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Sep 30

Effect of small magnetic fields on stimulated Raman scattering in the kinetic regime

Roman P. Lee, Benjamin Winjum, S.J. Spencer, Simon Bolaños, Mathieu Bailly-Grandvaux, Mario J.-E. Manuel, Frank S. Tsung, Farhat Beg, Warren B. Mori

Journal of Plasma Physics6 days agoAI, Modeling & Simulation Inertial Fusion & HEDP

Results from one- and two-dimensional particle-in-cell simulations – both of nonlinear electron plasma waves (EPWs) and of self-consistent stimulated Raman scattering (SRS) – are shown that explore effects of small magnetic fields ( omega Subscript c Baseline divided by omega Subscript p Baseline much less than 1 ω c / ω p ≪ 1 $\omega _c / \omega _p \ll 1$ ) oriented perpendicular to the direction of laser propagation on SRS across a range of laser intensities of relevance to inertial fusion energy. The magnetic field effect is strongly intensity dependent. The magnetic field raises the threshold for kinetic inflation and can suppress SRS entirely at intensities just above the unmagnetised threshold. At intermediate intensities, it reduces time-averaged reflectivity by factors of approximately two to three in one dimension. Far above threshold the suppressive effect largely disappears, and the reflectivity becomes comparable to, or modestly greater than, its unmagnetised value. Nevertheless, the instability retains distinct signatures in the presence of the magnetic field. The magnetic field promotes the detrapping of resonant electrons, enhances EPW damping, limits distribution-function flattening and the associated nonlinear frequency shift, and thereby produces a more regular recurrence of SRS bursts. Multi-dimensional effects weaken the contrast in EPW damping through transverse localisation and wavefront effects, but preserve the same qualitative intensity dependence and changes in recurrence behaviour. These results delimit the regime in which small transverse magnetic fields mitigate kinetic SRS and show that time-averaged reflectivity alone does not fully characterise their influence on the instability.

University of California Los AngelesGeneral AtomicsUniversity of California San DiegoLawrence Berkeley National Laboratory

Modulated deuteron spectra observed with the magnetic recoil neutron spectrometer at the National Ignition Facility

Bao Nguyen, Yousef Lawrence, Christopher Wink, Timothy Mark Johnson, Niels Vanderloo, Benjamin Reichelt, Amber Hennessy, Daniel Thomas Casey, Dave Schlossberg, Nathan Masters, et al.

Journal of Plasma Physics6 days agoControl & Diagnostics AI, Modeling & Simulation Inertial Fusion & HEDP

The Magnetic Recoil Spectrometer (MRS) at the National Ignition Facility is used to measure the neutron spectrum from deuterium–tritium-fuelled inertial confinement fusion implosions via neutron–deuteron elastic scattering and magnetic dispersion of recoil deuterons. From the MRS-determined neutron spectrum, the yield ( upper Y Subscript n Y n $Y_n$ ), apparent ion temperature ( upper T Subscript i T i $T_i$ ) and areal density ( rho upper R ρ R $\rho R$ ) are determined. However, anomalous energy modulations in recoil deuterons have been observed in several high-yield indirect drive experiments ( upper Y Subscript n Baseline tilde 10 Superscript 16 Baseline minus 10 Superscript 18 Y n ∼ 10 16 − 10 18 $Y_n\sim 10^{16}{-}10^{18}$ ). These observations raise concerns about their potential impact on the MRS-inferred performance metrics. Analytic calculations and particle-in-cell simulations are used to examine the possible beam–plasma instabilities, which indicate the two-stream instability as a plausible mechanism behind energy modulations. Based on a statistical analysis of synthetic deuteron spectra, the modulation-induced errors are found to be within the errors of the determined upper Y Subscript n Y n $Y_n$ , upper T Subscript i T i $T_i$ and rho upper R ρ R $\rho R$ values and thus do not have a significant impact on the MRS measurement.

Imperial College LondonLawrence Livermore National LaboratoryLos Alamos National LaboratoryMassachusetts Institute of TechnologyGeneral Atomics

Sep 25

Phase space resolved measurements of fast-ion density fluctuations and energy transfer induced by Alfvén eigenmodes

Jose Rueda-Rueda, Xiaodi Du, William W Heidbrink, Gregory G Howes, Pablo Oyola, Tommaso Barberis, Deyong Liu, Michael A Van Zeeland, Filipp Khabanov, Kyle Callahan

Nuclear FusionSep 25, 2026Plasma & Confinement Control & Diagnostics

We present phase-space resolved measurements of energetic particle (EP) fluctuations near Toroidal Alfvén Eigenmode (TAE) frequencies in the DIII-D tokamak. A newly rebuilt fast channel in the Imaging Neutral Particle Analyzer (INPA) system, with a significantly improved signal-to-noise ratio, enabled the detection of fluctuations at approximately 100-130 kHz, frequencies that were previously inaccessible. Coherent fluctuations with amplitudes of about 1\% were observed in localized regions of EP phase space. The secular energy exchange rate between the wave and the EP population is directly extracted from the observed INPA and electron density and temperature fluctuations, accounting for both parallel and perpendicular electric field components. Analysis reveals that poloidal energy exchange dominates by approximately two orders of magnitude over parallel and radial components, with energy transfer occurring through the interaction between drift velocities and perpendicular electric fields. The averaged energy exchange rates are of the order of 10 keV/s. A comparison with ASCOT orbit-following simulations confirms that the observed fluctuations originate from the passively viewed (trapped) particle population rather than from passing particles. The measured TAE radial structure shows good agreement with ideal MHD simulations using the NOVA code. These measurements provide unprecedented experimental validation of wave-particle energy transfer mechanisms.

Princeton Plasma Physics LaboratoryGeneral AtomicsUniversity of Wisconsin Madison

Sep 24

Demonstration of H-mode Error Field Identification in a Single Discharge via Island Healing

E. M. Bursch, A. Xie, J. L. Barr, S. K. Kim, N. C. Logan, S. M. Yang, J. G. Bak, W. Choi, Q. Hu, J. W. Juhn, et al.

arXiv (physics.plasm-ph)Sep 24, 2026Plasma & Confinement Control & Diagnostics

Non-disruptive compass scan error field identification via island healing is demonstrated on KSTAR in a single H-mode discharge. This method can allow for up to a 75% reduction in the operational time necessary for a complete compass scan by reducing the required discharges from four to one. Previous experiments on DIII-D, JET, and MAST-U have demonstrated the technique in Ohmic and L-mode but required multiple discharges and strong real-time n=1 magnetics diagnostics close to the plasma that are unlikely to be viable for future devices. The results presented here were made possible by implementing a new set of triggering algorithms relevant to fusion pilot plant operation into the KSTAR plasma control system, along with KSTAR's strong RMPs and long pulse lengths. This represents a significant step towards the disruption-free error field identification method being demonstrated for deployment on future disruption-averse tokamaks, including ITER and fusion pilot plants. Steps needed to close remaining gaps to viability are addressed.

General AtomicsKorea Institute of Fusion EnergyPrinceton Plasma Physics Laboratory

PQLS: A Quasilinear Gyrokinetic Transport Solver with a Bayesian Saturation-Rule Closure

F. Wilms, A. Agrawal, J. J. Freigang, T. F. Neiser, B. J. Frei, O. Meneghini

arXiv (physics.plasm-ph)Sep 24, 2026Plasma & Confinement AI, Modeling & Simulation

Quasilinear models make gyrokinetic turbulent-transport predictions sufficiently fast for integrated modelling, but their predictive capability is limited by two factors: the physical and geometrical applicability of the linear solver, and the validity of the saturation rule used to close the model. We present the Predictive Quasilinear Solver (PQLS), a quasi- linear gyrokinetic transport solver formulated in general magnetic geometry. Its implementation as an eigenvalue solver retains electromagnetic and collisional effects, provides access to dominant and subdominant modes and is differen- tiable with respect to all plasma parameters. Linear benchmarks against GENE reproduce the growth rates, frequencies, and eigenfunctions. We additionally formulate the saturation-rule closure as a Bayesian inference problem that distin- guishes uncertainty in its fitted coefficients from the residual model-form uncertainty. The approach is demonstrated by calibrating the SAT3 rule on PQLS quasilinear weights against published nonlinear CGYRO cases. In addition to improving the robustness of the calibration, the new method also quantifies the uncertainty in each of the fit coefficients. Such uncertainty is propagated through transport calculations to produce error-aware profiles that are compared to the ones obtained from the full gyrokinetic simulation, showing excellent agreement.

General AtomicsProxima FusionMax Planck Institute for Plasma Physics

Sep 18

Mechanism of Ionization Avalanche in Tokamak Microwave Gas Breakdown

Jinwoo Gwak, Yeongsun Lee, Jeongwon Lee, Won Ik Jeong, Hyun-Tae Kim, Yong-Seok Hwang, Min-Gu Yoo, Yong-Su Na

arXiv (physics.plasm-ph)Sep 18, 2026Plasma & Confinement Heating & Current Drive AI, Modeling & Simulation

Microwave breakdown driven by electron cyclotron (EC) waves provides a non-inductive route to plasma initiation in reactor-scale tokamaks. We introduce a three-dimensional Monte Carlo simulation that, for the first time, self-consistently treats nonlinear wave-particle interactions, atomic collisions, and guiding-center transport. The Monte Carlo simulation unveils the key role of parallel Brownian motion in the ionization avalanche mechanism. The predicted breakdown boundary is validated against KSTAR experiments. This work concludes that microwave gas breakdown will be successful under ITER-relevant conditions at a D$_2$ prefill pressure near 2 mPa with 1 MW of injected EC power.

United Kingdom Atomic Energy AuthorityGeneral AtomicsKorea Institute of Fusion Energy

Sep 15

Improved n=1 empirical error field penetration threshold scaling with Ohmic and L-mode conventional tokamak plasma discharges

Evan Maxwell Bursch, Jong-Kyu Park, Nikolas C. Logan, Feiyue Mao, Nengchao Wang, Carl Friedrich Benedikt Zimmermann, Richard J Buttery, Carlos Paz-Soldan, Matthew Pharr, Lidia Piron, et al.

Plasma Physics and Controlled FusionSep 15, 2026Plasma & Confinement Fusion Plant Engineering

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 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 fusion pilot plant.

Huazhong University of Science and Technology General Atomics

Neural network surrogate model for drift-kinetic energy perturbation for internal kink instability

Shuo Jiang, Lina Zhou, Yueqiang Liu, Yan Qiao, Zongqiang Li, Guangzhou Hao, Yong Wang, Xu Yang, Shuo Wang, Yutian Miao, et al.

Plasma Physics and Controlled FusionSep 15, 2026Plasma & Confinement AI, Modeling & Simulation

Multi-layer perceptron based neural networks (NNs) are trained to predict the ideal internal kink (IK) instability in tokamak plasmas, with particular emphasis on the drift-kinetic energy perturbation associated with thermal particles. The NN surrogate model is trained and validated on a semi-analytic database and tested on both held-out semi-analytic data and five EU DEMO design cases. A well-trained NN predicts the amplitude and phase of the perturbed drift-kinetic potential energy with the coefficient of determination (R²) reaching 0.88, and 0.98, respectively. The NN also achieves high R² values of 0.94 and 0.92 for the fluid growth rate and fluid perturbed potential energy, respectively. These results, together with low mean absolute errors, demonstrate the effectiveness of the semi-analytic training approach and establish a foundation for real-time IK stability monitoring and sawtooth period estimation in tokamak experiments. The physics-prediction reliability is further confirmed through case study of an EU DEMO negative-triangularity equilibrium, where the NN successfully reproduces the theoretical linear relationship between the fluid growth rate and the fluid perturbed potential energy and captures the dependence of IK instability on multiple plasma parameters. Collectively, these results confirm that the trained NN effectively captures essential physical characteristics of the IK instability.

General AtomicsSouthwestern Institute of Physics

Sep 14

Electromagnetic gyrokinetic analysis in Tokamak Energy’s pre-concept design of the ST-E1 fusion power plant

Alexandra V Dudkovskaia, Jeff Candy, Emily A Belli, Michail Savvas Anastopoulos Tzanis, Steven A.M. McNamara, Andrew Oakleigh Nelson, Yang Ren, Timothy Stoltzfus-Dueck, Carl Friedrich Benedikt Zimmermann

Nuclear FusionSep 14, 2026Plasma & Confinement Fusion Plant Engineering AI, Modeling & Simulation

ST-E1 is Tokamak Energy's fusion power plant concept. A series of reference flat-top plasma operating points were developed in [S McNamara et al. Nucl. Fusion 66 (2026) 086008]. The present work focuses on the low-density baseline conceptual power plant (BCP) operating point of [S McNamara et al. Nucl. Fusion 66 (2026) 086008], selected as representative of ST-E1, and investigates its micro-stability, turbulent transport and transport-informed optimisation using first-principles spectral flux-tube electromagnetic gyrokinetic simulations. Linear gyrokinetic calculations, involving electrons, thermal ions, impurities, and thermal and fast helium ions, complemented by dynamic mode decomposition to identify subdominant and stable drift-wave eigenmodes, reveal two principal long-wavelength electromagnetic instability branches: hybrid h-ITG modes driven by thermal gradients and FI-KBMs, i.e., kinetic-ballooning-like modes strongly influenced by plasma beta and fast-ion population. At shorter wavelengths, electron-temperature-gradient modes become unstable towards plasma edge while remaining stable in the core. A comprehensive sensitivity analysis identifies principal equilibrium parameters governing these instabilities. Building on these linear predictions, nonlinear electromagnetic gyrokinetic simulations quantify turbulent transport of thermal plasma and fast helium ions. The reference operating point is found to exhibit saturated, rather than runaway, electromagnetic turbulence, establishing it as a physically meaningful baseline for further optimisation. Turbulent transport is governed primarily by combined effects of plasma beta, safety factor and fast-ion fraction, with intrinsic-rotation-driven ExB shear providing complementary suppression. Although fast ions provide free energy driving FI-KBM branch, increasing fast-ion fraction is found to reduce saturated turbulent transport across all species through nonlinear self-organisation. To explore the resulting multidimensional parameter space, a physics-informed interpolation framework constrained and validated by nonlinear gyrokinetic simulations is developed to enable identification of transport-favourable operating points without exhaustive nonlinear parameter scans. This framework yields a family of transport-favourable operating points for ST-E1 low-density BCP equilibrium and provides quantitative targets for future self-consistent equilibrium optimisation.

Princeton Plasma Physics LaboratoryGeneral AtomicsOak Ridge National LaboratoryTokamak Energy

Sep 10

Effect of pedestal current on the density window for ELM suppression using n = 4 RMP in EAST

Xuemin Wu, Youwen Sun, Qun Ma, Shuai Gu, Manni Jia, Yueqiang Liu, Yifeng Wang, Cheng Ye, Pengcheng Xie, Alberto Loarte, et al.

Plasma Physics and Controlled FusionSep 10, 2026Plasma & Confinement AI, Modeling & Simulation

Existence of operational window in both edge safety factor and line averaged plasma density for suppression of ELMs using n=4 Resonant Magnetic Perturbations in low input torque plasmas has been observed in EAST experiment, in which q95 and plasma normalized beta (βN) close to that required in ITER high-Q operation. Here, n is toroidal mode number of the magnetic perturbation. In contrast to previous reports from other tokamaks, there is not only an upper density limit but also a lower one for accessing ELM suppression. Modelling results using the MARS-F code show that the RMP with linear plasma response has a peak at an intermediate density and decays as the density increases or decreases, which results in a minimal RMP field penetration threshold at the intermediate density. In this experiment, the observed lower density limit operationally manifests a sensitivity of the q-profile: different densities alter the edge current profile, which change the alignment of the eigenmode structure with the RMP coil configuration, causing a reduction of the resonant field in both low- and high-density cases, and hence making field penetration more difficult. The modelled window of the strongest resonant plasma response in terms of [⟨ne⟩, q95] agrees well with the observed ELM suppression in EAST. Peeling-ballooning modes stability analysis using the ELITE code shows that plasmas gradually approach peeling instability boundary caused by increase of edge bootstrap current as the plasma density decreases, which is consistent with the observation that ELMs come back again in lower density plasmas for fixed q95. These results indicate that linear modelling with full toroidal geometry can well predict the optimized RMP configuration for ELM suppression and reveal the important roles of pedestal plasma current, which need to be carefully considered in the application of high n RMPs for ELM suppression in future ITER.

General AtomicsHefei Institutes of Physical ScienceITER Organization

Sep 7

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

Yutong He, Alexey Arefiev, Mario Manuel, Hui Chen, Christopher Ridgers

arXiv (physics.plasm-ph)Sep 7, 2026Inertial Fusion & HEDP

The linear Breit-Wheeler (LBW) process ($γ+γ\rightarrow e^{-}+e^{+}$) 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.

Lawrence Livermore National LaboratoryUniversity of California San DiegoGeneral AtomicsInstituto Superior Técnico

Neoclassical tearing mode seeding by Alfvén eigenmode coupling in MAST-U

Kitt Cameron Medley Thomas, Laszlo Bardoczi, Kieran Gibson, Juan Ruiz Ruiz, Mykola Dreval, David Anthony Ryan, Kenneth G McClements, Clive A Michael

Nuclear FusionSep 7, 2026Plasma & Confinement Control & Diagnostics

We present evidence of tearing mode seeding by nonlinear three-wave coupling of Alfvén eigenmodes in a tokamak plasma. Here, a m,n=2,1 magnetic island appears to be seeded in a MAST-U discharge by coupling of n = 2 and n = 3 Alfvén eigenmodes, which is demonstrated by increased squared normalised bicoherence calculated from magnetic probes preceding tearing mode onset. This observation offers a new explanation for tearing modes that would be otherwise addressed as spontaneous, and indicates three-wave coupling can seed 2,1 magnetic islands in plasmas without pre-existing n ≥ 2 tearing modes. Further, this mechanism has the potential to be more prevalent in burning plasmas expected in future fusion devices.

General AtomicsKharkiv Institute of Physics and TechnologyCulham Centre for Fusion Energy

Exploration of an intrinsic low-collisionality, high performance, grassy ELM regime in the DIII-D tokamak

Zeyu Li, H Q Wang, Xi Chen, Xueqiao Xu, Rongjie Hong, Filipp Khabanov, V. S. Chan, Patrick H Diamond, Brian S Victor, Nami Li, et al.

Nuclear FusionSep 7, 2026Plasma & Confinement AI, Modeling & Simulation

Recent DIII-D tokamak experiments have demonstrated the integration of intrinsic low-collisionality grassy edge-localized mode (ELM) regime with high-performance hybrid core scenario, possibly offering a core-edge compatible solution for ITER and future fusion reactors. This regime features grassy ELMs (with ELM energy loss over pedestal stored energy <2%) at ITER-relevant pedestal top collisionality (ν_e^*~0.1), ITER similar shape, while maintaining high core confinement (H_98y2~1.5) in non-inductive hybrid scenarios. A small ELM-focused database is constructed to investigate the parametric dependence of the small/grassy ELM regimes. Access and sustainment of this regime appear to be favored by high poloidal beta (β_p>1.5), higher ratio of separatrix-to-pedestal density (n_(e,sep)⁄n_(e,ped) >0.4) and low pedestal top collisionality (υ_(e,ped)^*=0.1-0.4). In this scenario, the pedestal width exceeds the prediction from EPED–KBM scaling, consistent with the expectation of a turbulence-limited pedestal. Linear modeling using ELITE indicates that this grassy ELM regime is along the peeling boundary. Relative to a large-ELM phase, the grassy-ELM phase exhibits a broader inner-target heat-flux width and a substantially reduced ELM-induced transient heat-flux increment. These results motivate further evaluation of low-collisionality grassy ELMs as a potentially reactor-relevant operating regime, while full-duration sustainment and compatibility with divertor detachment remain unresolved.

University of California Los AngelesGeneral AtomicsUniversity of Wisconsin MadisonLawrence Livermore National Laboratory

Sep 3

Numerical studies of mode coupling induced by neoclassical toroidal viscous torque in error field penetration on EAST

Cheng Ye, Youwen Sun, Hui-Hui Wang, Yueqiang Liu, Pengcheng Xie, Jian Xu, Hui Sheng, Xin-Jian Wang, T Y Xia

Plasma Physics and Controlled FusionSep 3, 2026Plasma & Confinement Control & Diagnostics AI, Modeling & Simulation

Mode coupling induced by neoclassical toroidal viscous (NTV) torque governs error field penetration in toroidal plasmas , causing the $2/1$ penetration threshold to deviate from linear response prediction with respect to the RMP upper-lower coil phasing ($\Delta\phi_{UL}$). This behavior originates from nonlinear modulation of toroidal momentum transport by non-resonant three-dimensional magnetic field components. In this work, the characteristics of such mode coupling is systematically investigated using the MARS-Q code. Two aspects are examined. First, the dependence of mode coupling strength on key plasma parameters is analyzed. A larger momentum diffusivity ($\chi_M$) is found to strongly enhance mode coupling and invalidate linear response criteria, whereas at low $\chi_M$ linear prediction remains applicable. A NTV torque weighting factor based on linear response is identified as a qualitative indicator of mode coupling in the nonlinear field penetration. In addition, reduced resistivity ($\eta$), higher normalized beta ($\beta_N$), and larger inverse aspect ratio ($\epsilon$) all strengthen mode coupling. Second, a critical momentum diffusivity, $\chi_{M,crit}$, is introduced to characterize the impact of boundary-induced mode coupling on the $2/1$ penetration spectrum, with smaller value indicating stronger impact. The results show that $\chi_{M,crit}$ increases with initial plasma rotation, but decreases for rotation profile with reduced flow shear. Moreover, increasing $\eta$, as well as higher $\beta_N$ and $\epsilon$ also reduce $\chi_{M,crit}$, albeit through different physical mechanisms. Across broad range of numerical scenarios, these results highlight the essential role of mode coupling in error field control for future fusion devices.

Chinese Academy of SciencesGeneral Atomics

Sep 2

MGKDB: An IMAS-aligned multicode gyrokinetic simulation database for reproducible fusion turbulence modeling and data-driven analysis

Craig Michoski, David R. Hatch, Dongyang Kuang, Matthew Waller, Chris Holland, M. J. Pueschel, Joseph McClenaghan, Tom F. Neiser, Max T. Curie, Venkitesh Ayyar, et al.

arXiv (physics.plasm-ph)Sep 2, 2026Plasma & Confinement AI, Modeling & Simulation

Expensive fusion simulations are commonly preserved in code-specific formats that limit discovery, comparison, and reuse. We present the Multiscale GyroKinetic DataBase (MGKDB), an open-source software framework and curated archive that converts heterogeneous simulation campaigns into traceable scientific records. Each record links code-native inputs and outputs to provenance and quality metadata, an IMAS-aligned physics representation, and derived diagnostics, preserving model-specific evidence while enabling common-field queries. Production pathways support linear and nonlinear GENE and CGYRO calculations and reduced quasilinear TGLF evaluations. At the September 1, 2026 snapshot, MGKDB contained 1,068,089 records, nearly all of which included a populated gyrokinetics IMAS branch. The software is openly available, while access to the NERSC-hosted production records is managed. Three demonstrations show how these linked representations support scientific reuse. Standardized quantities stored in the Diagnostics branch enable population-scale analysis of archived linear modes; common input coordinates reveal coverage, redundancy, and campaign-driven sampling structure across a multicode collection; and record-level retrieval of native CGYRO inputs drives matched TGLF calculations and produces a traceable dataset for exploratory surrogate modeling. Together, these examples demonstrate how MGKDB supports archive characterization, candidate cross-code and cross-fidelity comparisons, campaign planning, and reproducible data-driven modeling without treating different models as automatically equivalent.

SophelioGeneral AtomicsMassachusetts Institute of TechnologyUniversity of California San Diego

Sep 1

Asymptotic mass and charge scaling for trace impurity transport in tokamaks

A. Tema Biwole, J. Candy, E. A. Belli, I. Sfiligoi, N. T. Howard, T. Odstrcil, P. Rodriguez-Fernandez

Physics of PlasmasSep 1, 2026Plasma & Confinement AI, Modeling & Simulation

Using an approximate quasilinear formulation of gyrokinetic theory, we derive a simple formula for the impurity particle transport in the limit of large charge za and mass ma. When za and ma are large and comparable, the flux decomposes into four separate contributions from curvature drift, parallel motion, density, and temperature gradients. In the extreme limit that ma≫za2, the flux obeys a simple 2-term scaling law proportional to za/ma. The results are limited to electrostatic gyrokinetic theory and are confirmed by direct nonlinear gyrokinetic simulations. The new findings contrast with historical emphases on charge-number scalings alone and have direct implications for tungsten transport in fusion plasmas. In particular, they indicate that most tungsten charge states, which share the same mass but differ in ionization, experience a mass-dependent suppression of impurity particle transport arising from finite-Larmor-radius effects in ion-scale turbulence.

General AtomicsMIT Plasma Science and Fusion CenterUniversity of California San Diego

The Fusion Equilibrium Challenge: Inferring Magnetic Geometry Without Magnetic Diagnostics

Tapan Ganatma Nakkina, Matthew Waller, Craig Michoski, Brian Sammuli, David R. Hatch, William Boyes, Mitchell Clark, Raffi Nazikian, Sterling Smith

arXiv (physics.plasm-ph)Sep 1, 2026Plasma & Confinement Control & Diagnostics AI, Modeling & Simulation

Next-generation fusion reactor devices such as SPARC, ARC, and CFETR will operate in extreme neutron environments that compromise the magnetic sensors traditionally used to reconstruct plasma equilibria. However, reliable knowledge of the plasma equilibrium--including magnetic flux surfaces, safety factor profiles, and shaping parameters--is indispensable for real-time control, disruption avoidance, and physics interpretation. The Fusion Equilibrium Challenge invites the NeurIPS community to confront a deceptively simple but scientifically rigorous inverse problem: reconstruct the two-dimensional poloidal flux function psi(R,Z) and a suite of scalar equilibrium parameters from non-magnetic diagnostics alone, namely external poloidal-field coil currents and Thomson-scattering electron temperature/density profiles. The challenge provides the first open-access, harmonized multi-machine benchmark for fusion, releasing a curated dataset of 9,113 DIII-D shots and 2,416 MAST shots--filtered for Thomson-diagnostic availability, feature completeness, and EFIT-reconstruction quality. Each shot is packaged into a standard Parquet file containing approximately 260 (DIII-D) / approximately 80 (MAST) EFIT flux maps and rich high-rate diagnostics. Two complementary awards reward intra-machine reconstruction fidelity (S_model) on DIII-D and zero-shot cross-machine generalization (G_ratio) to the topologically distinct MAST spherical tokamak. We argue that the challenge functions as a benchmark for reactor-ready equilibrium inference and as a probe of how far machine learning can be pushed toward truly machine-agnostic plasma state estimation.

SophelioGeneral Atomics

Aug 26

Effect of electron cyclotron waves on plasma with runaway electrons

Pavel Aleynikov, Alexander Franklin Battey, Carlos Paz-Soldan, Eric Matthias Hollmann, Andrey Lvovskiy, Claudio Marini, Daisuke Shiraki, Charles Lasnier

Nuclear FusionAug 26, 2026Plasma & Confinement Control & Diagnostics Heating & Current Drive

Runaway electrons generated during tokamak disruptions are a major concern for the safe operation of future fusion devices. The interaction of runaway electrons with waves has been proposed as a potential mechanism for their mitigation. This study investigates the effect of electron-cyclotron (EC) waves on post-disruption plasmas containing runaway electrons (REs). O- and X-mode EC waves are routinely used for plasma heating and current drive. However, these modes do not interact directly with relativistic electrons and cannot be injected into plasmas with densities exceeding their respective cutoff densities. In contrast, the internal slow X-mode (sX) can resonate with relativistic electrons and may therefore provide a pathway for their mitigation. We report DIII-D experiments designed to access the internal slow X-mode through O--sX conversion during post-disruption RE plateaus. ECH increased the background-plasma density, doubled the loop voltage, and strongly enhanced the RE synchrotron signal, demonstrating substantial ECH--plasma coupling and increased effective dissipation of the RE channel. Although ECH produced a pronounced response consistent with enhanced RE dissipation, we could not isolate conclusive evidence of resonant sX--RE interaction. Strong heating, ionization, and impurity-redistribution effects appear to dominate the response and may mask a direct wave--particle signature. Nevertheless, the results demonstrate the potential of ECH for RE control through background-plasma modification, with O--sX access offering an additional possibility of direct resonant interaction.

General AtomicsSwiss Federal Institute of Technology LausanneMax Planck Institute for Plasma Physics

Fast-ion enhanced modeling of neoclassical tearing modes at NSTX and DIII-D

James Yang, Eric Fredrickson, John W Berkery, Robert John La Haye, Mario Podesta

Plasma Physics and Controlled FusionAug 26, 2026Plasma & Confinement AI, Modeling & Simulation

A new framework for the solution of modified Rutherford equation including fast ions is successfully applied to interpret the tearing mode stability of two discharges NSTX #134020 and DIII-D #135861. The simulated island width growth rates are in better agreement with the measured island growth rate when the fast ions are included. While constants are multiplied to the polarization current contribution terms for NSTX #134020, no constants are necessary to match the simulated and measured island width growth rates for DIII-D #135861. The estimated island frequencies appear to provide an explanation of the different constants used in the two discharges. The gradient scale lengths suggest that the fast ion contribution can become significant in plasmas with flat thermal ion density profile and steep fast ion density profile.

Princeton Plasma Physics LaboratoryGeneral AtomicsSwiss Federal Institute of Technology Lausanne

Compact Experimental Negative TriAngUlarity Reactor (CENTAUR): A design study for a compact, affordable breakeven tokamak

The CENTAUR Collaboration, Samuel W. Freiberger, Evan Bursch, Javier Chiriboga, Hiro J. Farre-Kaga, Eliot Felske, Sophia Guizzo, John Labbate, Shreyas Seethalla, Frederick Sheehan, et al.

Plasma Physics and Controlled FusionAug 26, 2026Plasma & Confinement Materials & Plasma-Facing Components Blankets & Neutronics Magnets & Superconductors Fusion Plant Engineering Economics & Commercialization

This work presents the compact experimental negative triangularity reactor (CENTAUR), a low overnight cost, high-field tokamak, breakeven reactor design, achieving a predicted total fusion power of 40MW and scientific energy gain of 1.3. Ballooning stability calculations confirm that the device's pedestal is within the first stability regime, which is consistent with the expected ELM-free operation associated with negative triangularity (NT) plasmas. The geometry of the NT divertor allows for high fraction of radiated power (13.5$\%$) between the separatrix and plasma facing components. Heat transport modeling based on simulations of the edge region show heat loads into plasma facing components well below material limits. The magnet system employs rare-earth barium copper oxide (REBCO) high-temperature superconductors in 18 toroidal field coils, an hourglass-shaped central solenoid, and six poloidal field coils to support high-field ($B_0=10.9$ T) plasma confinement, shaping, and current drive. Neutronics analysis shows that a 12 cm $B_4C$ shield keeps superconducting magnet heating below the 33~K quench limit during 10 s, 40 MW DT pulses. With this shielding, the modeled fluence indicates HTS components can survive more than ten times the 3000-pulse design lifetime. Iteration of economic analysis in tandem with the technical design process allows CENTAUR to achieve its overnight cost goal of $\$$2B determined using a custom costing model that predicts a total overnight cost of $1.6$B$\pm0.2$B.

Princeton UniversityLawrence Livermore National LaboratoryPrinceton Plasma Physics LaboratoryGeneral Atomics