Recent Publications

Investigation of Carbon Impurity Behaviors on Tokamak Startup

Yanli Peng, Runqin Liu, Yang Li, Shali Yang

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

Ce Wang, Sven Van Loo, 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.

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

J. Gallego, P. Protopapas, A. Bustos, A. Alonso, S. Barquero, A. Baciero, I. Rivera, J. A. Moríñigo, R. Mayo-García

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, $κ_\perp(n,T)$. 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 $κ_\perp(n,T)$ 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.

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.

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.

Generalized Lawson-Cordey-Mills Accessibility of Fusion Ignition

Luis F. Delgado-Aparicio, Masayuki Ono, Jonathan E. Menard

JET neutron emissivity reconstruction using the Minimum Fisher Information method for 1 ms temporal resolution

Katarzyna Mikszuta-Michalik, Daniele Marocco, Basilio Esposito, Marco Riva, Gianluca Pucella

A tomography code based on the Minimum Fisher Regularisation method with a geometry description based on the field of view has been applied to reconstruct the neutron emissivity profiles from measurements performed in 2021 during the second JET deuterium-tritium campaign (DTE2). The Neutron Profile Monitor Upgrade installed at JET offers a unique opportunity for investigating the neutron emissivity in a full poloidal cross-section with unprecedented time resolution (~1 ms), thanks to the availability of digital data reprocessing and the high neutron emission that enables milisecond line-integrated raw data with low statistical error. Measurements of 14.1 MeV neutrons provided by the Bicron BC418 plastic scintillators installed in the JET neutron camera have been used.The analysis focuses on the effect of sawtooth crashes on the neutron emission profiles. Tomography results were compared with 1D reconstructions using two different methods and 10 ms time resolution. The use of the neutron camera data sampled at 1 ms enables the observation of hollowing of the neutron emissivity profiles during the sawtooth crash and their subsequent flattening.

Machine learning tomography for sparse-view soft X-ray diagnostics in MAST-U

Brian Steward, Marco Cecconello

Understanding magnetohydrodynamic (MHD) instabilities is crucial for advancing magnetic confinement fusion. Soft X-Ray (SXR) tomography enables reconstruction of the spatial structure of these instabilities from one dimensional line integrated measurements, however traditional tomography methods struggle with sparse diagnostic arrays. The MAST-U tokamak has a sparse camera array of 28 non-intersecting lines of sight (LoS), making traditional tomography techniques challenging. We present a machine learning (ML) tomographic method using a 50 layer residual neural network (ResNet) trained on synthetic data simulated by the transport code TRANSP. ML tomography is compared with traditional minimum Fisher information tomography, and with second derivative smoothing tomography. The ML technique shows a 78 to 94 times median improvement in the mean square error (MSE) over traditional 28 LoS reconstructions of synthetic data, at a computational speed which is 144 to 294 times greater (25 ms on GPU hardware). Notably, ML tomography with 28 LoS outperforms traditional methods even when provided with three times as much data in the form of 84 intersecting synthetic LoS. Validation on experimental data from MAST-U demonstrates accurate reconstructions of peaked and broad plasma SXR profiles, and the evolution of sawtooth instabilities are visualized with ML tomography. The sub-30 ms reconstruction time enables real-time plasma diagnostics, opening new possibilities for MHD monitoring in fusion devices.

Kinetic modeling of lithium impurity transport in the HIDRA stellarator during controlled lithium evaporation

Steven Gula, Nina Mihajlov, Giovanni Diaz, Arnav Goyal, R Maingi, D Andruczyk, Davide Curreli

Post-operational imaging of the Hybrid Illinois Device for Research and Applications (HIDRA) stellarator following controlled lithium evaporation reveals distinct streak patterns of lithium deposition along the vacuum vessel wall. These localized structures exhibit strong alignment with magnetic field topology, suggesting that impurity transport in the scrape-off layer (SOL) is governed primarily by the three-dimensional magnetic geometry and associated electrostatic fields. In this work, large-scale kinetic simulations of trace lithium impurity transport are used to investigate the formation of these deposition structures. The magnetic field topology is reconstructed using a Biot-Savart law approach, from which a normalized flux-surface parameter is computed and used to prescribe the background helium plasma profiles. Electron density and plasma potential are modeled as normalized flux-surface-dependent fields scaled by representative scalar measurements and analytical sheath-based estimates, with the electric field obtained from ∇V p . These reconstructed fields are used to compute lithium ion trajectories and generate spatial maps of deposition on the vessel wall. The simulations reproduce streak-like deposition features aligned with magnetic flux tubes intersecting the wall, and the simulated deposition locations and orientations show qualitative agreement with experimental observations. These results demonstrate that magnetic field structure plays a dominant role in shaping impurity transport and deposition in HIDRA and establish a predictive framework for modeling impurity transport and surface deposition in plasma-surface interaction experiments involving liquid-metal plasma-facing components.

Positive sheath generation in front of fishscaled divertor plates

Choong-Seock Seock Chang, Seung-Hoe Ku, Xin Zhang, Trenton Brewer, Nicolas Lopez, Chris Marsden

Fish-scale divertor plates are becoming popular in the design of next-generation magnetic fusion reactors to make the edge of one divertor tile sits safely behind the shadowed profile of the preceding tile, hence to protect the leading edge from extreme heat deposition. Recently, ST40 tokamak has observed an extremely narrow and peaked divertor heat-load footprint near the separatrix strike point on the fish-scale plates, on top of the usual ion-drift-width scale footprint. This observation raised concern over the severely localized burn even when the well-known ion-drift-width scale burn issue can be resolved [X. Zhang et al., Nucl. Mater. Energy 41, 101772 (2024)]. In this report we demonstrate that the lost ion gyro-orbits to the tilted tile edges can lead to a significant positive-potential sheath at a practical magnetic field incidence angle that is much greater than the usual incidence angle for the ion gyro-sheath formation on flat strike-surface. This could make the kinetic electron heat-flow spilled over from the confined region down the divertor legs to gain a significant kinetic energy amplification and allow a highly localized heat-load peak under a low edge turbulence condition. Possible intrinsic mitigation mechanism is discussed. This issue should be considered carefully in the design of next generation magnetic fusion reactors.

Experimental evidence for increased particle fluxes due to a change in transport at the separatrix near density limits on Alcator C-Mod

Marco Andrés Miller, Jerry W Hughes, Thomas Eich, George R Tynan, Peter Manz, Amanda E Hubbard, Brian LaBombard, Jamie Dunsmore

Experimental inferences of cross-field particle flux at the separatrix, Γ ⊥ sep , show rapid growth near H-mode and L-mode density limits at high magnetic field on Alcator C-Mod. Increases in Γ ⊥ sep correlate well with proximity to high density operational boundaries as proposed by the separatrix operational space model. Γ ⊥ sep grows as the L-mode density limit and the H-L-mode back transition boundaries are approached, consistent with expectations of plasma instability-driven turbulence suggested by theory, confirming the power dependence of density limits. Γ ⊥ sep is well-organized by the characteristic wavenumber for resistive ballooning mode turbulence, k RBM , from interchange-drift-Alfvén fluid turbulence theory, with additional dependence on the cylindrical safety factor, q cyl , yielding an empirical limit to plasma operation of k RBM 2 q cyl = 1. This limit corresponds to the point where the perpendicular heat flux, Q ⊥ , reaches the level of the parallel heat flux, Q ∥ , i.e. Q ⊥ ≈ Q ∥ , beyond which point thermal equilibrium is not satisfied, resulting in a fold catastrophe.

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

Atul Kumar, Dhyanjyoti Nath, Wouter Tierens, Jeremy D. Lore, Andrei Pshenov, Tom Wauters, Andrea Galvan, Davide Curreli, Syun'ichi Shiraiwa, Nicola Bertelli, et al.

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.

All field strengths are possible locally in Boozer coordinates

Taylor J. Klotz, Nathan Duignan, Joshua W. Burby, James D. Meiss

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.

A composite impurity-pressure index for assessing startup readiness during plasma recovery on EAST

Shuqi Yang, Yaowei Yu, Tao Zhang, Xiang Zhu, G Z Zuo, Xiang Gao

Reliable restart after vessel venting is controlled by the burn-through power balance: the plasma must ionize the residual neutrals, dissociate molecules and raise the electron temperature before ionization, charge-exchange and impurity-radiation losses exhaust the available ohmic or auxiliary power. In practice, however, operators usually do not know whether the wall has recovered until a shot has already been attempted. Six EAST plasma-recovery campaigns from 2023 to 2025 are analysed, and a pre-shot impurity-pressure index is introduced to combine the neutral pressure measured 2–3 s before breakdown with a weighted residual-gas-analyser proxy for nitrogen- and oxygen-bearing species. In this dataset, the index organises discharge duration, stable-shot probability and the loop voltage during the first 50–200 ms more clearly than either pressure or composition alone. Under the standard EAST startup condition used here, stable discharges become much more likely when the index falls to around 1 × 10-7 Pa. In the second 2025 campaign, the shot-by-shot evolution also shows threshold-like burn-through behaviour: the maximum line-averaged electron density in 0–0.2 s stays low at large index values, overshoots in a transition interval, and then settles to a more stable level as the wall recovers. Across campaigns, post-recovery values cluster much more tightly than the recovery paths themselves, indicating that the index characterizes the startup-ready wall state rather than a particular conditioning route. The metric therefore provides a practical pre-shot indicator of whether EAST has recovered sufficiently for reproducible stable startup.

Design and commissioning of a tangential Gas Puff Imaging diagnostic on the HL-3 tokamak

W.C. Wang, Z.H. Huang, Z.B. Shi, Y. Hu, J. Cheng, K.Y. Yi, W.M. Xie, Y. He, N. Wu, W. Zhao, et al.

Electromagnetic filament coalescence as magnetic island merging with diamagnetic effects

Souvik Mondal, N Bisai, Abhijit Sen, Indranil Bandyopadhyay

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.

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

Hong Qin, 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.

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.

Runaway electron control by self-excited waves

Kun Huang, 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.

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

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.

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.

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.