Recent Publications

Efficiency of non-resonant photon trap for future experiments on neutralisation of negative hydrogen ion beam

Magomedrizy Gadjimuradovich Atlukhanov, Alexander Vladimirovich Burdakov, Sergey Sergeevich Popov, Yuri Alexandrovich Trunev, Dmitry Ivanovich Skovorodin, Alexander Alexandrovich Kasatov, Igor Vladimirovich Shikhovtsev, Victor Viktorovich Kurkuchekov

Budker Institute of Nuclear Physics SB RAS, Novosibirsk State University, Novosibirsk State Technical University

This paper presents experimental investigations of a non-resonant photon trap featuring adiabatic radiation confinement, designed for the photoneutralisation of negative hydrogen ( H − / D − ) ion beams in fusion plasma injection heating systems. The trap is formed by two monolithic dielectric mirrors with high reflectivity. The study focuses on the methodology and results of measuring the confinement time and integral optical losses. The diagnostic approach is based on analysing the decay rate of radiation within the optical cavity following abrupt interruption of the pump source, utilising wire attenuators for signal reduction. The influence of the laser beam angular divergence on the accumulation efficiency is examined. The obtained data confirm the adiabatic nature of the photon confinement, demonstrate consistency between the measured losses and the specified mirror coating parameters and validate the feasibility of controlling the density of the photon ensemble within the trap. This work confirms the technical feasibility and potential of this approach for the development of high-efficiency photoneutralisation systems.

Geometric numerical discretisation of electromagnetic quasineutral models

Nishant Narechania, Emil Poulsen, Eric Sonnendrücker

Max Planck Institute for Plasma Physics, Technical University of Munich

In this work, the geometric electromagnetic particle-in-cell (PIC) framework, GEMPICX , is extended to solve the quasineutral, fully kinetic Vlasov–Maxwell equations on dual grids using mimetic finite differences. The discrete action principle is derived, taking into account the duality between the grids. The temporal derivative of the electric field does not directly appear in the dynamical system for the quasineutral model. Hence, a discretised curl–curl equation is used to implicitly obtain the electric field at every time-step. This also circumvents the need to obtain electric potentials. A Lagrange multiplier is used to maintain the discretised divergence of the current density at machine zero.

On shear-Alfvén wave-induced energetic ion transport in optimised stellarators

Alexey Romanovich Knyazev, Alexandra L. Lachmann, Alan Geoffrey Goodman, Abdullah Hyder, Michael Czekanski, Donald Spong, Elizabeth J. Paul

Columbia University, Max Planck Institute for Plasma Physics, Cornell University, Oak Ridge National Laboratory

In this work, we investigate prompt ion drift orbit losses caused by shear-Alfvén waves (SAWs) in quasi-symmetric and quasi-isodynamic (QI) stellarators optimised for equilibrium confinement of energetic particles (EPs). We use the ideal reduced magnetohydrodynamic (MHD) model for SAW perturbations and study their impact on the collisionless EP drift dynamics. We present a semi-analytical model for resonance between the passing EP and SAW, generalised to arbitrary quasi-symmetric configurations including the quasi-poloidal case relevant to QI equilibria. Analysis reveals that an increase in the number of field periods upper N Subscript normal f normal p N f p $N_{\mathrm{fp}}$ suppresses stochasticity in quasi-helical (QH) and quasi-isodynamic, but not quasi-axisymmetric (QA) stellarators. We show that wave-induced transitions between passing and trapped orbits cause significant losses in QA and QH, but not in QI configurations. For the considered equilibria at scales relevant to fusion power plants, we numerically determine the SAW amplitudes needed to induce prompt loss of fusion-born alpha particles. Using the weighted Birkhoff averaging technique, we confirm that the onset of prompt losses across all orbit classes occurs with the onset of stochasticity in ion motion. This motivates extending the stochasticity-onset criterion beyond passing orbits in future work.

GPU-MPI parallelisation for QuickPIC, a three-dimensional quasi-static particle-in-cell algorithm

Yueran Tian, Yueluo Wang, Thamine Dalichaouch, Viktor Decyk, Frank S. Tsung, Warren B. Mori, Weiming An

University of California Los Angeles, Beijing Normal University, Peking University

QuickPIC is a quasi-static PIC program for simulating plasma wakefield acceleration. It supports a hybrid parallel simulation on multiple central processing units via OpenMP and message passing interface (MPI). We now present a version of QuickPIC that runs on graphics processing units (GPU), with MPI for communication between different GPUs, based on the UCLA Parallel Partice-In-Cell framework. By rewriting relevant Fortran77 functions with CUDA C, we port the two-dimensional particle and field computations onto GPU platforms. QuickPIC-GPU supports simulation with a single GPU for up to a resolution of 1024 cubed 1024 3 $1024^3$ and 25 particles per cell to avoid time spent on cross-processor communications, and simulation with multiple GPUs for larger problem sizes.

Firewall effect on electron acceleration by R-waves and parallel electric fields

Hye Lin Kang, Young Dae Yoon, Myung-Hoon Cho, Gunsu S. Yun

Pohang University of Science and Technology, Asia Pacific Center for Theoretical Physics

We report an unanticipated electron dynamics in a classical setting of a uniform magnetic field, a parallel electric field and a right-handed circularly polarised wave (R-wave). The setting admits a natural trajectory that a particle accelerated by the electric field reaches a Doppler–shifted cyclotron resonance and becomes trapped in the resonance space. Remarkably, once it becomes resonantly trapped, the electron undergoes reversal of parallel acceleration together with perpendicular energisation, despite the parallel electric field remaining constant. This counterintuitive behaviour has important implications for particle scattering in various laboratory and space plasmas. Applied to fusion devices, particle-in-cell simulations show that an externally injected R-wave can act as a firewall suppressing further runaway-electron acceleration.

The machine learning approach to moment closure relations for plasma: a review

Samuel Burles, Enrico Camporeale

Queen Mary University of London, University of Colorado

The requirement for large-scale global simulations of plasma is an ongoing challenge in both space and laboratory plasma physics. Any simulation based on a fluid model inherently requires a closure relation for the high-order plasma moments. This review compiles and analyses the recent surge of machine learning (ML) approaches developing improved plasma closure models capable of capturing kinetic phenomena within plasma fluid models. We survey two methodological families: neural network surrogates (from multi-layer perceptrons to Fourier neural operators, the latter recently reproducing both linear and nonlinear Landau damping online within a fluid solver) and equation discovery methods such as sparse regression; and organise the studies by whether they are tested offline against reference data or online within a time-evolving solver. We outline the challenges associated with ML closures, including off-diagonal pressure-tensor accuracy, generalisation beyond the training distribution and stable integration into large-scale simulations, and the directions future research might take to address them.

Strong gradient neoclassical transport in the plateau regime

Silvia Trinczek, Felix I. Parra, Peter J. Catto, Iván Calvo

Princeton Plasma Physics Laboratory, Massachusetts Institute of Technology, CIEMAT

Strong gradient regions in tokamaks such as the pedestal or internal transport barriers are regions of reduced turbulence where neoclassical transport can play a dominant role. In pedestals, gradient lengths comparable to the ion poloidal gyroradius have been measured. Standard neoclassical theory can miss important strong gradient effects in these regions because it assumes that the gradient length scales of density, temperature and potential are larger than the ion poloidal gyroradius. We extend plateau regime neoclassical theory into regions of gradients of the order of the ion poloidal gyroradius to capture strong gradient effects on transport processes in the pedestal and internal transport barriers. The fundamental idea behind our new framework is to keep a scale separation between the orbit widths and the gradient length scales by performing a large aspect ratio expansion. In the plateau regime, strong gradients cause poloidal variation that is in–out as well as up–down asymmetric. We study two different test cases assuming either radial force balance or the absence of turbulence and show that strong gradient effects can enhance or reduce standard neoclassical theory predictions in the plateau regime in strong gradient regions.

Interpretation of high-harmonic fast-wave propagation in the scrape-off layer of NSTX-U as a geometrically bounded-waveguide mode

Seung-Gyou Baek, Nicola Bertelli, Ricardo Antonio De Levante Rodriguez, Paul Thaddeus Bonoli, Syun’ichi Shiraiwa

MIT Plasma Science and Fusion Center, Princeton Plasma Physics Laboratory

This paper revisits the onset of high-harmonic fast-wave (HHFW) propagation in the scrape-off layer (SOL) plasma of the NSTX/NSTX-U spherical tokamak, motivated by past HHFW heating and current drive experiments and modelling. Previously, the fast-wave propagation in the SOL was correlated with the opening (suppression) of the fast-wave right-hand cutoff layer in front of the antenna. In this work, the SOL propagation is interpreted as a geometric waveguide mode guided by the SOL geometry, whose radial width can be comparable to the wave’s perpendicular wavelength in the HHFW regime. A two-dimensional circular model is first employed using full-wave solvers to characterise the poloidal eigenmode structures and to clarify their relationship to annulus resonance. By progressively adding a tokamak-like magnetic-field configuration, starting from a uniform axial field, the effects of magnetic-field gradients and pitch on the SOL waveguide mode are characterised. The poloidal mode numbers supported by the SOL plasma agree well with analytic estimates, indicating that an anisotropic plasma in a bounded geometry selectively supports and amplifies the resonant poloidal mode number. Additionally, two-dimensional axisymmetric NSTX-U simulations demonstrate that the SOL eigenmode features identified in the circular model persist in the experimentally relevant configurations. A control approach based on lengthening the wave perpendicular wavelength relative to the SOL width, including a higher magnetic (B)-field operation, is discussed. The analysis here shows the key role of a bounded geometry in interpreting HHFW eigenmode coupling and propagation in the NSTX-U SOL plasma.

On the evolution of a large-amplitude, weakly collisional electron plasma wave

Archis Joglekar, Alec Thomas

University of Michigan – Ann Arbor, Ergodic LLC, Pasteur Labs

Vlasov–Poisson–Fokker–Planck (VPFP) simulations of large-amplitude electron plasma waves, where the bounce frequency is much larger than the collision frequency, omega Subscript upper B Baseline much greater than nu Subscript italic ee ω B ≫ ν ee $\omega _B \gg \nu _{\textit{ee}}$ , show that the evolution of these waves exhibits three phases: (i) a short-lived trapping phase during which collisional effects are minimal; (ii) a long-lived detrapping phase during which collisional effects are most influential; (iii) a short-lived Landau damping phase where the effect of collisions becomes minimal again. While the dispersion relation during the trapping and Landau damping phase is well known, the wave behaviour during the detrapping phase is not as well understood. The simulations show that during the detrapping phase, the interplay between weak electron–electron collisions and strong wave–electron interactions results in an increasing frequency shift further from the linear root, omega Subscript EPW ω EPW $\omega _{\text{EPW}}$ . At the conclusion of the detrapping phase, the distribution function is nearly Maxwellian, the frequency shift rapidly diminishes and the wave damps at a larger rate than the Landau damping rate. Empirical fits to the damping rates, frequency shift enhancement rate and the lifetime of the plasma waves are provided as functions of collision frequency, wavenumber and wave amplitude.

A comparison of cusp and mirror topologies for beta right arrow normal infinity β → ∞ $\beta \rightarrow \infty$ magnetic confinement

Brendan James Sporer

TAE Technologies, University of Michigan

The magnetic confinement topologies potentially capable of macro-stability with core plasma pressure greatly exceeding the vacuum magnetic field pressure (or ‘ beta right arrow normal infinity β → ∞ $\beta \rightarrow \infty$ ’) are discussed. An emphasis is placed on application to magnetic fusion energy (MFE) given the perceived advantages. The particle loss rate from highly diamagnetic plasmas in open-field topologies is quantified, including a generalised model for the gas-dynamic regime. It is concluded that, regardless of the topology type, attractive fusion energy gain from such a system is impossible unless (i) the scrape-off-layer can be made much thinner than one thermal ion gyroradius through ambipolar effects, (ii) sufficient stability exists in the stochastic phase space of certain topologies so as to increase the average number of ion orbits before loss by tilde ∼ $\sim$ 1000 times over the isotropic prediction or (iii) electrostatic or centrifugal effects are used to recover adiabatic confinement. Outside of MFE, high- beta β $\beta$ topologies offer many alternative applications in plasma science. The required magnet bore to limit perpendicular particle loss in a point cusp is estimated. Additionally, the unique spherical multipole topology is reviewed and comments are made on the use of cusp topologies for ‘electromagnetic’ confinement efforts.

Confinement performance predictions for a high field axisymmetric tandem mirror – CORRIGENDUM

Samuel Frank, Jesse Viola, Yuri Petrov, Jay Anderson, Dominick Bindl, Bodhi Biswas, Juan Francisco Caneses-Marin, Doug A. Endrizzi, Kieran Furlong, Robert Harvey, et al.

Status of the GDT experiment

Peter Bagryansky, Elena Soldatkina, Andrey Meyster, Vadim Prikhodko, Alexander Solomakhin, Vladimir Maximov, Alexander Khilchenko, Yury Kovalenko, Evgeniy Shmigelsky, Olga Korobeynikova, et al.

Budker Institute of Nuclear Physics

The Gas Dynamic Trap (GDT) facility at the Budker Institute of Nuclear Physics continues experimental studies in support of the Gas-Dynamic Multiple-Mirror Trap (GDMT) project – a prospective neutron source and thermonuclear reactor concept based on open-ended magnetic confinement. During the 2024–2025 upgrade campaign, key engineering systems and diagnostic suites were substantially modernised to address critical physics and technology challenges for the GDMT development. Three key research and development directions were pursued. First, a conductive wall stabilisation system was designed to suppress magnetohydrodynamic (MHD) instabilities at high plasma beta. Second, a new second-harmonic X-mode electron cyclotron resonance heating (ECRH) system operating at 54.5 GHz with 0.8 MW power was commissioned. Preliminary experiments demonstrated increased electron temperature compared with non-ECRH heated plasmas. Third, proof-of-principle experiments on bulk plasma fuelling using a Marshall gun injector were successfully performed, supporting further development towards a high-repetition-rate injector. These results provide essential experimental validation and technological groundwork for the GDMT project, demonstrating progress towards high-beta plasma operation, efficient heating schemes and particle balance control in open magnetic traps.

Dependence of field-reversed configuration formation in collisional merging experiments on the mirror magnetic field strength

Satsuki Ishiwata, Loren Steinhauer, Tomohiko Asai

Nihon University, TAE Technologies Inc

Collisional merging experiments on field-reversed configurations (FRCs) in the FRC amplification via translation–collisional merging (FAT-CM) device have suggested that the mirror magnetic field strength influences the final merged plasma state. Analytical models do not account for mirror magnetic fields at the ends of the confinement section. To explore their effect, experiments were conducted at FAT-CM, where the strength of mirror magnetic fields were varied. Increasing the mirror field strength was found to promote relaxation to an FRC with larger radius and shorter axial length. Based on these observations, the dependence of equilibrium states on the mirror field strength and the resulting plasma parameters were investigated using a model that reconstructs fully two-dimensional equilibria primarily constrained by wall-mounted magnetic probe measurements. Consistent with the experimental results, computed equilibria with stronger mirror fields converged to plasma states with a larger radius and shorter length. Further, these results showed a bifurcation of topologies with a closed FRC core for higher mirror field and an open-field high- beta β $\beta$ mirror for lower field. This result agreed with internal magnetic probe data which confirm the FRC-to-high- beta β $\beta$ mirror transition. This demonstrates the importance of employing fully two-dimensional modelling of plasma equilibria.