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Physics Physics Plasma Physics

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Sunday 13 September

Plasma Physics

Superradiant Thomson Scattering via Oscillating Quasiparticles

Qianyi Ma, Yuhui Xia, Zhenan Wang et al. (+4 more)

The recently proposed concept of generalized superradiance (GS) allows for the generation of coherent radiation without the need for complex compression or prebunching of relativistic electrons. However, the quasiparticles in current GS schemes are formed through local electron accumulation and their sizes exceed 100 nm, restricting the achievable radiation wavelength. In this study, we demonstrate a realization of narrow quasiparticles with <10 nm widths through sheet-crossing. When an energy chirped electron beam collides with an intense laser pulse, lower energy electrons at the front slip back, forming an accelerating quasiparticle. These quasiparticles undergo transverse oscillations within the laser field, thereby extending GS emission from the Cherenkov regime into the synchrotron regime. Three-dimensional particle-in-cell simulations indicate the production of gigawatt-class chirped radiation in the 10-100 nm range. The proposed scheme is compatible with existing Thomson scattering facilities, paving the way for the generation of coherent ultrafast radiation.
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All authors Qianyi Ma, Yuhui Xia, Zhenan Wang, Letian Liu, Zhiyan Yang, Xinlu Xu, Xueqing Yan

Friday 11 September

Plasma Physics

Hydrogen and Lithium Isotope Analysis via Optical Spectroscopy of Laser-Produced Plasmas

Elizabeth Kautz, Caleb Clark, Nusrat Karim et al. (+2 more)

Accurate detection and quantification of light-element isotopes, particularly hydrogen (H) and lithium (Li), are essential across chemistry, materials science, geology, nuclear energy, forensics, and defense applications. Their isotopic compositions provide insight into fuel cycles, materials degradation, mass transport, and environmental processes. Hydrogen and Li isotopes are especially relevant in nuclear environments because neutron-induced reactions directly connect their isotopic inventories. Despite their importance, analysis of these light isotopes remains challenging due to their low atomic mass, high mobility, and, in some cases, radiological constraints. While some existing analytical techniques can detect and measure H and/or Li, they typically require extensive sample preparation, laboratory-based instrumentation, or extended analysis times that preclude real-time monitoring capabilities. To address these limitations, optical spectroscopy of laser-produced plasmas offers a promising solution, enabling field-deployable, standoff sensing of isotopic signatures with real-time analysis capability and no sample preparation requirement. This Topical Review summarizes fundamentals and recent advances in H and Li isotopic analysis using optical spectroscopy, with emphasis on laser-produced plasmas generated by laser ablation of solid targets. Key concepts relevant to isotope-resolved optical measurements are introduced, including isotope shifts, fine and hyperfine structure, and line-broadening mechanisms. Progress in optical emission, laser induced fluorescence, and absorption spectroscopy is reviewed for H and Li isotopes.
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All authors Elizabeth Kautz, Caleb Clark, Nusrat Karim, Mark C. Phillips, Sivanandan S. Harilal

Plasma Physics

Sensitivities of electron-scale core transport in MAST Upgrade

B.S. Patel, T. Adkins, S. Blackmore et al. (+2 more)

Integrated modelling of the Mega Ampere Spherical Tokamak Upgrade (MAST-U) indicates that turbulent electron heat transport can dominate over ion transport across a range of operating regimes. Only a limited set of instabilities are able to produce this behaviour, with the primary candidates being microtearing modes (MTMs) and electron temperature gradient (ETG) driven modes. This work investigates electron-scale core transport in two L-mode plasmas and one H-mode plasma on MAST-U using local gyrokinetic analysis. The linear and nonlinear sensitivities of ETG modes are examined, with particular focus on their dependence on electron temperature gradients and ExB shearing rates. In L-mode discharges, ETG modes are found to be linearly unstable over a broad radial region and can drive experimentally relevant levels of electron heat transport, particularly towards the outer core. The transport is strongly sensitive to both the electron temperature gradient and the ExB shear, with nonlinear simulations showing stiff transport and good agreement with experimental estimates within uncertainty. Towards the core, however, ETG-driven transport is reduced and is insufficient to fully explain the observed anomalous heat flux. In contrast, in the H-mode plasma ETG modes are found to be stable or only weakly unstable in the core, resulting in negligible electron-scale transport. This is attributed to higher equilibrium pressure gradients and reduced electron temperature gradients, both of which stabilise ETG turbulence. Following the collapse of the core rotation profile, ETG modes can become unstable towards the outer core, but typically remain insufficient to account for the full level of transport. Overall, ETG turbulence can play a significant role in setting electron heat transport in L-mode plasmas, particularly at larger radii, but is unlikely to dominate in high-performance H-mode conditions.
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All authors B.S. Patel, T. Adkins, S. Blackmore, D. Kennedy, C. Vincent

Plasma Physics · Cross-listed from physics.flu-dyn

A Variational Formulation of the MHD Induction Equation

Ahmed Farooq

We present a variational formulation of the induction equation in magnetohydrodynamics (MHD) based on Gauss's principle of least constraint. The central result is the Euler–Lagrange equation $\mathbf{Z}_B = -\nabla h_m$, where $h_m = \mathbf{A}\cdot\mathbf{B}$ is the magnetic helicity density. This reveals that the magnetic helicity gradient $\nabla h_m$ acts as the constraint force maintaining the solenoidality of the magnetic field, exactly as the pressure gradient $\nabla p$ maintains incompressibility in Taha et al.'s pressure-gradient minimization principle. The magnetic helicity density—which, though gauge-dependent locally, yields a gauge-invariant variational principle—naturally emerges as the Lagrange multiplier enforcing $\nabla\cdot\mathbf{B}=0$. At the solution, the field minimizes the norm of the magnetic helicity gradient $\|\nabla h_m\|^2$. This establishes a structural analogy: magnetic helicity is to the magnetic field as pressure is to velocity. The variational principle connects to Woltjer's theorem, Taylor relaxation, and the Hamiltonian structure of MHD.
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Plasma Physics

Nonuniform Hydrodynamic Plasma-Wave Instability in a Gated Electron Channel

Yuhui Zhang

We report a theoretical and numerical study of plasma-wave instability in a current-driven InGaAs/GaAs gated electron channel. A finite drain current drives the channel away from a uniform plasma-wave cavity: the carrier density and plasma velocity decrease toward the drain, while the electron drift velocity increases. This current-induced spatial nonuniformity modifies both the propagation time and amplification of plasma waves and becomes increasingly important as the sonic regime is approached. Using a hydrodynamic description combined with the unified charge-control model, we determine the nonuniform steady state and analyze the resulting plasma-wave frequency and instability growth rate. The oscillation frequency is found to be governed by wave propagation through the entire channel rather than by the local drain conditions, leading to a pronounced current-induced frequency reduction that is not captured by a uniform-channel approximation. The instability results from the competition between boundary-induced plasma wave gain and distributed losses due to momentum relaxation and electron viscosity. The finite-viscosity theory shows good quantitative agreement with the simulation data over the subsonic regime and regularizes the singular behavior of the inviscid description near the sonic point. At higher currents, the dynamics become increasingly sensitive to the near-sonic structure and contact boundary conditions, indicating that the eventual suppression of the instability cannot be attributed to bulk hydrodynamics alone.
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Plasma Physics

First Experimental Evidence of Helicon Current Drive

J.B. Lestz, R.I. Pinsker, B. Van Compernolle et al. (+10 more)

Helicon current drive is an attractive solution for driving current to sustain steady state tokamak operation in reactor conditions. Dedicated DIII-D experiments have been conducted with a MW-level helicon system and successfully demonstrated core power deposition and current drive with helicon waves launched via a traveling wave antenna. The profile of the measured electron temperature response to helicon power injection is in good agreement with time-dependent integrated modeling that incorporates ray tracing and the effects of thermal transport simultaneously. When the helicon power is injected continuously to drive co-$I_p$ current, the reconstructed safety factor profile flattens significantly faster and sawteeth are triggered earlier than in comparison shots where the helicon is replaced by a comparable amount of electron cyclotron heating. Calculation of the helicon-driven current profile yields a peaked profile in the core, consistent with the observed power deposition profile and in good agreement with ray tracing predictions. Taken together, these experimental results represent strong evidence for the first definitive observation of auxiliary current drive due to helicon waves on any device.
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All authors J.B. Lestz, R.I. Pinsker, B. Van Compernolle, S.X. Tang, A. Dupuy, A.M. Garofalo, L. McAllister, C.P. Moeller, C.C. Petty, M. Porkolab, M.P. Ross, G. Rutherford, A. Torrezan

Thursday 10 September

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

Physics-Informed Drift Diagnosis for Laser-Plasma Accelerator Operations

Ou Labun, Calin Hojbota, Mara Klebonas et al. (+4 more)

Laser-plasma accelerators (LPAs) sustain accelerating gradients of order $100\,\mathrm{GV/m}$, but routine operation remains difficult: electron beam metrics drift over an operating shift, and the root physical cause is often invisible to the available diagnostics. We formulate LPA operation as a latent state-space model in which three effective interaction-point variables, the normalized laser amplitude $a_0$, the normalized plasma electron density $\tilde n_e$ and the residual pulse chirp $\mathcal{C}$, are inferred from routine electron beam observations by an extended Kalman filter. The emission model, which maps the latent state to the diagnostics, is kept structurally separate from the {transition} model, which describes how the latent state evolves between shots. The separation supports diagnosis in two stages, one asking which latent variable moved and one asking what moved it. The implemented emission model is a toy model, yielding an expected performance in line with current facilities and using 3D blow-out regime dependencies where relevant. We conduct synthetic sessions to test the effectiveness of the detection and attribution protocols, finding that attribution is limited by excitation rather than by shot count or diagnostic resolution. Because the construction needs only a set of physical latent variables, an emission model and a family of hardware-derived transition models, it transfers to other drift-prone subsystems. We argue that the accuracy of the whole procedure is limited by the emission model rather than by the inference method.
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All authors Ou Labun, Calin Hojbota, Mara Klebonas, Mike Downer, Rafal Zgadzaj, Phil Franke, Lance Labun

Plasma Physics

Experimental Plasma Density Profiles Determined Through Measurements of the Magnetosonic Wave Speed

Cameron Kuchta, Jan Egedal, Abhishek Mhatre et al. (+4 more)

Information on plasma density in laboratory plasmas is commonly acquired using either Langmuir probes or optical diagnostics. Here, we present an alternative approach, inferring the density profile from magnetic measurements of a plasma wave. In particular, during the process of creating a reconnecting current layer for magnetic reconnection experiments in the Big Red Ball, the reconnection drive first launches a large amplitude fast magnetosonic wave. The propagation of the wavefront is measured with high spatial and temporal resolutions by in situ magnetic diagnostics. Given a known uniform background magnetic field strength and the known dispersion relation of the wave, we here show how the characteristics of the wavefront can be applied to determine the initial plasma density profile.
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All authors Cameron Kuchta, Jan Egedal, Abhishek Mhatre, Paul Gradney, Joseph Olson, Xinyu Yu, Cary B. Forest

Plasma Physics

Physics-Informed Neural Networks to Infer the Perpendicular Energy Conductivity in the Scrape-Off Layer of Stellarator Devices

J. Gallego, P. Protopapas, A. Bustos et al. (+6 more)

In this work, we develop an inverse Physics-Informed Neural Network (PINN) framework to infer the dependence of the scrape-off layer (SOL) perpendicular heat conductivity on plasma density and temperature, $\kappa_\perp(n,T)$. The method combines radial profile measurements of electron density and temperature with the residual of a reduced one-dimensional SOL transport equation, so that the inferred conductivity is constrained by both the measurements and the underlying transport model. Three neural networks are trained simultaneously: two reconstruct the temperature and density profiles as functions of the radial coordinate and transported power, while a third represents the effective conductivity as a function of the local density and temperature. The framework is first validated using synthetic data generated from a prescribed conductivity function, allowing the inferred $\kappa_\perp(n,T)$ to be compared directly with the ground truth. The model recovers the imposed functional dependence with errors below $10~\%$ in the data-constrained region. Bootstrap resampling is shown to provide a practical indicator of prediction reliability and consistency. A scan in the number of plasma profiles used for training and the number of radial measurement positions per profile identifies a practical trade-off between reconstruction accuracy and data availability. Finally, the method is applied to an experimental dataset from the TJ-II stellarator obtained with the helium-beam diagnostic. This exploratory application provides an initial estimate of the effective SOL conductivity and illustrates the potential of inverse PINNs for extracting transport information from plasma edge measurements.
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All authors J. Gallego, P. Protopapas, A. Bustos, A. Alonso, S. Barquero, A. Baciero, I. Rivera, J. A. Moríñigo, R. Mayo-García

Plasma Physics

kobra: a new Vlasov code intended for plasma-wall modeling

Sebastian Konewko, Nathan Maestracci and Sven Van Loo

In a fusion device plasma-wall interactions \edit{on the sheath scale} can be modeled as a collisionless problem. When modeling these regions particle-in-cell codes suffer from statistical error originating from undersampling the velocity space. On the other hand, Vlasov codes do not have this issue as they evolve the full distribution function. Here, we present a new finite-volume Vlasov code, kobra, equipped with adaptive-mesh refinement to reduce computational effort. Currently, the code solves the Vlasov-Poisson equations. We validate our code in 1d1v and 1d2v using established benchmarks, i.e. the two-stream instability, Landau damping, the Dory-Guest-Harris instability, and also a classical electrostatic plasma sheath. We find that the code reproduces the theoretical properties of these problems well. More importantly, the adaptive grid provides a computational gain that is likely to scale to higher dimensional, plasma-wall simulations.
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Plasma Physics

Modeling of plasma transport during edge-localized mode in tokamak using a kinetic Vlasov-Poisson code

Ce Wang, Sven Van Loo and Geert Verdoolaege

A kinetic parallel transport code KOBRA based on a finite-volume method is developed to study edge localized mode (ELM) plasma transport from the mid-plane to divertor targets. The large scale separation between the Debye length (~cm) and the connection length (~10-20 m) leads to prohibitive computational cost in full 6D simulations. To alleviate this, an adaptive-mesh refinement (AMR) strategy is employed. Comparisons with uniform-grid simulations show that AMR accurately reproduces the characteristic ELM dynamics, including the rapid rise and slow decay of divertor fluxes, as well as the early-time peak induced by fast electrons. Analysis of the electron distribution and self-consistent electric field reveals that AMR efficiency is closely linked to phase-space evolution. Overall, AMR achieves comparable physical accuracy while reducing memory usage by 30-40% and accelerating computations by up to a factor of two, demonstrating its effectiveness for high-dimensional kinetic ELM simulations.
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Plasma Physics

Particle-resolved pathways to energetic-ion formation in a fluctuating low-current hollow-cathode plume

Baisheng Wang, Zilong Peng, Yinjian Zhao et al. (+1 more)

Energetic-ion formation in a low-current hollow-cathode plume is investigated using experiments, self-consistent electrostatic particle-in-cell (PIC) simulation, and particle-resolved analysis. Retarding potential analyzer measurements show a substantial energetic-ion population over discharge currents of 0.8-3.5 A, while probe measurements reveal broadband plume fluctuations. Two-point phase-derived frequency-wavenumber measurements do not resolve a continuous ion-acoustic dispersion branch within the principal apparent-wavenumber interval. Because the inferred wavenumber is obtained from a cross-spectral phase defined modulo 2pi, the fluctuation diagnostics do not provide an unambiguous modal attribution for the energetic-ion population. A representative PIC plume, used as a qualitative kinetic reference, likewise develops broadband time-dependent electrostatic fluctuations together with a nonthermal energetic-ion population. Particle-resolved analysis shows that the energetic outflow is dominated by ions generated through ionization inside the plume, while source localization biases access to distinct trajectory and escape families. Matched field controls further show that time-averaged and frozen fields strongly suppress access to high-energy trajectories relative to the full time-dependent field over the analyzed interval. At the single-particle level, ion kinetic-energy gain is determined by electrostatic-field work accumulated along the actual trajectory, with different escape families exhibiting distinct radial and axial work contributions. These results establish a source-trajectory-field-work pathway for energetic-ion formation that can be identified without first assigning the fluctuating plume to a unique resolved plasma mode.
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All authors Baisheng Wang, Zilong Peng, Yinjian Zhao, Zhongxi Ning

Wednesday 9 September

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

Wave Emission and Absorption in a Near-Sun Proton-Cyclotron Wave Storm

Kristopher G Klein, Daniel Verscharen, Mihailo Martinovic et al. (+5 more)

Quantification of energy transport and dissipation in weakly collisional heliospheric plasmas that are far from local thermodynamic equilibrium is an outstanding scientific problem. A central challenge is determining how non-Maxwellian velocity-space structure affects damping and emission of coherent ion-scale waves, especially compared to simplified analytical models for background plasma velocity distributions. In this work, we study the damping and emission of parallel-propagating proton cyclotron waves for two models of proton velocity distributions measured by the SPAN-I instrument on board Parker Solar Probe during an extended storm of waves with left-hand polarization in the solar wind at a heliocentric distance of 30.1 solar radii. Using the measured velocity distribution rather than a two-component bi-Maxwellian model predicts instabilities consistent with the observed coherent waves. For intervals in which both models predict net damping, the observed VDF model yields weaker damping in 90% of cases, with a reduction in the integrated heating rate of 0.44 relative to the bi-Maxwellian model. These results suggest that simplified analytical velocity distribution models may overestimate cyclotron damping and underestimate wave emission in the near-Sun solar wind.
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All authors Kristopher G Klein, Daniel Verscharen, Mihailo Martinovic, Niranjana, Ali Rahmati, Roberto Livi, Davin Larson, Michael Stevens

Plasma Physics

High-charge, highly polarized positron beams generated from a laser-driven nanowire-array target

De-Sheng Zhang, Cui-Wen Zhang, Kun Xue et al. (+4 more)

The generation of high-charge, highly polarized positron beams in the interaction of a linearly polarized laser pulse with a nanowire-array target is investigated. Here, laser-driven electrons emit high-energy photons through nonlinear Compton scattering (NCS), which subsequently produce electron–positron pairs through the nonlinear Breit–Wheeler (NBW) process. We model this interaction using two-dimensional spin-resolved quantum electrodynamics particle-in-cell (QED-PIC)} simulations. At positron birth, the sign of $S_z$ is statistically correlated with that of the local $B_z$. The spatiotemporal field structure arising from the laser–nanowire interaction strengthens the correlation between the birth spin sign and the direction of the subsequent transverse Lorentz impulse, thereby limiting cancellation between opposite-spin contributions at a given angle. The results show that the average polarization degree reaches $|\bar S_z|\approx0.46$, and the positron charge satisfying $|\bar S_z|>0.3$ is approximately $308\,\mathrm{nC}$. Parameter scans reveal that the high-polarization positron charge is maximized at intermediate target densities and nanowire periods. Such a source could enable polarization-sensitive studies of strong-field QED and spin-dependent phenomena in high-energy and materials physics.
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All authors De-Sheng Zhang, Cui-Wen Zhang, Kun Xue, Feng Wan, Xue-Ren Hong, Jian-Xing Li, Bai-Song Xie

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

Connecting Dynamo Theory with DNS Data: A Computational Analysis of $\alpha$ and $\beta$ effects

Kiwan Park

We investigate the influence of current helicity on the turbulent magnetic diffusivity $\beta$ using three complementary derivations of the $\alpha$ and $\beta$ coefficients, based on the large-scale magnetic field $\overline{\mathbf{B}}$, the turbulent velocity $\mathbf{u}$, and the turbulent magnetic field $\mathbf{b}$. Applying these coefficients to raw DNS data, we reconstruct $\overline{\mathbf{B}}$ and compare the results with the original simulations. In the kinematic regime all models agree well with the DNS data. In the nonlinear regime, however, $\beta_{\mathrm{vv-vw}}$ alone produces unbounded growth of $\overline{\mathbf{B}}$. Including the contribution from turbulent magnetic fields ($\beta_{\mathrm{bb+jb}}$) suppresses this unphysical growth and restores agreement with the DNS results. We find that kinetic helicity drives $\beta$ more negative, while current helicity shifts it back toward zero. Weighted combinations of the coefficients further show that the $\beta$ effect dominates the evolution of $\overline{\mathbf{B}}$ throughout, whereas the $\alpha$ effect becomes important mainly for sustaining the field in the nonlinear regime. The corresponding IDL analysis scripts are provided to facilitate practical implementation of the theoretical models.
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Plasma Physics

A polar-harmonic unified gas-kinetic scheme for magnetized ion dynamics from cyclotron kinetics to the Hall-Pedersen constitutive limit

Yixiao Wang, Zhigang Pu, Xing Ji et al. (+1 more)

Magnetized ion transport in weakly ionized plasmas ranges from gyroangle-dependent kinetics to Hall-Pedersen drift-diffusion as collisionality and magnetization vary. We develop a polar-harmonic unified gas-kinetic scheme (PH-UGKS) for the ion Vlasov-BGK equation in a uniform magnetic field. The scheme evolves the full ion distribution by coupling a conservative density update to exponential evolution of its nonequilibrium component. Exact collision-rotation integration in gyroangle Fourier space is combined with a time-averaged kinetic flux that incorporates spatial transport and electric acceleration, together with a compact Hall-Pedersen correction to the density flux. The scheme conserves ion number, and analysis establishes second-order temporal consistency and asymptotic preservation of the Hall-Pedersen density limit at fixed magnetization. Numerical tests reproduce ion Bernstein dispersion and Dory-Guest-Harris growth rates and resolve changes in the gyroharmonic spectrum as the collision-to-gyrofrequency ratio varies. The driven ion-flux response agrees with an independent characteristic-Volterra reference, including finite-frequency departures from the instantaneous Hall-Pedersen relation. In collisional tests, accurate responses are obtained with time steps far larger than both the collision time and the gyroperiod. Fixed-resolution density tests confirm convergence to the corresponding Hall-Pedersen discretization. The same kinetic formulation thus connects kinetic response and macroscopic transport without switching to a fluid solver or subcycling microscopic time scales.
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All authors Yixiao Wang, Zhigang Pu, Xing Ji, Kun Xu

Tuesday 8 September

Plasma Physics

All field strengths are possible locally in Boozer coordinates

Taylor J. Klotz, Nathan Duignan, Joshua W. Burby et al. (+1 more)

The magnetic field strength is a central design object in the theory of magnetic confinement of plasma. Desirable confinement properties such as quasi-symmetry and omnigenity are characterized by special forms of the field strength when it is given in the particular coordinate system called “Boozer coordinates." We ask a basic realizability question: which functions of Boozer coordinates can arise as the magnitude of a magnetic field that is written in Boozer coordinates? Using exterior differential systems and the Cartan-Kähler theorem, we show that every positive analytic function is locally realizable. Thus there is no local analytic obstruction to prescribing the field strength in Boozer coordinates; though further restrictions might arise from global geometric or additional physical constraints.
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All authors Taylor J. Klotz, Nathan Duignan, Joshua W. Burby, James D. Meiss

Plasma Physics

RF-Specific Tungsten Erosion and Global Transport in ITER under Neon Seeding

Atul Kumar, Dhyanjyoti Nath, Wouter Tierens et al. (+15 more)

Ion cyclotron radio-frequency heating (ICRH) is a key auxiliary heating system in ITER, but high-power RF operation can enhance plasma-material interactions through rectified RF sheath potentials on antenna structures and nearby plasma-facing components. We present the first predictive application of the STRIPE (Simulated Transport of RF Impurity Production and Emission) framework to assess RF sheath-driven tungsten (W) erosion and global impurity transport from the ITER ICRH antenna under ITER-relevant neon-seeded conditions. STRIPE couples SOLPS-ITER plasma backgrounds, full-wave RF sheath calculations, geometry-specific ion energy-angle distributions, sputtering physics, and three-dimensional impurity transport. Simulations predict RF sheath potentials of 1 to 3 kV on antenna limiter sidewalls, increasing gross W erosion by about a factor of 64 relative to thermal sheath conditions and producing a gross source of 3.34e18 W atoms per second. Erosion is governed by RF-modified ion energy-angle distributions together with local plasma flux rather than sheath voltage alone. About 10 percent of sputtered W is locally redeposited, giving a net source of 3.01e18 W atoms per second. The RF-induced antenna source remains about three orders of magnitude smaller than the thermal divertor source and more than two orders of magnitude smaller than the integrated thermal main-chamber source. After 100 ms, about 22 percent of the mobile W inventory resides within the SOLPS-covered confined-plasma region, corresponding to an annular W concentration of 1.70e-6. These results indicate that the ITER ICRH antenna is unlikely to dominate the total W source budget under the conditions considered and demonstrate the need for coupled modeling of RF waves, sheaths, sputtering, redeposition, and global impurity transport.
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All authors Atul Kumar, Dhyanjyoti Nath, Wouter Tierens, Jeremy D. Lore, Andrei Pshenov, Tom Wauters, Andrea Galvan, Davide Curreli, Syun'ichi Shiraiwa, Nicola Bertelli, Guillaume Urbanczyk, Sebastijan Brezinsek, Onkar Sahni, Mark Shephard, Walid Helou, Xavier Bonnin, Klaus Schmid, Volodymyr Bobkov

Plasma Physics

On the modeling of oblique firehose instabilities in regularized Kappa plasmas using ALPS

D. L. Schröder, M. Lazar, H. Fichtner et al. (+2 more)

In-situ measurements in space plasmas indicate that the velocity distributions of charged particles are not in thermal equilibrium, deviating from a standard Maxwellian mainly due to anisotropies and suprathermal populations which enhance high-energy tails. Although the Standard ąppa}-Distribution (SKD) is well established in modeling these non-equilibrium distributions, its application is often viewed controversially due to certain unphysical implications, in particular divergent velocity moments. To address these issues, the Regularized ąppa}-Distribution (RKD) was introduced. Such advanced, in general anisotropic RKDs are invoked here for the first time to investigate oblique firehose instabilities, including those induced by the temperature anisotropy of electrons and protons, the dominant species in space plasmas. In weakly collisional plasmas, both of these instabilities are expected to play significant roles in the self-regulation of the macroscopic properties of space plasmas (e.g. the expanding solar wind) reported by observations. Is not yet possible to resort to a general dispersion tensor related to RKD plasmas (whose derivation is still a challenge), instead the Arbitrary Linear Plasma Solver (ALPS) is exploited here. The unstable solutions obtained for the already established Maxwellian & SKDs successfully validate the capability of ALPS. For RKDs, the instabilities confirm the stimulating effect of suprathermal populations, with lower ąppa} values generally enhancing the firehose growth rates. For the oblique electron firehose instability, RKDs substantially modify the competition between periodic & aperiodic branches only at intermediate angles & can sustain significantly increased growth rates of especially aperiodic modes at highly oblique propagation beyond both the Maxwellian unstable regime & the one that is accessible with SKDs.
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All authors D. L. Schröder, M. Lazar, H. Fichtner, K.G. Klein, D. Verscharen

Plasma Physics

Demonstration of a 10-metre-long discharge plasma source for plasma wakefield acceleration

C. Amoedo, N. Lopes, N. E. Torrado et al. (+13 more)

The AWAKE experiment at CERN uses high-energy proton bunches to drive plasma wakefields capable of accelerating electron bunches up to hundreds of GeV in a single plasma. Realising this potential requires a plasma source that can extend to tens of metres while maintaining high longitudinal uniformity and reproducibility. Towards this goal, we report on the first experimental characterisation and beam-based validation of a 10-metre-long pulsed-DC discharge plasma source (DPS) used as an alternative to the laser-ionised rubidium vapour plasma source (VPS). The DPS reliably delivered plasma densities in three noble gases (He, Ar, Xe) at electron densities between 10$^{14}$ and 10$^{15}$ cm$^{-3}$. Proton bunch self-modulation was observed with all three gases, and the resulting modulation frequency was used as a beam-based diagnostic of the plasma density, showing good agreement with laboratory interferometry measurements. The event-to-event modulation frequency was demonstrated to be reproducible to within 0.63%, comparable to that of the VPS. These results establish the DPS as a valid alternative plasma source for beam-driven wakefield experiments, and support its further development towards longer plasma lengths. The present design is tailored to AWAKE requirements, but the underlying discharge concept provides a route to long, reproducible plasma sources with straightforward density control through discharge-beam timing for other beam-plasma experiments.
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All authors C. Amoedo, N. Lopes, N. E. Torrado, F. Silva, P. Muggli, L. Verra, M. Turner, G. Zevi Della Porta, M. Bergamaschi, A. Clairembaud, J. Mezger, F. Pannell, N. Z. van Gils, E. Gschwendtner, A. Sublet, the AWAKE Collaboration

Plasma Physics

Generalized Similarity Theory for Plasmas

Yangyang Fu

A generalized theory of plasma similarity is established based on the scaling of the Boltzmann equation coupled with the full set of Maxwell's equations. The predicted similarity scalings are demonstrated through first-principles particle-in-cell and fluid simulations across diverse operational regimes, including collisionless plasmas with two-stream instabilities, pure electron diodes spanning the classical to relativistic regimes, electromagnetic-wave-driven plasmas, and discharge plasmas ranging from low to high ionization regimes. The generalized theory reveals the intrinsic scale-invariant nature of plasmas under specified conditions, rooted in the symmetry and scaling transformations of the governing equations.
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Monday 7 September

Plasma Physics

Scalable Production of Lead-212 and Actinium-225 Generators with Fusion Neutrons

J. F. Parisi and A. Rutkowski

Targeted alpha therapy will require a large expansion of ${}^{212}$Pb and ${}^{225}$Ac production. We propose neutron- and photon-driven routes that convert ${}^{230}$Th, ${}^{231}$Pa, ${}^{232}$Th, and ${}^{237}$Np into generator parents ${}^{228}$Th and ${}^{229}$Th. The most direct ${}^{225}$Ac route is ${}^{230}$Th(n,2n)${}^{229}$Th. A 10 MW deuterium-tritium (D-T) neutron source irradiating thorium with a 27% ${}^{230}$Th isotopic fraction accumulates about 250 Ci of ${}^{229}$Th in six months, initially enough for five million ${}^{225}$Ac dose-equivalents per year, and its 7916 yr half-life makes it useful for millennia. This requires only $\sim$2 gigawatt-days of D-T fusion operations. Thermal-neutron irradiation of tens of grams of ${}^{230}$Th produces gram quantities of ${}^{232}$U, a decades-long source of ${}^{228}$Th for millions of ${}^{212}$Pb dose-equivalents per year. Alternate routes proceed through ${}^{231}$Pa: fusion neutrons produce 1 to 2 tonnes of ${}^{231}$Pa per gigawatt-year from ${}^{232}$Th, and a thermal reactor converts up to 0.4 g of ${}^{232}$U per gram of ${}^{231}$Pa. Each gram of stored ${}^{232}$U produces 10 mg (8.2 Ci) of ${}^{228}$Th per year. Subsequent ${}^{228}$Th(n,$\gamma$)${}^{229}$Th converts 0.07 to 0.34 g of ${}^{229}$Th per gram of ${}^{228}$Th. We also analyze the ${}^{237}$Np(n,2n)${}^{236\mathrm{m}}$Np route to ${}^{228}$Th. These pathways scale from kilowatt to megawatt D-T sources and could secure millions of ${}^{212}$Pb and ${}^{225}$Ac dose-equivalents per year.
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Plasma Physics

Runaway electron control by self-excited waves

Kun Huang and Boris Breizman

Runaway-electron avalanches in tokamak plasmas can be limited by kinetic instabilities driven by the non-Maxwellian runaway distribution. We formulate a reduced model for the quasi-steady state in which the total plasma current and bulk electron temperature are prescribed, while the inductive electric field is determined self-consistently from the partition between Ohmic bulk current and runaway-electron current. Because the wave growth time is short compared with the current-decay time, we consider a marginal-stability regime, in which whistler-wave drive by the runaway electrons balances collisional damping. The resulting states separate into three regimes: a subcritical Ohmic regime without an avalanche, an avalanche regime in which runaway growth relaxes the inductive field to the avalanche threshold, and an instability-regulated regime in which self-excited whistler waves enhance momentum-space diffusion and limit the runaway current. In the instability-regulated regime, the whistler wave spectrum forms a narrow ridge, and low-energy runaway electrons carry most of the runaway current.
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Plasma Physics · Cross-listed from astro-ph.HE

Two-Phase Structure of Synchrotron-Cooling-Unstable Relativistic Plasma

Agnieszka Wierzchucka, Pablo J. Bilbao, Robert J. Ewart et al. (+2 more)

Using analytic theory, radiative particle-in-cell (PIC) simulations, and fluid simulations, we show that relativistic, synchrotron-cooling, collisionless, high-$\beta$ pair plasmas filament into a two-phase medium. This process occurs through the interplay of the synchrotron cooling instability (SCI) with the synchrotron firehose instability (SFHI). One phase has high plasma~$\beta$ and is infested with small-scale firehose fluctuations, which scatter particles and pin the pressure anisotropy to the firehose-marginal level. The other phase has much lower~$\beta$, causing the suppression of firehose modes and thus allowing large pressure anisotropies. We propose a fluid model for this two-phase plasma, which we use to study the linear and nonlinear evolution of the SCI and SFHI, and to predict the emergence time of the two-phase structure.
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All authors Agnieszka Wierzchucka, Pablo J. Bilbao, Robert J. Ewart, Dmitri A. Uzdensky, Alexander A. Schekochihin

Plasma Physics

Hamiltonian Particle Dynamics in Fusion Plasmas: Orbital Tomography and Spectrum Analysis for Energy and Momentum Transport under Resonant Non-Axisymmetric Perturbations

Yiannis Antonenas

This thesis investigates the impact of resonant mode-particle interactions on transport and confinement in toroidal fusion plasmas. Using analytical and numerical approaches, we study how intrinsic and externally applied magnetic perturbations affect plasma particles with different kinetic characteristics, focusing on resonant interactions with guiding-center motion. We develop a novel, computationally efficient method based on Action-Angle variables to identify resonance locations and characterize resonances in guiding-center phase space, including the number of islands in resonance chains and the formation of transport barriers. Using the drift center (DC) approximation, we derive analytical expressions for orbital frequencies and the kinetic $q$ factor for large-aspect-ratio (LAR) equilibria. These results provide the conditions for mode-particle resonances, which can strongly influence particle, momentum, and energy transport and, consequently, plasma confinement. The analytical results are validated against numerical simulations, demonstrating an efficient tool for predicting transport barriers and energetic-particle behavior under non-axisymmetric perturbations. We further extend the orbital-frequency analysis from LAR equilibria to numerically reconstructed, realistic equilibria using a computationally efficient, semi-analytical geometrical method applicable to arbitrary unperturbed equilibria. Finally, for the LAR equilibrium, we extend the analysis to time-dependent perturbations and demonstrate the emergence of Arnold diffusion in guiding-center phase space, highlighting the role of the Arnold web in particle transport. Overall, this work advances the understanding of mode-particle resonant interactions and provides computational tools for predicting transport and confinement in realistic fusion configurations.
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Plasma Physics

Thresholdless IBW Emission and Alpha-to-Thermal-Ion Energy Channeling via Pole Resonance of Fusion-Product Ions

Hong Qin and Nathaniel J. Fisch

Fusion reactions can release a substantial fraction, and in aneutronic reactions nearly all, of their energy as the kinetic energy of fusion-product ions. In magnetized plasmas, these energetic ions can form ring distributions in velocity space. Such distributions can drive Dory-Guest-Harris (DGH) electrostatic instabilities, but these self-instabilities require a finite energetic-ion density and can be stabilized by a thermal background. We show that the same background can instead enable a distinct instability mechanism: a cyclotron pole of the minority energetic-ion susceptibility destabilizes a stable ion Bernstein wave (IBW) eigenmode. When the energetic-ion harmonic is distinct from the thermal-ion harmonics, exact root-pole resonance is thresholdless in the ideal collisionless limit. A species-resolved power balance shows that the energetic ions supply the free energy while the thermal plasma receives it. In the LAPD proton–alpha example, $99.96\%$ of the alpha-particle power loss enters the coherent proton response. This self-excited, ion-directed transfer provides a possible linear building block for alpha-particle energy channeling in proton-Boron11 fusion.
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Plasma Physics · Cross-listed from astro-ph.SR

R-matrix calculations for opacities: V. Temperature-density dependence of photoabsorption cross sections and opacity spectra of oxygen ions O VI and O VII

Divya Chari, Sultana N Nahar and Anil K Pradhan

We present R-matrix photoabsorption cross sections for Li-like oxygen O VI and He-like oxygen O VII and examine how plasma broadening modifies them as a function of temperature and density in astrophysical and laboratory high-energy-density (HED) plasma sources. All atomic systems are subject to plasma environment effects, and the propagation of radiation depends on photoabsorption via bound-bound transitions as spectral lines and autoionizing resonances in bound-free photoionization cross sections. We identify and illustrate low and high temperature-density limits for the onset of plasma broadening in O VI and O VII, demonstrating general features and methodology applicable across a broad range of plasma conditions. Calculations are presented along two representative isotherms corresponding to the solar base of convection zone (BCZ) at $T = 1 \times 10^6$ and $2 \times 10^6$ K, and electron densities $N_e = 10^{18}$-$10^{23}$ cm$^{-3}$. Autoionizing resonances progressively dissolve into the continuum with increasing electron density at each isotherm, flattening and merging into the background cross sections at BCZ conditions, at much lower densities than bound-bound line features require. Illustrative examples are given for energy regions containing Rydberg resonance series, including large photoexcitation-of-core (PEC) resonances. Comparisons with previous Opacity Project results reveal that (i) the R-matrix photoionization cross sections cover a much higher energy range where a significantly richer spectrum of autoionizing resonances are present that are not included in OP, and (ii) the corresponding monochromatic opacities show significant quantitative differences. This work is generally applicable to modeling HED plasmas in astrophysics, and to the analysis of transmission spectra in laboratory experiments on inertial confinement fusion (ICF) devices.
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Plasma Physics

Self-organized positron reorienting and pinching mechanism for the experimental detection of the linear Breit-Wheeler process

Yutong He, Alexey Arefiev, Mario Manuel et al. (+2 more)

The linear Breit-Wheeler (LBW) process ($\gamma+\gamma\rightarrow e^{-}+e^{+}$) is a fundamental prediction of quantum electrodynamics, but yet to be observed under laboratory conditions using real photons. In recent years, a few experimental schemes utilizing high-intense ($\sim10^{22}$W/cm$^2$) laser-plasma interactions to observe the LBW process have been proposed. However, a high level of signal-to-noise-ratio are expected in these schemes, hindering the first-ever experimental detection of the LBW process by real photons. In this paper, we present a simple experimental setup which could enhance the expected positron signals by 2-3 orders of magnitude compared to previously proposed schemes, reaching the level of $10^{6}$MeV$^{-1}$str$^{-1}$. Moreover, such high positron signal is achieve in the direction opposite to the laser propagation, where a significantly quieter background is expected compared to the previously focused direction of laser propagation. The key to achieve this result is a newly discovered self-organized positron reorienting and pinching mechanism, enabled by the in-situ strong plasma fields from the laser-plasma interaction.
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All authors Yutong He, Alexey Arefiev, Mario Manuel, Hui Chen, Christopher Ridgers

Plasma Physics

Dual-decomposition multi-GPU particle-in-cell method for cylindrical plasmas: Batched Fourier-mode multigrid and balanced particle slabs

Yinjian Zhao, Xi Chen and Yingjie Chen

Three-dimensional electrostatic particle-in-cell (PIC) simulations combine irregular particle operations with a globally coupled Poisson solve, whose preferred parallel decompositions conflict. We introduce a dual-decomposition multi-GPU method: complete azimuthal Fourier modes are owned during the field solve, whereas spatial slabs own particles. Exact full-spectrum diagonalization and batched matrix-free geometric multigrid keep collectives outside the V-cycle, after which every GPU reconstructs the full field. Peer migration, cell reordering, warp-aggregated deposition, and capacity-constrained dynamic cuts restore particle locality and balance. CPU/GPU Poisson solutions agree to relative L2 errors below 6.9e-16; all 65 physical modes on eight V100 GPUs are solved in 9.350 ms with relative errors below 8.2e-15. For an identical 512*128*400 problem containing 707,788,800 particles per species, the complete PIC loop reaches 0.149108 s per step and strong-scales from five to eight GPUs with 91.81% efficiency. A separate cross-resolution production study shows approximately threefold aggregate particle-update and Poisson-cell throughput relative to a single RTX 5090 calculation; this measures refined-problem capability rather than a hardware speedup or formal convergence order.
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Plasma Physics

Electromagnetic filament coalescence as magnetic island merging with diamagnetic effects

Souvik Mondal, N Bisai, Abhijit Sen et al. (+1 more)

We investigate the nonlinear coalescence of two current-carrying ELM filaments using a three-dimensional electromagnetic fluid model. In the flat-density limit, the coalescence exhibits magnetic island-like reconnection, characterized by X-point formation, current-sheet development, and Sweet-Parker-like resistive scaling. Introducing a blob-like density perturbation modifies the reconnection dynamics: while the peak reconnection rate remains nearly unchanged for weak perturbations, it decreases and is increasingly delayed for larger density amplitudes. Analysis of the induction equation reveals a transition from resistive to increasingly density-dependent advective dynamics. Finite density perturbations also enhance the post-compression rebound, or sloshing, of the filaments. The sloshing amplitude increases with the density-gradient pressure force, establishing density perturbation as an additional control parameter for both reconnection and filament sloshing. These results highlight the coupled electromagnetic and pressure-driven dynamics governing the nonlinear evolution of ELM filaments in the tokamak edge.
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All authors Souvik Mondal, N Bisai, Abhijit Sen, Indranil Bandyopadhyay