys. Rev. Lett. 88 205002(2002)), and later revisited by Yu et al. (J. Plasma Phys. 88 905880306 (2022)), this work explores axi-symmetric equilibria using two planar vertical field coils and two non-planar intertwined coils. Neoclassical optimization studies are performed in a single stage approach using the DESC stellarator equilibrium solver (Dudt and Koleman, Phys. Plasmas 27 (2020) 102513). The physical parameters used as input were the toroidal magnetic flux function $\psi$, the rotational transform $\iota(\rho)$ as a function of the normalized flux function $\rho$ as the radial coordinate, the number of field periods $N_{FP}$, and an initial equilibrium assumption, given the major radius $R_0$ and the minor radius $a$. Once the equilibrium given by the coil configuration is defined, an optimization is made for quasi-symmetry. In this study, a triple product metric as defined by Dudt et al., (J. Plasma Phys. 89 (2023) 95589020) is used as a local error indicator, which is evaluated without resorting to Boozer coordinates. The goal is to optimize the effective ripple modulation amplitude ${\epsilon}_{eff}^{3/2}$, thus decreasing the neoclassical transport in the low collisionality $1/\nu$ regime. We show a sample of quasi-axisymmetric configurations obtained, both for the vacuum field and with finite pressure, which have reasonably good neoclassical transport in the sense of Boozer coordinates. In the best case scenarios the final rotational transfrom is inverted after optimization.

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physics.plasm-ph 2026-05-04 2 theorems

Gaps shorten wall time for low-n modes in segmented shells

Diffusion wall time in toroidally segmented shell aka Armadillo

Standing-wave currents vanishing at toroidal gaps add a quadratic resistivity correction, matching 3D calculations within 10%.

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An analytical expression for the diffusion wall time of a toroidally segmented conducting shell (the Armadillo configuration) is derived by extending the continuous-shell formulation to include the non-axisymmetric current pattern imposed by the presence of toroidal gaps. The segmentation constrains the toroidal current to follow a standing-wave structure that vanishes at the gap locations, introducing a correction to the effective resistivity that grows quadratically with the number of gaps and competes with the intrinsic toroidal scale of the mode. As a result, the wall time decreases rapidly for low toroidal-number modes, more gradually for intermediate ones, and only for sufficiently large segmentation in the high-n regime. The analytical formula shows agreement within 10% against 3D electromagnetic numerical calculations. The resulting expression provides a compact tool for estimating the wall time of segmented conducting structures surrounding the plasma, with direct applications to MHD stability and control in both RFPs and tokamaks.

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physics.plasm-ph 2026-05-04

General boundaries allow realistic 3D plasma equilibrium models

Computational boundary specification in 3D fixed-boundary magnetohydrodynamic equilibrium modeling

Placing the computational boundary in the vacuum region outside the edge relaxes requirements that it be a flux surface or isobar for fixed-

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Outside the core of the plasma, the plasma current and pressure rapidly transition to zero in a scrape-off or edge region or plasma-vacuum interface. However, existing tools for fixed-boundary magnetohydrodynamic equilibria in 2D and 3D domains $\Omega$ typically prescribe the computational boundary $\partial\Omega$ interior to this transition layer. We (1) argue that a more realistic and robust assumption is to define the computational boundary exterior to this transition layer, in a vacuum-like region where $J|_{\partial\Omega} \sim p|_{\partial\Omega} \sim 0$, (2) show that, without this boundary change, existing coil optimization routines for 3D toroidal equilibria (stellarators) should be changed to match free-boundary equilibrium requirements, and (3) derive an algorithm for a fixed-boundary 3D equilibrium solver compatible with a very general computational boundary, with conditions $B \cdot n|_{\partial\Omega} \neq 0$ (not necessarily a flux surface), $p|_{\partial\Omega} \neq \text{const.}$ (not necessarily an isobar), and $J \times n|_{\partial\Omega} \neq 0$.

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physics.plasm-ph 2026-05-04 3 theorems

Density triggers conductor-insulator crossover in ultracold plasmas

Conductor-Insulator Crossover in the Steady-State Ultracold Plasmas

Collective ionization-recombination balance produces a sharp switch from Rydberg-gas insulator to conducting plasma at a critical density.

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We present a theoretical model of the ionization-recombination balance in the ultracold Rydberg gas-plasma mixture, which is caused by the collective processes rather than by individual interparticle interactions. This should be well relevant to the steady-state ultracold plasmas obtained in the recent experiment [B. Zelener, et al. Phys. Rev. Lett. 132, 115301 (2024)]. As follows from our calculations, there should be a sharp crossover from the insulating phase (Rydberg gas) to the conducting one (plasma) with increase in the particle density, which closely resembles Mott transition in the condensed-matter physics.

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physics.plasm-ph 2026-05-04

Hundreds of gyrokinetic simulations map power-dependent shaping effects

High-throughput full-f gyrokinetics of the tokamak boundary

Unsupervised full-f runs in TCV-like geometry find triangularity controls SOL temperature at low power but edge gradient at high power.

abstract click to expand

Full-f global gyrokinetic simulations of the plasma boundary have until now required heroic computational efforts and case-by-case expert intervention, precluding systematic parameter scans. Here we demonstrate a paradigm shift: hundreds of independent, concurrent, and unsupervised full-f boundary gyrokinetic simulations in a geometry inspired by the Tokamak \`a Configuration Variable (TCV), covering both the closed flux surface region and the open-field-line scrape-off layer (SOL) while scanning triangularity, elongation, and heating power. All simulations are evolved much longer than the turbulence relaxation time until the steady state is reached. Analysis of the steady-state profiles reveals that the impact of plasma shaping on confinement is strongly power dependent: at low power, triangularity primarily controls the SOL ion temperature, while at high power it mostly affects the edge ion temperature gradient. The low-power hot SOL observed for positive triangularity is explained by a neoclassical trapped-ion mechanism in which triangularity modifies the field-line arc length between banana turning points and the high-field-side limiter, altering the interaction with cold neutral-ionization regions. Fingerprint analysis of turbulent transport categorize the simulations in a regime dominated by ion temperature gradient (ITG) or trapped electron modes (TEMs), confirmed by dedicated local linear gyrokinetic calculations. The generated open data represents a previously unobtainable resource. It can serve both as a benchmark for boundary transport models, and as a training dataset for data-driven methods in fusion foundation and surrogate models.

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physics.plasm-ph 2026-05-04

Turbulence generates the flow that triggers L-H transition at lower power

The L-H transition in tokamaks: power threshold, density minimum and toroidal-field asymmetry

First-principles simulations derive scaling laws matching empirical thresholds and explain why one toroidal-field direction requires lessaux

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The physical mechanism underlying the L--H transition in tokamaks has remained an open problem for over forty years. We present three-dimensional flux-driven two-fluid simulations in a diverted geometry that exhibit a confinement transition at lower power in the favourable toroidal-field configuration. The simulations show that electromagnetic drift-wave turbulence spontaneously generates a sheared $\bm{E}\times \bm{B}$ flow responsible for transport suppression. The toroidal-field-direction asymmetry arises from time-reversal symmetry breaking by finite collisionality, as demonstrated by a quasilinear calculation of the turbulent momentum flux. First-principles scaling laws are derived for the L--H power threshold in both density branches, the density minimum, and the minimum power, all matching or surpassing existing empirical scalings.

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physics.plasm-ph 2026-05-04

MHD generators exceed 1000 MW/m³ at 0.1 atm with cesium

Profiles of the Power Density and Other Properties of Hydrogen Magnetohydrodynamic Generators at Conditions

Ideal power density in hydrogen plasma channels reaches exceptional levels when pressure drops and cesium is the seed.

abstract click to expand

Hydrogen and some of its derivatives (such as e-methanol, e-methane, and e-ammonia) are promising energy carriers that have the potential to replace conventional fuels, thereby eliminating their harmful environmental impacts. An innovative use of hydrogen as a zero-emission fuel is forming weakly ionized plasma by seeding the combustion products of hydrogen with a small amount of an alkali metal vapor (cesium or potassium). This formed plasma can be used as a working fluid in supersonic open-cycle magnetohydrodynamic (OCMHD) power generators. In these OCMHD generators, direct-current (DC) electricity is generated straightforwardly without rotary turbogenerators. In the current study, we quantitatively and qualitatively explore the levels of electric conductivity and the resultant volumetric electric output power density in a typical OCMHD supersonic channel, where thermal equilibrium plasma is accelerated at a Mach number of two (Mach 2) while being subject to a strong applied magnetic field (applied magnetic-field flux density) of five teslas (5 T), and a temperature of 2300 K (2026.85 {\deg}C). We varied the total pressure of the pre-ionization seeded gas mixture between 1/16 atm and 16 atm. We also varied the seed level between 0.0625% and 16% (pre-ionization mole fraction). We also varied the seed type between cesium and potassium. We also varied the oxidizer type between air (oxygen-nitrogen mixture, 21-79% by mole) and pure oxygen. Our results suggest that the ideal power density can reach exceptional levels beyond 1000 MW/m3 (or 1 kW/cm3) provided that the total absolute pressure can be reduced to about 0.1 atm only and cesium is used for seeding rather than potassium.

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physics.plasm-ph 2026-05-01

Vertical jogging paces ELMs to halve divertor heat flux

Characterization of ELM Pacing via Vertical Jogs on DIII-D

Oscillations at 20 Hz raise ELM rate from natural 5 Hz, shrink energy loss per event, and cut peak heat loads by factor of two.

abstract click to expand

Edge localized mode (ELM) pacing via vertical plasma oscillations or jogging has been successfully demonstrated on DIII-D. Rapid vertical movement of the plasma toward the X-point has been shown to effectively trigger ELMs. By vertically oscillating the plasma at a rate of 20 Hz, the ELM frequency increased from $\sim$5~Hz, the natural ELM frequency in similar DIII-D discharges, to 20~Hz. Downward jogs have been observed to trigger multiple ELMs in one cycle. ELMs triggered at higher than natural frequencies lead to smaller decreases in stored energy, from ~10\% to as little as below 1\%. As a consequence, the peak heat flux to the divertor has been observed to be reduced by a factor of $\sim$2. In addition, a reduction in the carbon impurity concentration has been observed. During downward jogs in the lower single null (LSN) configuration, the X-point movement is slower and smaller than the top of the plasma. As a result, a reduction in the plasma cross section and hence volume has been observed. To understand the mechanism of ELM triggering by jogging, a toy model of the edge toroidal current has been built and tested with DIII-D experiment data. The experimental data and model suggest that when the plasma moves down towards the X-point, a net positive toroidal current is locally induced in the edge region. ELITE stability analysis suggests that this current pushes the plasma state across the peeling side of the peeling-ballooning stability boundary into the unstable region triggering ELMs.

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physics.plasm-ph 2026-05-01

Two energy cascades in FLR-MHD set ion heating rate via helicity barrier

Determination of turbulent heating rate and relaxed states in finite Larmor radius magnetohydrodynamic turbulence with helicity barrier

Exact laws show the difference between the rates equals the turbulent heating of ions in imbalanced plasma turbulence.

abstract click to expand

Finite Larmor radius magnetohydrodynamics (FLR-MHD) provides a hybrid model of plasma that explains how turbulent energy cascade extends to sufficiently small parallel length scales, potentially leading to perpendicular heating of the ions in the solar corona and the solar wind. In this work, we derive exact laws for the cascades of energy and generalized helicity in fully developed FLR-MHD turbulence. In large and small scale limits, we obtain the exact laws for reduced MHD and electron reduced MHD turbulence respectively. Unlike ordinary or reduced MHD turbulence, a global stationary state is shown to be absent in the case of a strong imbalance between the Elsasser variables. This is due to the so-called helicity barrier, which leads to two separate stationary energy cascades with different cascade rates. Our derived exact laws enable us to calculate these two cascade rates and therefore their difference, which effectively provides the heating rate of the ions. In addition, we also derive alternative Banerjee-Galtier forms for the exact laws and hence obtain the relaxed states of FLR-MHD turbulence using the framework of recently proposed principle of vanishing nonlinear transfer. The relaxed states show alignment between the velocity and magnetic field fluctuations. However, due to strong anisotropy, no Beltrami alignment is possible for velocity and magnetic fields. Similarly to the exact laws, the relaxed states of reduced and electron reduced MHD emerge in the large and small scale limits, respectively.

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physics.plasm-ph 2026-05-01

Exponential chirp boosts plasma wakefields past 58 GV/m

Chirp-controlled plasma wake excitation by an exponential laser pulse in underdense plasma

Nonlinear phase variation across the pulse envelope strengthens accelerating fields more than linear or quadratic chirps in fluid and PIC

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The excitation of plasma wakefields driven by chirped laser pulses is investigated using a reduced relativistic fluid Poisson model supported by fully relativistic particle in cell (PIC) simulations. The study considers exponential, linear, quadratic, and unchirped phase-modulated laser drivers propagating in an underdense plasma. Numerical solutions of the governing equations demonstrate that exponential chirping produces enhanced wakefield amplitudes compared to polynomial and unchirped cases due to nonlinear phase variation across the pulse envelope. The analytical predictions are validated using quasi cylindrical PIC simulations performed under identical plasma and laser parameters. The simulations reveal strong chirp dependent wakefield modification, with positively chirped pulses generating peak accelerating fields exceeding 58 GV per m, accompanied by pronounced density compression and enhanced electron momentum gain. These results demonstrate that exponential chirping provides an effective mechanism for controlling wakefield strength and improving plasma based particle acceleration.

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physics.plasm-ph 2026-05-01

Resistivity derivatives disturb current density at tokamak cold fronts

Electrothermal Dynamics of Cold Front in Impure Tokamak Plasmas

Reaction term in the diffusion equation creates narrow-layer increases and decreases that compete with heating and radiation

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Current density perturbations induced by radiative collapse, which is a possible mechanism governing tokamak plasma disruptions, have been investigated using a reaction-diffusion model. The reaction term of the current diffusion equation, which depends on the first and second radial derivatives of the electrical resistivity profile, produces a strong disturbance in the current density profile in a narrow layer of the cold front. While the current density locally increases in the region where the electron temperature gradient is steep, it decreases behind the cold front in the region where the electron temperature profile exhibits a pronounced concave-down curvature. The electrothermal dynamics driven by such a shape of the current density perturbation and the competition between Ohmic heating and impurity radiation are simulated by the tokamak transport code INDEX.

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physics.plasm-ph 2026-05-01

Mildly relativistic correction fixes runaway current overestimation in tokamak startup

Self-consistent modelling and qualitative comparison of mildly relativistic runaway electron dynamics with a closed flux surface formation model during tokamak startup

DYON-RE model predicts KSTAR plasma parameters and radiative temperature during open-to-closed field transition, supporting ITER startup

abstract click to expand

A model for mildly relativistic Runaway Electrons (REs) is developed in a reduced-kinetic form and qualitatively compared with radiation characteristics observed in KSTAR ohmic startup. The mildly relativistic correction not only alleviates runaway current overestimation but also accounts for the partial parallel confinement of the initial runaway seed under an open-field configuration during early burn-through. The model is self-consistently integrated in the state-of-the-art predictive plasma initiation code DYON (Hyun-Tae Kim et al 2022 Nucl. Fusion 62 126012), hereafter referred to as DYON-RE. DYON-RE provides an improved RE confinement model during the transition from an open to a closed magnetic configuration by employing a model-based description of closed flux surface formation validated in multi machines. We show prediction capability of DYON-RE in two representative discharges among KSTAR ohmic startups. DYON-RE reliably predicts key plasma parameters such as plasma current, density, and temperature and also implies the characteristic behavior of the radiative temperature measured by electron cyclotron emission diagnostics in agreement with experimental results. The proposed model offers a framework for designing runaway-free ohmic startup scenarios in CPD and ITER. Future experimental validation will further refine its predictive capabilities and broaden its practical application.

0

physics.plasm-ph 2026-05-01

Refined scaling lowers uncertainty for tokamak error field thresholds

Improved n=1 Empirical Error Field Penetration Threshold Scaling with Ohmic and L-Mode Conventional Tokamak Plasma Discharges

Ohmic and L-mode database expansion yields a conservative bound with tighter projections for future device design.

abstract click to expand

This paper presents an updated n=1 error field penetration threshold scaling, which increases fit quality compared to previous error field scaling laws, is produced from an expanded database, and exhibits reduced uncertainty in projections to future conventional tokamaks. It improves confidence in tokamak engineering tolerances, which are a significant driver of cost and time constraints on device construction. We add J-TEXT data, new JET data, and create the scaling using only conventional tokamak Ohmic and L-mode experiments. Since H-mode plasmas are more resilient to error field penetration, this scaling predicts what is likely the most dangerous regime of error field penetration for new tokamak designs. These decisions improve confidence in the error field penetration threshold scaling and its application in the construction and design decisions of any future conventional tokamak or FPP.

0

physics.plasm-ph 2026-05-01

Neural operator predicts plasma properties with 0.1-1% error

DeepPropNet: an operator learning-based predictor for thermal plasma properties

DeepPropNet learns mappings for SF6-N2 and C4F7N mixtures and plugs directly into plasma simulation codes.

abstract click to expand

Thermal plasma properties play a critical role in plasma simulations and plasma-related applications. However, their strong nonlinear dependence on temperature, pressure, and gas composition makes accurate and efficient evaluation challenging. In this work, an operator learning-based model, termed DeepPropNet, is proposed for fast prediction of thermodynamic and transport properties of thermal plasmas. Two architectures are developed, including a single-property model (S-DeepPropNet) and a Mixture of Experts (MoE)-based multi-property model (MoE-DeepPropNet). The proposed models learn the nonlinear mapping from plasma operating conditions to physical properties based on high-fidelity datasets. The MoE architecture enables efficient multi-property prediction within a unified framework. Predictions are performed for binary SF6-N2 and ternary C4F7N-CO2-O2 mixtures. The results show that the proposed models achieve high accuracy, with relative L2 errors on the order of 10-3 to 10-2, while maintaining strong generalization capability under unseen conditions. The applicability of DeepPropNet is further demonstrated by coupling with finite volume method (FVM) and physics-informed neural networks (PINNs). The results indicate that DeepPropNet provides an efficient and scalable approach for plasma property prediction and plasma simulations.

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physics.plasm-ph 2026-04-30

New code speeds X-ray Thomson scattering analysis in dense matter

X-Ray Diagnostics Analysis Verification and Exploration (xDAVE) Code for the Prediction and Interpretation of X-Ray Thomson Scattering Experiments

xDAVE uses the Chihara model to fit spectra and plan experiments, validated on OMEGA beryllium shots.

abstract click to expand

X-ray Thomson scattering (XRTS) is a common diagnostic used in the warm dense matter (WDM) regime to estimate plasma parameters like density, temperature and charge state. Experimental analysis typically relies on a forward model to obtain estimates for these parameters, as the measured spectrum is a convolution of the dynamic structure factor (DSF) and the source-instrument function. The Chihara decomposition, where the spectrum is separated into contributions from bound and free electrons, is commonly used to estimate DSFs in the WDM regime, as it allows for the fast calculation of DSFs and therefore can easily be applied in a large-scale parameter optimization. Due to the limited availability of XRTS codes, in this work we present the ``\textbf{X}-ray \textbf{D}iagnostics, \textbf{A}nalysis, \textbf{V}erification and \textbf{E}xploration`` (\texttt{xDAVE}) code, designed to quickly estimate DSFs using the Chihara decomposition and analyse experimental spectra. The code is validated by re-analysing an experiment with isochorically heated beryllium at the OMEGA Laser Facility. In addition, we demonstrate the applicability of the code to plan experiments and predict scattering spectra through the coupling to a ray-tracing code. Lastly, the importance of accounting for the energy-dependence of spectrometer instrument functions is demonstrated by comparing ray-tracing simulations to the standard convolution for strongly compressed Beryllium shots at the National Ignition Facility similar to previously published results.

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physics.plasm-ph 2026-04-30

Plasma gratings match optical theory for pulse compression

Dispersive Properties of Plasma Diffraction Gratings: Towards Plasma-Based Laser Pulse Compression

0.005 deg/nm dispersion measured in 10-micron structures supports compact petawatt-to-exawatt laser designs

abstract click to expand

The standard architecture for a high-peak-power femtosecond laser is chirped pulse amplification using diffraction gratings for compression; the damage threshold of the compression gratings limits current lasers to multi-petawatt peak power. Plasma gratings have orders-of-magnitude higher damage tolerance than conventional optics, so plasma gratings with sufficiently high optical quality could allow the construction of ultra-high-power femtosecond lasers. Here, we present experimental measurements of the angular dispersion, angular bandwidth, and diffraction angles of ionization-based plasma transmission gratings and show that both the dispersive and the diffractive properties of these gratings are in close agreement with optical theory and simulations. Gratings with a period of 10.2 microns are found to have an angular dispersion of approximately 0.005 degrees/nm. The dispersion and bandwidth of these gratings suggest plausible designs for a plasma-grating-based compressor and indicate a pathway to compact lasers with petawatt to exawatt peak power.

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physics.plasm-ph 2026-04-30

Curvature rotation rate predicts coil non-planarity in QI stellarators

Exploring the link between coil non-planarity and magnetic surface geometry across a dataset of QI stellarators

Analysis of many plasma boundaries shows that how principal curvatures change across the surface largely sets the required coil shape.

abstract click to expand

Stellarator fusion devices confine plasma by means of complex, non-planar electromagnetic coils. Understanding how the shape of the plasma boundary determines the required complexity of the coil set is a central open question in stellarator design, with direct implications for engineering feasibility and the prospects of building next-generation fusion power plants. In this work we address this question using a large data-driven study. Starting from the Constellaration dataset of quasi-isodynamic (QI) stellarator plasma boundaries, we compute a set of filamentary coil configurations using constrained optimisation within SIMSOPT, and define quantitative coil-complexity metrics (torsion, SVD non-planarity score, inboard-side inclination angle, spectral width) together with a rich set of surface and magnetic geometry features (second fundamental form, principal-direction rotation rate, surface curvatures, and magnetic axis properties). Univariate and multivariate statistical analyses, reveal a strong, central role of the surface geometry: the principal-direction rotation rate of the plasma boundary is the single best predictor of coil non-planarity, while a Random Forest model using up to four surface features achieves R2 = 0.882 for the same target. These results provide quantitative evidence that the rate of change of the principal curvatures cross the plasma boundary are the primary drivers of coil non-planarity in this dataset of quasi-isodynamic stellarators.

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physics.plasm-ph 2026-04-30

The paper derives an analytical theory for how intense lasers excite relativistic surface…

Theory of Relativistic Surface Plasmon Excitation on Smooth Surface by High-Intensity Laser

Analytical theory derives a driven wave equation for relativistic surface plasmons, demonstrating that surface curvature and laser…

abstract click to expand

We present a classical theory of relativistic surface plasmon (RSP) excitation at a smooth plasma-vacuum interface driven by either a ponderomotive force or an electric field of an intense laser pulse. Starting from Maxwell equations coupled to a cold-fluid plasma response, we derive a general driven wave equation for the RSP and solve it analytically. We show that an infinite planar surface enforces conservation of the in-plane wavevector. A finite longitudinal interaction length or axial modulation supplies a finite kz spectrum, while cylindrical curvature replaces one continuous transverse in-plane wavenumber by a discrete azimuthal mode index m. This partially relaxes the planar in-plane constraint, while axial phase matching remains controlled by the longitudinal spectrum of the drive. The excitation strength is controlled by the overlap between the drive and the surface eigenfield, which is determined by the surface geometry. This provides a general principle for controlling RSP excitation. We also show that relativistic effects can substantially modify the dielectric response and can be preliminarily verified by particle-in-cell simulations. Within the local relativistic dielectric model, the overlap-normalised planar source saturates at large a0, and cylindrical curvature partially alleviates this reduction before strong surface softening develops. The role of surface geometry is analysed. A cylindrical surface can sustain an on-axis accelerating field, enabling highly nonlinear wakefield generation for particle acceleration. In addition, the cylindrical geometry imposes a precise mode-selection rule that provides intrinsic control over RSP excitation. Axisymmetric ponderomotive drive selects fundamental mode m=0. A linearly polarised laser field selects a superposition of m=+1 and m=-1 modes, and a circularly polarised laser field selects a single helical mode.

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physics.plasm-ph 2026-04-30

Plasma tail dechirps LWFA electrons to 3.4% energy spread

Plasma dechirper and lens for electron beams from laser wakefield acceleration in a tailored density profile

Down-ramp and 10 mm tail after plateau acceleration cut chirp and divergence, producing 190 MeV beams with 0.46 mrad rms divergence.

abstract click to expand

Achieving high-quality electron beams from laser wakefield accelerators critically relies on density tailoring to control electron dynamics during injection, acceleration, and extraction. We report on the experimental observation of electron beam acceleration and shaping, in transverse momentum and longitudinal phase space, controlled by plasma density tailoring in a gas cell. Electron beams with a FWHM charge of 40 pC at an energy of 190 MeV, 3.4\% energy spread and an rms divergence of 0.46 mrad, corresponding to a transverse momentum spread of 0.2 $m_e c$, have been measured. These beams have a peak spectral brightness of up to 8 pC/MeV/mrad. Simulations using experimental parameters as input show that acceleration in the plasma plateau leads to chirped electron beams which then undergo transverse momentum spread reduction in a plasma down-ramp followed by dechirping in a 10~mm long plasma tail, leading to the measured peaked spectra. %\textcolor{blue}{A control experiment with and without the LPT confirms these results.} The comparison of experimental results with and without long plasma tail confirms this analysis.

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physics.plasm-ph 2026-04-30

Nitrogen gas injection in the EAST tokamak with a full metal wall completely suppresses…

Nitrogen-induced ELM suppression and confinement improvement in the EAST tokamak with a full metal wall

Nitrogen seeding achieves ELM-free H-mode in EAST with improved confinement by driving a dissipative trapped electron mode at the pedestal…

abstract click to expand

This paper reports the achievement of an ELM-free H-mode regime with confinement improvement enabled by nitrogen (N2) seeding on the Experimental Advanced Superconducting Tokamak (EAST) with a full metal wall. Following N2 injection, large Edge-Localized Mode (ELM) bursts are completely suppressed, while global energy confinement is significantly enhanced, with the H98 factor increasing from approximately 0.9 to 1.2. A distinct edge coherent mode (ECM), localized at the pedestal foot (psi_N ~ 0.99), is identified using O-mode Poloidal Correlation Reflectometry and AXUV diagnostics. This mode operates within a frequency range of 20-50 kHz with a poloidal wavenumber of k_theta ~ 0.54 cm^-1. Linear gyrokinetic simulations performed with the CGYRO code reveal a dominant instability that quantitatively matches the experimental measurements. Detailed scans of parameters identify this mode as a Dissipative Trapped Electron Mode (DTEM). The energy and particle transport driven by this pedestal-foot DTEM effectively regulates the edge gradients, preventing the pedestal from crossing the Peeling-Ballooning stability boundary and sustaining a stationary ELM-free state. These findings provide a physical basis for an integrated scenario to maintain high confinement and protect plasma-facing components in future steady-state fusion reactors.

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physics.plasm-ph 2026-04-29

FPGA runs real-time ML to forecast disruptive plasma events

FPGA-Accelerated Real-Time Diagnostics at DIII-D Using the SLAC Neural Network Library for ML Inference

Live diagnostic signals are fed to a neural network on hardware whose output guides magnetic coils to suppress predicted disruptions in atok

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In this work, we demonstrate the deployment of a hardware-accelerated machine learning (ML) inference system integrated into a real-time processing at the DIII-D tokamak fusion reactor. The team has successfully deployed an AMD/Xilinx KCU1500 field-programmable gate array (FPGA) into the realtime Plasma Control System (PCS) nodes that receives the live Beam Emission Spectroscopy (BES) signal used for Edge Localized Mode (ELM) forecasting. The FPGA hosts a dense neural network using the SLAC Neural Network Library (SNL) that has been trained to infer the likelihood of disruptive ELM conditions. This likelihood then feeds a separate plasma controller that uses Resonant Magnetic Perturbation coils to suppress the predicted disruptive condition. The SNL allows for on-the-fly updates of the neural network weights and biases without requiring full hardware resynthesis for the FPGA. Judicious design of the neural-network architecture can further allow for the hot-swapping of multiple classification tasks to be executed on the single FPGA, significantly enhancing the real-time adaptability of the system for context-aware control strategies that respond in real-time to evolving reactor conditions. These adaptive weights naturally support continuous model refinement and seamless task switching during live experimental operation. This use case is chosen as a high rate signal processing example that can serve as a template for general ML-based reactor diagnostic processing for active reactor control systems. We see this as an essential development for achieving reactor relevant operation in future continuous operation fusion devices.

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physics.plasm-ph 2026-04-29

Analytic tensor yields emission formulas for tokamak runaways

Non-thermal electron cyclotron emission during runaway plateau in tokamak disruptions from an analytic hot plasma dispersion tensor

Closed-form expressions for cyclotron radiation and instability drive allow direct calculation during disruption plateaus even without wave–

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We derive an analytic hot plasma dispersion tensor for particle distribution functions characterized by Gaussian pitch-angle distributions. The formalism provides direct analytic expressions for non-thermal electron cyclotron emission coefficients and kinetic instability drive rate. We show the verification of the solutions using the KIAT and SYNO codes. The results offer possible mechanisms that could generate non-thermal electron cyclotron emission during tokamak disruption experiments, even when kinetic instability onset is forbidden.

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physics.plasm-ph 2026-04-29

Imaginary time extracts temperature from X-ray scattering without models

Model-free interpretation of X-ray Thomson scattering measurements

The correlation function in imaginary time gives direct access to temperature, normalization, and elastic weight from XRTS data.

abstract click to expand

X-ray Thomson scattering (XRTS) has emerged as a widely used diagnostics for extreme states of matter in a great variety of situations, and over a broad range of parameters. The standard approach for the interpretation of XRTS measurements is given by the forward modeling approach, where the electronic dynamic structure factor $S_{ee}(\mathbf{q},\omega)$ is computed from a suitable theoretical model, convolved with the combined source-and-instrument function, and then matched with the experimental observation, treating a-priori unknown parameters such as the mass density, temperature and ionization state as free fit parameters. Very recently, it has been suggested that this inherent model dependence can be avoided by analyzing XRTS spectra in terms of the imaginary-time correlation function (ITCF) $F_{ee}(\mathbf{q},\tau)$ [Dornheim \textit{et al.}, \textit{Nature Commun.}~\textbf{13}, 7911 (2022)], giving one model-free access to the temperature, normalization, Rayleigh weight, as well as a number of other properties. Here, we present a comprehensive review article on these developments, including accessible discussions of the method's theoretical background in terms of Feynman's imaginary-time path integral picture of statistical mechanics as well as its remaining limitations, in particular with respect to the source-and-instrument function of the experimental set-up. In addition, we discuss new chances for the further development of this framework by utilizing emerging capabilities for high-repetition XRTS experiments with meV resolution over spectral ranges of tens of eV at state-of-the-art x-ray free electron laser (XFEL) facilities such as the European XFEL in Germany.

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physics.plasm-ph 2026-04-29

Webcam AR overlays live particle paths on fusion fields

Augmented reality system for visualising magnetic field topology and charged-particle trajectories in magnetic fusion plasmas

Simulation steps match camera frames so users inspect 3D magnetic islands by simply moving the camera.

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A cost-effective augmented reality (AR) system is presented for visualising three-dimensional magnetic field structures and charged-particle trajectories in magnetically confined fusion plasmas. The system presented in this study integrates an orbit-following simulation code with a marker-based AR framework using a web camera and the OpenCV library. By synchronizing the time step of the simulation with the frame rate of the camera, the trajectories are continuously updated and superimposed in real time onto the camera image. Through the interactive operation of manipulating the web camera, users can observe three-dimensional structures, such as magnetic islands, from various positions and viewing angles. Simultaneously, the visualisation results can be shared by multiple people through a display. Such a shared AR environment supports an intuitive understanding of three-dimensional spatial structures that involve a high cognitive load. It also enables collaborative reasoning based on common visual information in research on magnetic confinement fusion, where researchers and students have diverse backgrounds in physics, engineering, and related fields.

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physics.plasm-ph 2026-04-29

Axial magnetic field enhances and hybridizes plasma wakefields from relativistic beams

Theoretical Analysis and PIC Simulations of Electromagnetic Wakefields Excited by Relativistic Beams in Magnetized Plasmas

Green function analysis and simulations show magnetization boosts amplitudes and adds coherent radial oscillations for better beam control.

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This study presents theoretical and numerical investigation of the coupled longitudinal and radial wakefields excited by ultrarelativistic electron beams propagating through a cold plasma channel subjected to an external axial magnetic field. A fully causal three dimensional Green function formalism is developed directly from the linearized Maxwell fluid equations in the presence of the magnetized plasma dielectric tensor. This unified framework captures the complete electromagnetic response, including the induction of a transverse plasma current and the resulting hybridization of longitudinal charge separation dynamics with cyclotron driven transverse motion. The analytical treatment reveals how magnetization modifies the effective restoring forces, enhances wake amplitudes, and reshapes the radial focusing defocusing structure of the wake. To validate the theoretical predictions and explore realistic parameter regimes, extensive three dimensional particle in cell simulations are performed using the EPOCH code across wide ranges of plasma density, magnetic field strength, beam Lorentz factor, transverse beam radius, and longitudinal current profiles. The simulations demonstrate excellent quantitative agreement with the analytical Green function solutions, confirming that increasing plasma density substantially amplifies the initial wake amplitude while accelerating the damping of higher order oscillations. Application of an external magnetic field induces coherent high frequency radial oscillations, strengthens focusing forces, and produces a hybrid eigenmode whose properties are absent in the unmagnetized limit. Variations in the driver Lorentz factor lead to rapid convergence toward a universal ultrarelativistic wake structure, while the transverse beam profile controls the radial extent and balance between longitudinal acceleration and transverse focusing.

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physics.plasm-ph 2026-04-29

Right-handed laser polarization boosts plasma second-harmonic output

Second Harmonic Generation Through Backward Raman Scattering in Magnetized Plasmas Driven by Circularly Polarized Intense Lasers

Resonant circular polarization strengthens the Raman-to-harmonic cascade in magnetized plasmas while the opposite handedness suppresses it.

abstract click to expand

A fluid-based theoretical framework is developed to describe the nonlinear cascade linking primary BRS-driven plasma wave amplification, oscillating two-stream instability (OTSI), nonlinear current generation within a self-formed ponderomotive channel, and radiation of the secondary electromagnetic mode. Systematic parameter studies reveal a strong sensitivity of the entire cascade to the relative handedness of laser polarization and axial magnetic field direction, as well as to cyclotron resonance strength. Resonant right-handed circular polarization significantly enhances ponderomotive expulsion, channel depth, BRS and OTSI growth rates, nonlinear current density, and the amplitude of the secondary harmonic, whereas non-resonant left-handed polarization effectively suppresses these processes. Fully kinetic particle in cell simulations using EPOCH, together with macroscopic finite-element modelling in COMSOL Multiphysics, corroborate the polarization- and magnetization dependent wake modulation and channelling efficiency across a wide range of laser wavelengths, pulse durations, and plasma densities. The temporal evolution and saturation of OTSI, resilience to density inhomogeneities, and wavenumber-resolved resonance tuning illustrate axial magnetization as a flexible control mechanism for adjusting the intensity, spectral position, bandwidth, and stability of multi peak Raman spectra. These findings demonstrate that cyclotron resonance and polarization control are effective methods for manipulating nonlinear Raman emission in high-intensity laser magnetized plasma interactions, offering predictive insights for forthcoming kinetic simulations and experimental implementations.

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physics.plasm-ph 2026-04-29

New closure embeds exact kinetic scaling in plasma fluid equations

Wave-number-dependent closure condition for fluid moment equations

Mapping Padé coefficients to kinetic roots preserves dispersion relations at all scales in fluid models.

abstract click to expand

Fluid models offer crucial computational efficiency for plasma simulations, yet accurately capturing kinetic effects like Landau damping remains a fundamental challenge. While conventional closures (e.g., Hammett-Perkins and Hunana) are widely used, their fidelity relative to exact kinetic response degrades significantly depending on the perturbation wave number. Here, we propose a novel wave-number-dependent closure condition for the three-moment fluid equations that explicitly preserves the primary dispersion relation. By mapping Pad\'e approximant coefficients directly to the kinetic roots of the collisionless Vlasov-Poisson system, we derive an analytical closure that rigorously embeds exact kinetic scaling across all spatial scales. We further demonstrate that this framework readily extends to collisional plasmas via the BGK model. This deterministic approach precisely captures the long-term macroscopic evolution of fluid moments and field energy, offering a rigorous foundation for high-fidelity fluid modeling.

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physics.plasm-ph 2026-04-28

Quasilinear model matches nonlinear ion fluxes from linear inputs

Quasilinear flux model consistent with gyrokinetic ordering

When ion and electron temperature gradients are comparable, the ordering-derived saturation reproduces both spectrum and magnitude of full-6

abstract click to expand

We propose a quasilinear (QL) flux model in which the saturation amplitude is uniquely determined using multiscale gyrokinetic ordering relations. The model is fully self-contained within a linear framework and does not rely on calibration against nonlinear simulations or mixing-length estimates. The wavenumber-dependent flux is given in ion gyro-Bohm units with a weighting factor of $|k_\theta \rho_i|$, such that its area integral in the log-linear scale yields the total flux, as employed in multiscale simulations. In systems with comparable ion and electron temperature gradients, the QL ion energy flux reproduces nonlinear simulation results in both its wavenumber dependence and absolute magnitude. In contrast, the QL electron flux is predominantly generated at electron scales, indicating that the shift of electron-scale transport toward ion scales observed in nonlinear Gsimulations is not captured within the present linear framework. We argue that the relation $Q_i\sim Q_e$, obtained as a closed conclusion of the QL model, may be predictive of simulation results if the area-integrated flux is conserved in nonlinear energy cascade process.

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physics.plasm-ph 2026-04-28

Skewness scales with Knudsen number in solar wind electrons

Knudsen number as a non-thermal parameter: possible origin of skewness in space plasma distributions

Model derives δ ∼ K_N, showing how weak collisions produce asymmetry in non-thermal plasma distributions.

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Non-Maxwellian distributions and their origins in space plasma have attracted significant attention due to their prevalence and impact on various astrophysical and space-related phenomena. This paper presents a theoretical study of the consequences of incorporating a Skew-Kappa distribution to describe the non-thermal electron distribution in the solar wind. By introducing a Krook-like term into the Boltzmann equation to represent collision effects, we investigate the dependence of the skewness parameter on plasma macro-dynamics. Our analysis focuses on understanding the departure from thermal equilibrium and the statistical behavior of the plasma under the influence of collisional processes. By analyzing the Boltzmann transport equation adapted to space plasma, we derive expressions for the skewness parameter as a function of plasma parameters and the collision effect. Our results provide valuable information on the relationship between skewness, collisional dynamics, and the statistical properties of space plasmas, namely $\delta \sim K_N$, the relationship between the skewness parameter and the effective Knudsen number. This study contributes to a deeper understanding of non-Maxwellian distributions and their role in astrophysical and space plasma phenomena.

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physics.plasm-ph 2026-04-28

ITER 60-second current ramp feasible if plasma hot enough

A simple model of current ramp down in the ITER tokamak

Cylindrical model predicts tearing modes only for faster shutdowns that could lock and disrupt the plasma.

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The controlled ramp down of the toroidal plasma current in the ITER tokamak is simulated using a simple model that employs cylindrical geometry. The magnetohydrodynamical (MHD) stability of the plasma throughout the whole current ramp is also calculated. The only potentially unstable MHD mode is the m=2/n=1 classical tearing mode. The envisioned 60 second ramp down of the plasma current in ITER is found to be perfectly feasible, provided that the plasma is sufficiently hot at the start of the ramp. However, attempts to ramp down the current on a significantly faster time scale are predicted to excite 2/1 tearing modes that are likely to lock to the vacuum vessel, and trigger a disruption.

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physics.plasm-ph 2026-04-27

Microwire arrays yield 3.67×10^7 n/sr/J neutrons with 1 PW laser

Efficient Generation of Neutrons Based on Ultrashort Laser-driven Direct Acceleration in Microwire-Array Targets

Optimal wire spacing boosts protons above 1 MeV, enabling compact high-repetition sources via beryllium converter.

abstract click to expand

We report on an experimental demonstration of efficient neutron generation based on direct laser acceleration in microwire-array targets irradiated by ultrashort (tens of femtoseconds) laser pulses. The optimal array period was identified, at which the maximum proton energy and the number of protons with energies exceeding $1~\mathrm{MeV}$ were significantly increased. Using a $1~\mathrm{PW}$, $\sim25~\mathrm{fs}$ laser at a moderate intensity of $\sim10^{20}~\mathrm{W/cm^2}$, a high neutron yield of up to $(8.33\pm0.84)\times10^{6}~\mathrm{n/sr/J}$ was detected from the LiD converter via $^7\mathrm{Li}(p,n)$ and $\mathrm{D}(p,n+p)$ nuclear reactions. Self-consistent integrated simulations reproduced the experimental results and predicted that with a Be converter, a forward pulsed neutron source with an unprecedented yield per joule of $3.67\times10^{7}~\mathrm{n/sr/J}$ can be obtained under identical laser conditions. This type of neutron source is favorable for applications that require a high repetition rate utilizing compact and economical laser systems.

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physics.plasm-ph 2026-04-27

X-ray Thomson scattering diagnoses extreme matter states

Overview of X-ray Thomson scattering measurements of extreme states of matter

Overview catalogs experiments, analysis methods and thermodynamic inferences from laser and XFEL facilities.

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Since its first successful applications in the early 2000s, x-ray Thomson scattering (XRTS) has emerged as one of the most successful tools for the diagnostics of extreme states of matter in the laboratory. By sampling the dynamic structure factor of the electrons, XRTS is capable of giving detailed insights into the atomic-scale physics of the matter probed. Moreover, thermodynamic parameters, like the mass density, temperature, and ionization state, are routinely inferred from XRTS measurements, providing a comprehensive characterization of the sample probed. In addition, the dynamic structure factor is of considerable interest in its own right as it contains information on other effects such as the plasmon shift, miscibility between species, electronic states and potential transitions between these states. In this work, we provide an extensive overview of previous XRTS experiments at both traditional laser and X-ray free electron laser facilities, including information about the probed material (elements, conditions), scattering geometry, analysis methods as well as corresponding references. In addition, we briefly discuss the advantages and shortcomings of widely used analysis methods for XRTS spectra and reflect on upcoming future developments in XRTS experiments and theory.

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physics.plasm-ph 2026-04-27

Plasma current sets tokamak fusion triple product as I_p squared

Revisiting confinement scalings and fusion performance with a perspective optimized for extrapolation

Optimized scalings show quadratic dependence for both triple product and power, implying higher currents needed for high-field reactor goals

abstract click to expand

Recent advances in high-temperature-superconductor technology have made substantially higher toroidal magnetic fields technologically accessible, reopening the design space for compact, high-field tokamak reactors. Because reactor performance projections remain anchored to empirical confinement scalings, the recent update to the ITPA global H-mode confinement database raises an important question: what does the present experimental record and its uncertainty imply for the path to reactor-grade fusion performance? In this work, we revisit confinement extrapolation from an explicitly extrapolation-oriented perspective and, to complement its implications in terms of a direct reactor performance measure, present a cross-machine empirical scaling for fusion power. We systematically search for a minimally complex confinement scaling that optimizes the tradeoff between variance capture and extrapolative robustness. We find that low-order models centered near $N=3$ to $N=4$ optimize this tradeoff, with plasma current, machine size, heating power, and elongation emerging as the dominant engineering levers, together with an empirically inferred confinement penalty associated with metallic walls. Recast in reactor-performance terms, the results indicate that both the fusion triple product and fusion power are governed primarily by plasma current: the triple product scales approximately as $I_p^2$, and the empirical fusion power scaling exhibits a similarly near-quadratic dependence over a survey of the highest performing discharges across several machines. Projecting to reactors, these results suggest that high-field devices with metal walls may require higher plasma current than standard IPB98$(y,2)$-based expectations imply, and that gigawatt-class tokamak performance likely demands operation at $I_p \gtrsim 20\mathrm{MA}$.

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physics.plasm-ph 2026-04-27

Stacked shocks approach isentropic compression in spheres

Scaling laws of multi-shock implosions toward the quasi-isentropic limit

As the number of converging shocks rises, final density increases while entropy and Rayleigh-Taylor growth are suppressed, confirmed by self

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We present a unified theoretical and numerical framework for self-similar multi-shock implosions achieving ultrahigh compression in a uniform solid spherical target. Extending the classical Guderley model to N stacked, spherically converging shocks, we derive selfsimilar solutions and the scaling law for the final density. One dimensional Lagrangian hydrodynamic simulations confirm this relation over a broad range of parameters, from the weakly to the strongly nonlinear regime. The results show that cumulative compression increases systematically with the number of stacked shocks while entropy generation is strongly suppressed, asymptotically approaching a quasi isentropic limit as N increases infinity. This volumetric scheme strongly suppresses the Rayleigh Taylor instability that plagues shell based implosions and thus provides a robust, largely instability-resistant compression pathway applicable to inertial confinement fusion and other high energy density systems. The framework bridges similarity theory with realistic multi-shock dynamics, guiding the design of advanced laser-driven compression schemes.

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physics.plasm-ph 2026-04-27

PINN surrogate predicts plasma sheath profiles across parameters

A Deep Learning Approach to Describing the Plasma Sheath

Trained once on governing equations, the network rapidly outputs density and potential profiles for any input conditions instead of solving

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Despite their ubiquity, the rich physics present in a plasma sheath has inhibited the development of a generally applicable description of this critical region. The present study utilizes a physics-informed neural network (PINN) to evaluate a hierarchy of models of the plasma sheath. Unlike traditional deep learning methods, PINNs use the governing PDEs to constrain the predictions of a neural network, and thus do not require any experimental or simulation data to train. In this work, we utilize a PINN to identify the parametric solution to fluid models of different physics fidelity of the plasma sheath. While the offline training time of the PINN is often longer than a traditional solver, once trained, the PINN is able to efficiently predict the sheath profiles across a broad range of parameter regimes, thus yielding an effective surrogate of the plasma sheath.

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physics.plasm-ph 2026-04-27

ITER tungsten wall survives unmitigated VDE heat loads

3D modelling of thermal loads during unmitigated vertical displacement events in ITER and JET

Workflow validated on JET predicts no melting and maps 3D energy deposition for ITER first wall during vertical displacements

abstract click to expand

Predicting three-dimensional thermal loads during tokamak disruptions is essential for ITER yet remains weakly developed. We present a physics-based workflow that couples MHD simulations of vertical displacement events with field line tracing on a realistic 3D first wall model and a transient wall thermal response. The approach is validated against JET discharges with beryllium main chamber armour, reproducing key global dynamics, non-axisymmetric current features, and the occurrence (or absence) of melting, thereby building confidence in the methodology. We then apply the same workflow to ITER-relevant conditions with tungsten (W) armour, consistent with the new 2024 ITER re-baseline, to assess disruption heat loads and their 3D localization. The resulting analysis demonstrates the resilience of the ITER W first wall against these events and provides predictions for the energy deposition and current flow profiles. Beyond these studies, the workflow enables scenario-by-scenario estimates of disruption-induced thermal loading, allowing to assess the disruption-budget consumption for these events in future devices.

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physics.plasm-ph 2026-04-27

Pinching driver beam injects spin-polarized electrons

Pinching injection in wakefields for spin-polarized electron beams

Simulations show 50 percent spin preserved due to injection geometry, easing use of polarized targets in wakefields.

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Pinching of the driver beam in plasma wakefield acceleration is generally considered an unwanted effect that needs to be mitigated. Here, we propose that this effect can be utilized for the injection of spin-polarized electron beams from hydrogen halide targets into wakefields. Particle-in-cell simulations show that the electron spin is preserved on a level of 50% for a wide range of parameters due to the injection geometry. The presented injection scheme provides a possible pathway to alleviate some of the restrictions associated with pre-polarized hydrogen halide targets.

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physics.plasm-ph 2026-04-24

Mobile ions extend Weibel instability growth through channel merging

Ion Channel Dynamics in Temperature-Dependent Weibel Instability Saturation

Simulations show ions merge channels at late times, raising magnetic energy while electrons equilibrate faster than ions.

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We present 1X2V continuum Vlasov-Maxwell simulations of interpenetrating plasma beams with mobile ions. While the early-time evolution is similar to the stationary-ion case, the late-time dynamics are dominated by the ion-Weibel instability. As ion channels merge, the magnetic energy increases and the magnetic structures extend further along the beam direction. Electrons rapidly reach thermal equilibrium, whereas ions retain distinct bulk velocities for much longer and thermalize more slowly. These results are relevant to collisionless shock formation in astrophysical compact objects and laser-plasma experiments. Wind/SWE observations place all four simulated cases in the firehose/Weibel-unstable region of the proton temperature anisotropy diagram, and MMS1 observations of a quasi-perpendicular bow shock ($\theta_{Bn}\approx83^\circ$, $M_A\approx27$) show a qualitatively similar electron-ion thermalization disparity.

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physics.plasm-ph 2026-04-24

Hybrid model couples kinetic ions to drift-kinetic electrons

Towards hybrid kinetic/drift-kinetic simulations in 6d Vlasov codes

An implicit solver enforces quasi-neutrality and produces ion-scale zonal flows without electron stiffness in six-dimensional Vlasov codes.

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Simulating fully kinetic, two-species plasmas is computationally challenging due to the stiff multiscale dynamics of electrons and ions. While enforcing a quasi-neutral time evolution mitigates this stiffness, it requires an electric potential that consistently maintains this constraint. In this work, we present an implicit approach to determine this electric field self-consistently within the semi-Lagrangian, fully kinetic BSL6D code. We employ a hybrid two-species model that couples kinetic ions with massless, drift-kinetic electrons, enabling an implicit treatment of the latter. Notably, the model captures the generation of ion-scale zonal flows. Beyond the algorithmic description, we provide a proof of second-order time-splitting error convergence under specific regularity assumptions. A key feature of our approach is an error-balancing mechanism: we demonstrate that the field solver achieves the required accuracy of the electric field by automatically adjusting the error of certain moments of the distribution function. Furthermore, we provide a comprehensive analysis of semi-Lagrangian interpolation errors to ensure robustness against the steep density and temperature gradients characteristic of tokamak edge plasmas.

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physics.plasm-ph 2026-04-24

Phase mixing turns collisionless particle paths into apparent diffusion

Collisionless Phase Mixing Mimics Diffusive Transport in Radiation Belt Observations

Spacecraft sampling of neighboring drift shells converts localized structures into decorrelating signals within a few periods.

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Since the dawn of the space age, observations of energetic particles in planetary radiation belts have been interpreted within a diffusive transport framework, even though the processes that populate and deplete these belts produce highly structured and spatially localized distributions. This exposes a fundamental problem: how can coherent phase-space structures evolving under collisionless dynamics give rise to observational signatures that appear consistent with diffusion-based transport? Here we show that diffusion-like behaviour can arise from an observational phase-mixing effect, independent of stochastic wave-particle transport. As spacecraft sample neighbouring drift shells while particles undergo electromagnetic drifts, spatially localized drift-phase structures are converted into rapidly decorrelating temporal signals, making them observationally indistinguishable from stochastic processes. We show that the effective lifetime of these structures is only a few drift periods, preventing the resolution of fine-scale structure. These results demonstrate that collisionless dynamics can mimic diffusive transport on short timescales, limiting the inference of particle acceleration processes and biasing transport estimates. This calls for a reassessment of diffusion-based interpretations of radiation belts at Earth, across the solar system, and in the radiation belts of ultra-cool brown dwarfs.

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physics.plasm-ph 2026-04-24

Exact virial coefficient benchmarks PIMC for hydrogen plasma EoS

The virial expansion of the Hydrogen equation of state in comparison to PIMC simulations: the quasiparticle concept, IPD, and ionization degree

Combining analytical low-density results with simulations yields precise equation of state and reveals medium effects on ionization.

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The properties of plasmas in the low-density limit are described by virial expansions. Analytical expressions are known for the lowest virial coefficients from Green's function approaches.Recently, accurate path-integral Monte Carlo simulations were performed for the hydrogen plasma at low densities by Filinov and Bonitz [Phys. Rev. E 108 (2023)055212], which made a comparison of the virial expansions and the derivation of interpolation formulas possible. The exact expression for the second virial coefficient is used to test the accuracy of the PIMC simulations and the range of application of the virial expansions.To describe plasmas in a wider range of density and temperature, the concept of quasiparticles is considered. Medium modifications of free and bound states are obtained from the spectral function. Mean-field effects are presented, such as exchange terms, Pauli blocking and screening. The density expansions of the quasiparticle shifts is considered. The combination of PIMC simulations with benchmarks from exact virial expansion results allows us to obtain precise results for the EoS in the low-density range. At low densities, the results are compared with the Saha equation to introduce the medium-dependent ionization potential. The relation to the Beth-Uhlenbeck formula and concepts such as the Mott effect, ionization potential depression (IPD), and ionization degree are discussed. The limits of current PIMC results for hydrogen plasmas are shown. Further improvements of the PIMC simulations are required to compare with analytical benchmarks.

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