Recent Publications

Optical diagnostic design for measuring the radiation front with a mid-leg pumped divertor on DIII-D

Gilson Ronchi, Robert S. Wilcox, Jonathan H. Yu, Andreas Holm, Filippo Scotti, Adam McLean, Morgan Shafer, Joshua Hicok, Tyler Elsey

Oak Ridge National Laboratory, General Atomics, Lawrence Livermore National Laboratory

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

Princeton University Plasma Physics Laboratory, General Atomics, Ecole Polytechnique Federale de Lausanne

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.

Non-dimensional confinement scaling in similar negative triangularity plasmas on the DIII-D and TCV tokamaks

Alessandro Marinoni, Colin Chrystal, Stefano Coda, Reinart Coosemans, Claudio Marini, Mario Podesta, Olivier Sauter, Matteo Agostini, Max E Austin, Emily A Belli, et al.

General Atomics, Ecole Polytechnique Federale de Lausanne, Columbia University, University of California San Diego, Consorzio RFX

Similarity experiments were performed on the DIII-D and TCV tokamaks to explore the scaling of energy confinement in negative triangularity plasmas using non-dimensional variables. Near up-down symmetric plasmas with large top-bottom averaged negative triangularity were created in a lower single null configuration, with the shape of the separatrix being closely matched between the two devices. The normalized energy confinement is found to weakly improve at increasing collisionality and, between the two devices, shows a machine size scaling behavior between Bohm and gyro-Bohm. Engineering scaling on a large DIII-D dataset is in agreement with the non-dimensional experiment.

Integration of X-point radiator divertor operation with high beta hybrid core plasmas in DIII-D

H Q Wang, Xinxing Ma, Zeyu Li, Roberto Maurizio, Anthony W Leonard, Filippo Scotti, Qiming Hu, Dan M Thomas, Andrea M Garofalo, Siye Ding, et al.

General Atomics, Peking University, Lawrence Livermore National Laboratory, Princeton Plasma Physics Laboratory, University of Tennessee Knoxville

Recent DIII-D experiments have demonstrated the compatibility of complete divertor detachment with a high-beta core using the high beta hybrid scenario plasmas with an ITER-similar shape and nitrogen impurity seeding. With complete divertor detachment (near-zero divertor particle flux and temperature), the radiation peaks inside the X-point indicating the achievement of the X-Point Radiator (XPR) regime, which is a highly dissipative divertor operation scenario that may be attractive for future reactors. SOLPS-ITER modelling with full drifts is able to qualitatively reproduce the experimental measured boundary plasma conditions and radiation patterns from attached to XPR detached divertor state for these high-beta hybrid plasmas. Experiments found that when the radiation peak is inside but close to the X-point, as noted ‘shallow XPR’, complete divertor detachment and high-beta high-confinement core (βN ~3.0, H98~1.25) could be simultaneously achieved. However, this plasma remains ELMing with giant ELMs (W/W ~ 3-4%). With stronger N2 impurity injection, the plasma enters a ‘deep XPR’ regime where the radiation peak is close to inboard side of the pedestal and core radiation is about a factor of 2 higher. With deep XPR, the ELMs are strongly mitigated. However, the confinement is significantly reduced to H98<1.0, which is attributed to the 50% lower pedestal pressure and 30% colder pedestal temperature. SOLPS-ITER simulations highlight the key role of impurity radiation, neutrals and divertor closure in the formation of an XPR and the effects of drifts on the distribution of plasma and radiation near the X-point, all of which are important for the divertor design and operation in future reactors. The modelling also exhibits qualitatively good agreement with experimental observations on the pedestal performance responding to the radiation dynamics, which provides physics insight on the core-edge integration between divertor dissipation and high-performance core that is critically important for future tokamak fusion reactors.

SOLPS-ITER modeling of a dedicated divertor for negative triangularity operation on DIII-D

J.D. Lore, F. Scotti, A. Marinoni, K.E. Thome, C. Lasnier, M. Shafer, D. Truong, H.Q. Wang

Lawrence Livermore National Laboratory, Oak Ridge National Laboratory, General Atomics, University of California San Diego

Achievement of a high-density, high-confinement, and high-beta tokamak plasma regime in DIII-D, and implications for a lower-current path for ITER and FPP

Andrea M Garofalo, Bart Van Compernolle, Siye Ding, Jeremy M Hanson, Christopher Thomas Holcomb, Tomas Odstrcil, Nathan Jordan Richner, Shengyu Shi, H Q Wang, Torrin Bechtel, et al.

General Atomics, Columbia University, Lawrence Livermore National Laboratory, Oak Ridge Associated Universities, Princeton Plasma Physics Laboratory

Experiments on DIII-D have demonstrated a density-confinement synergy that enables sustainment of high performance in a previously unattained parameter regime of simultaneous very high energy confinement quality (H 98y2 ≥ 1.5), very high line-average density Greenwald fraction (ƒ Gr = πa 2 <n>/I P ≥ 1.4), and high toroidal beta (β T ≥ 3%). Tokamak operation in this regime is essential for a compact steady-state FPP, as well as for Q=10 with 500 MW of fusion power in ITER at I P << 15 MA. These experiments leveraged the knowledge that, in the high-poloidal-beta (β P ) regime, impurity and density gradients can enhance turbulence stabilization caused by high α MHD (α MHD ~(dβ P )⁄dr). This was described by theoretical predictions and gyrokinetic transport simulations [M.T. Kotschenreuther et al, 2024 Nucl. Fusion, 64 076033], and later confirmed by experiments on DIII-D [S. Ding et al, 2024 Nature 629 555]. To increase both β P and β T , the new experiments increased the ideal-wall stability β N -limit by using a smaller plasma-outer wall distance and higher triangularity in the plasma cross section (top/bottom average δ~0.9), enabled by the recent “shape & volume rise” (SVR) modification to the DIII-D divertor. The higher triangularity also contributed to achieving higher ƒ Gr by enabling higher pedestal density. At high density, the pedestal is ballooning limited and exhibits small and frequent ELMs, while the divertor is near detachment even without any impurity seeding. High plasma performance was attained and sustained reproducibly, with the eventual terminations brought about by an MHD mode destabilized as the current profile slowly continued to evolve. A path to stationary fully noninductive operation might include ECH injection to reduce both core impurity accumulation and the electron collisionality, thus increasing the bootstrap current. These experiments provide the first experimental demonstration of the ƒ Gr , H 98y2 , and β T values required simultaneously for ITER Q = 10 at I P < 10 MA, pointing to practical ways to improve the energy confinement in a fusion reactor.

Linear and quasi-linear plasma response to resonant magnetic perturbations during ELM mitigation in HL-3

N. Zhang, Y. Q. Liu, G. Z. Hao, J. M. Yu, T. F. Sun, G. Q. Dong, Yi Liu, L. Wang, J. Huang, M. Y. He, et al.

Southwestern Institute of Physics, General Atomics, Sichuan Technology and Business University, Tsinghua University

Active mitigation of edge-localized modes (ELMs)with the n=1 (n is the toroidal mode number) resonant magnetic perturbation (RMP) has recently been achieved for the first time on the HL-3 tokamak. The linear and quasi-linear plasma responses to RMP fields are numerically investigated by utilizing the MARS-F [Liu et al., Phys. Plasmas 7, 3681 (2000)] and MARS-Q [Liu et al., Phys. Plasmas 20, 042503 (2013)] codes. The linear results show that RMP induces a strong edge-peeling response which facilitates the ELM mitigation. A −50° phase shift for the n=1 coil current between the upper and lower rows of the RMP coils presents the optimal coil phase. MARS-Q quasi-linear results show that: (i) without involving perturbation mode near the plasma edge, the applied RMP has minor side effects on both the toroidal momentum and radial particle transport in this HL-3 case; (ii) allowing weak perturbation mode together with RMP produces finite flow damping and density pump-out level comparable to experiments; and (iii) the modeled flow damping and density pump-out is not very sensitive to the assumed resistivity model (Spitzer vs uniform resistivity). We found that both the neoclassical toroidal viscosity and resonant electromagnetic torques play important roles in the plasma toroidal momentum transport in HL-3.

Understanding carbon sourcing and transport originating from the helicon antenna surfaces during high-power helicon discharge in DIII-D Tokamak

Atul Kumar, Dhyanjyoti Nath, Wouter Tierens, Jeremy D Lore, R S Wilcox, Gilson Ronchi, Morgan W Shafer, Aditya Y Joshi, Onkar Sahni, Mark Shephard, et al.

Oak Ridge National Laboratory, Rensselaer Polytechnic Institute, General Atomics, University of Wisconsin-Madison

The high-power helicon wave system in the DIII-D tokamak could potentially introduce new plasma--material interaction (PMI) challenges owing to rectified RF sheath potentials that develop near the antenna and surrounding plasma-facing components. We present the first application of the STRIPE (Simulated Transport of RF Impurity Production and Emission) framework to helicon-induced PMIs, extending previous STRIPE studies of ICRH antennas by incorporating net erosion, local re-deposition, and three-dimensional global impurity transport. The integrated workflow couples SOLPS, COMSOL, RustBCA, GITR, and GITRm to simulate carbon erosion, re-deposition, and impurity transport for two experimentally constrained DIII-D H-mode helicon operating scenarios with different antenna--plasma gaps, coupled RF powers, and edge plasma conditions. COMSOL predicts rectified RF sheath potentials of 1--5~kV localized near the lower portion of the antenna, where the magnetic field intersects the surface at grazing incidence. Carbon self-sputtering dominates the erosion source, whereas RF-accelerated D$^+$ ions contribute approximately 1\% of the total gross erosion. The smaller-gap operating scenario exhibits substantially stronger gross erosion, enhanced local re-deposition ($\sim$12\%), and a larger confined carbon inventory owing to increased plasma accessibility and broader RF sheath coverage. Comparison with available DIII-D measurements shows no distinct change in the global carbon signal that correlates with the helicon RF pulse, consistent with the simulations indicating that the helicon-generated carbon source remains small compared with the existing background carbon inventory under the present graphite-wall operating conditions. These results demonstrate the capability of STRIPE to integrate RF sheath modeling, plasma transport, surface interaction physics, and three-dimensional impurity transport for the interpretation of helicon-induced PMIs. The study further identifies the principal sources of modeling uncertainty, including grazing-angle RF sheath physics, slow-wave resolution, plasma-background extrapolation, and trace-impurity transport assumptions, providing a framework for future validation and model development.

First experimental realization of robust negative triangularity plasma control in a spherical tokamak

Andrey Lvovskiy, Charles Vincent, Himank Anand, Anders S Welander, Sam Blackmore, Martin Kochan, Graham McArdle, Scott Alan Silburn, Andrew Oakleigh Nelson

United Kingdom Atomic Energy Authority, General Atomics, Columbia University

In this work, we present the experimental realization of a plasma with a steady negative triangularity in a spherical tokamak (MAST-U) for the first time. An average triangularity of -0.1 was maintained for 100 ms in a double-null plasma with a plasma current of 600 kA, a neutral beam injected power of 3.2 MW, a normalized β of 2, and an elongation of 2 on the MAST-U spherical tokamak. Despite many constraints, this was made possible due to the development of a dedicated plasma shape control scheme and the validation and assessment of magnetic shape controllers in simulations. This effort lays the foundation for physics exploitation of the negative triangularity plasma in a spherical tokamak.

Multi-field turbulence and transport barrier measurements and validation of predictive codes for high-performance, negative triangularity ELM-free DIII-D plasmas

Guiding Wang, Terry L Rhodes, Julius Damba, Rongjie Hong, William A Peebles, Quinn Pratt, L Zeng, Max E Austin, Kathreen E Thome

General Atomics, University of California - Los Angeles, Princeton Plasma Physics Laboratory

A recent DIII-D campaign demonstrated high confinement, ELM-free plasmas in strongly shaped, diverted negative triangularity (NT) configurations. This paper presents new multi-field turbulence and flow measurements near the edge to understand the physics of edge transport of these plasmas. Correlation electron cyclotron emission and Doppler backscattering measurements provide electron-temperature fluctuation levels, density-fluctuation levels, radial correlation lengths, and poloidal turbulence velocity profiles. The measurements show a modest edge electron temperature/pressure pedestal, a localized poloidal-velocity well near the pedestal-top region, and the peaking of both temperature and density fluctuation levels near the region of minimum poloidal velocity shear, consistent with edge-transport-barrier formation. Linear TGLF calculations indicate a transition from predominantly ITG-like modes in the inner region to TEM-like modes near the pedestal top, approximately coincident with the observed increase in fluctuation levels. These results provide new quantitative constraints for testing predictive models of turbulence and transport in high-performance NT tokamak plasmas.

Experimental Demonstration of Reactor-Relevant Adaptive Density Control on the DIII-D Tokamak with Coordinated Gas and Pellet Fueling

Hassan Al Khawaldeh, Andres Pajares, Sai Tej Paruchuri, Vincent Graber, Tariq Rafiq, Eugenio Schuster, June-Woo Juhn, T Pederson, Daisuke Shiraki, Francesca Turco

Lehigh University, General Atomics, Korea Institute of Fusion Energy, Oak Ridge National Laboratory, Columbia University

Precise regulation of plasma density is crucial for achieving and maintaining fusion-relevant conditions in reactor-grade tokamaks. Tokamak reactors will utilize both gas puffing and pellet injection as standard fueling mechanisms. However, the coordinated use of gas puffing and pellet injection within feedback control frameworks is largely unexplored, underscoring the necessity to develop and validate dedicated strategies on existing machines. These strategies must address the various challenges associated with both actuators, including actuation delays, unknown fueling efficiencies, and the coexistence of continuous-time and discrete-time dynamics. To overcome these challenges, which surpass the capabilities of traditional empirically tuned Proportional-Integral-Derivative (PID) control, an indirect adaptive control algorithm is proposed in this work for the regulation of the line-averaged electron density through coordinated gas puffing and pellet injection. After initial validation in simulations with a multiple-reservoir global particle model, the controller was successfully implemented and tested on the DIII-D tokamak. Experimental results demonstrate robust density tracking under reactor-relevant scenarios, showcasing the controller's ability to seamlessly coordinate actuators while handling disturbances and evolving actuator constraints. This work provides a critical step forward in the control of future fusion reactors.

Access and Limits of RMP ELM Suppression with n =1 Fields in DIII-D

Priyansh Lunia, Nils Leuthold, Nikolas Logan, Carlos Paz-Soldan, Daniel Alexander Burgess, Evan Maxwell Bursch, Richard J Buttery, Qiming Hu, SangKyeun Kim, Jong-Kyu Park

Columbia University, Princeton Plasma Physics Laboratory, General Atomics, Seoul National University

This work reports on DIII-D experiments aimed at extending resonant magnetic perturbation (RMP) suppression of edge localized modes (ELMs) to n =1 fields, where n is the toroidal mode number. Modeling of the 3D ideal MHD plasma response to the RMPs using the GPEC code is used to quantify edge and core resonant fluxes, guiding experimental strategies to increase plasma resilience against core error field penetration, optimize multi-coil phasing, and explore higher q 95 operation. In DIII-D, ELM mitigation is regularly observed across a wide range of n =1 RMP scenarios. A ~100 ms phase of complete ELM suppression was achieved at q 95 ~3.9 using an odd-parity coil configuration. The suppressed phase exhibited clear signatures of RMP ELM suppression, including the elimination of D α spikes, increased pedestal rotation, enhanced magnetic response, and elevated broadband density turbulence. An optimized coil configuration for edge-to-core resonant flux did show increased edge resonance indicated by increased density pumpout, but did not yield RMP ELM suppression. At q 95 ~5.1, a bifurcation to a grassy-like ELM regime occurred, while large type-I ELMs persisted. These results demonstrate progress in experimental access to n =1 RMP ELM suppression in DIII-D, motivating further study for robust access. This work also highlights the potential role of 3D edge stability as well as rational surface alignment in RMP ELM suppression access, which has important implications for the use of low- n RMPs in future reactor-scale devices.

Toroidal phase of post-disruption runaway electron loss to wall in presence of applied 3D fields in DIII-D

Xue Bai, Eric M. Hollmann, Yueqiang Liu, Claudio Marini

University of California, General Atomics

The toroidal distribution of runaway electrons (REs) striking the centre post in DIII-D post-disruption RE final loss events is simulated utilizing the MARS-F code combined with the REORBIT module, by tracing the guiding-centre drift orbits of test REs in the presence of external (applied) plus internal (intrinsic instability) 3D fields. To better recover the experimental results, three different equilibria with different safety factor profiles are adopted. Dominant resistive magnetohydrodynamic instabilities are found to be m / n = 2/1, 3/2 and 1/1 modes (where m and n are the poloidal and toroidal mode numbers). Externally applied n = 1 resonant magnetic perturbations (RMPs), with both even and odd parity configuration, are considered in each equilibrium. Locking of MHD instability to the applied RMPs is simulated by minimizing the total perturbed magnetic energy in the plasma. The simulated toroidal locations, where most REs strike in the presence of an edge m / n = 2/1 instability and the n = 1 RMPs in even parity, are found to agree well with experiments. Modelling also captures the measured n = 2 RE loss pattern on the wall for experiments where an edge m / n = 3/2 instability is dominant instead of the m / n = 2/1 mode. The key overall findings include ( a ) the measured global RE toroidal impact pattern in experiments can be explained by a locked resistive kink instability, ( b ) the toroidal phase of the locked mode is well predicted by minimizing the total perturbed magnetic energy, and ( c ) this edge locked mode determines the toroidal phase of the peak RE wall impact.

Active control of ion-ITB using RMP in the EAST tokamak

H. Sheng, Y.W. Sun, T.H. Shi, S. Gu, H.H. Wang, X. Jian, C. Ye, Y.Y. Li, M.N. Jia, Y.Q. Liu, et al.

Institute of Plasma Physics, Chinese Academy of Sciences, University of Science and Technology of China, Enn Science and Technology Development Co., Ltd, General Atomics

Resonant magnetic perturbations (RMPs) have been demonstrated to trigger sustainable internal transport barriers in the ion temperature channel (ion-ITB) in the EAST tokamak (Sheng et al 2025 PRX Energy 4 043012). This paper further examines the diverse effects of RMPs on ion-ITB. In addition to triggering and sustaining ion-ITB using RMPs, the barrier can also be actively controlled by adjusting the phasing, toroidal mode number ( n ), and current of the RMP coils. The formation of ion-ITBs exhibits robust reproducibility, achievable solely by adjusting the RMP coil configuration with all other discharge settings held fixed. Comparative results indicate that high- n RMPs are more favorable for ion-ITB formation than low- n RMPs. Both experimental and modeling results demonstrate that strong magnetic perturbations in the core region suppress the formation of the ion-ITB in these cases. Preliminary results from RMP-modulated ITB experiments reveal that fishbone is not essential for ion-ITB formation, nor is sawtooth essential for its collapse, whereas the plasma current profile appears likely to play an important role. Overall, these results demonstrate that RMPs can serve as an effective method for controlling ITBs and investigating their underlying physics.

Investigating long-duration plasma operation with the international multi-machine CICLOP database

Xavier L Litaudon, Ernesto A Lerche, Olaf Grulke, Christopher Thomas Holcomb, Juan Huang, Marcin Jakubowski, Hyun-Seok Kim, Pierre Manas, Tomohiro Morisaki, Francesca Turco, et al.

IRFM, Max-Planck-Institut für Plasmaphysik, Chinese Academy of Sciences, General Atomics, Culham Science Centre

Combined high-fusion performance and long-pulse operation is one of the key integration challenges for fusion energy development in magnetic devices. Addressing these challenges requires an integrated vision of physics and engineering aspects with the purpose of simultaneously increasing time duration and fusion performance. Since the previous 2023 IAEA Fusion Energy Conference, significant progresses have been made in tokamaks and stellarators including very recent achievement in duration and/or performance. These progresses are reviewed by analyzing the experimental data provided by 10 tokamaks and two stellarators. The published database [Litaudon X. et al 2024 Nucl. Fusion 64 015001], which initially included data up to January 2022, has been significantly updated for the 2025 IAEA Fusion Energy Conference to incorporate the latest 2023-2025 experiments (up to May 2025) including recent records performance with new entries provided by DIII-D, EAST, JET, KSTAR, WEST, and W7-X. The update dataset has been gathered and coordination have been provided by the IEA-IAEA international CICLOP group (Coordination on International Challenges on Long duration OPeration). An overview of the recent progress toward long pulse operation analysing the CICLOP database is provided in this publication.

Drift-kinetic effects of tungsten on plasma response to RMP in ITER

Yueqiang Liu

General Atomics

Effects of high-Z (Z is the particle charge number) tungsten impurity ions on the plasma response to the resonant magnetic perturbation (RMP) field are numerically investigated for the ITER 15 MA baseline scenario, where the tungsten contribution to the plasma response is computed with a drift-kinetic model while the bulk thermal particle contributions follow the fluid approximation. The study yields three highlights: (i) the drift-kinetic contribution of the tungsten impurity exerts minor influence on the plasma response compared to that computed by the pure fluid model without tungsten; (ii) a new figure of merit, based on the resonant spectrum perturbation at the plasma boundary surface, results in different optimal coil phasing compared to that previously obtained by maximizing the edge-peeling plasma response; (iii) the optimal n = 3 RMP (for ELM control, n is the toroidal mode number) is found to induce a large tungsten particle influx near the plasma edge associated with the neoclassical toroidal viscosity. The study thus provides useful data on the compatibility of the full tungsten wall with RMP ELM control in ITER.

The Data Fusion Labeler (dFL): Challenges and solutions to data harmonization, labeling, and provenance in fusion energy

Craig Michoski, Matthew Waller, Brian Sammuli, Zeyu Li, Tapan Ganatma Nakkina, Raffi Nazikian, Sterling Smith, David Orozco, Dongyang Kuang, Martin Foltin, et al.

Sophelio, General Atomics, University of Texas, Hewlett Packard Enterprise

Fully generalized, turbulent trace impurity transport with Gkeyll and Flan in the DIII-D far-SOL

S A Zamperini, T N Bernard, S Andersen, A C D Hoffmann, D L Rudakov, T Abrams

General Atomics, San Diego State University, Princeton Plasma Physics Laboratory, University of California

The Monte Carlo trace impurity turbulent transport code Flan is introduced for the first time. Flan follows impurities in a turbulent background plasma from Gkeyll using the Lorentz force to resolve the full particle gyro orbit. Collisions are handled using the Nanbu collision algorithm (Nanbu 1997 Phys. Rev. E 55 4642–52), and ionization/recombination is handled via ADAS coupling. The far-SOL of a generic DIII-D L-mode is simulated with and without the collision model to show how collisions affect radial tungsten transport. Anomalous diffusion coefficient ( D r ) and pinch velocity ( v p ) profiles are extracted from fits to the results. With collisions, D r and v p are between 0–1.0 m 2 s −1 and −100–100 m s −1 , respectively. Without collisions, D r and v p are between 0–0.3 m 2 s −1 and −50–50 m s −1 , respectively. Exponential fits to the radial W density profiles and experimental data from W deposition along a collector probe are in good agreement, demonstrating Flan as a useful interpretive modeling tool. Additional simulations show that impurity transport away from the wall increases with atomic number, though it is not clear why. Flan has the potential to better interpret existing data and improve reactor scale predictions of core contamination because the underlying physics model is very general and does not rely on arbitrary user-defined transport coefficients.

TORQSNet: a physics-inspired spectral neural network for predicting toroidal torque density in tokamak devices

Hanyu Zhang, Yueqiang Liu, Xu Yang, Zheng-Xiong Wang

Dalian University of Technology, General Atomics, Chongqing Technology and Business University

Fast prediction of torque density profiles driven by three-dimensional (3D) magnetic fields is required for accurate real-time control of plasma toroidal rotation. A toroidal torque spectral neural network (TORQSNet) is developed to enable rapid and accurate prediction of radial profiles of neoclassical toroidal viscosity (NTV) torque, electromagnetic torque, and Reynolds stress torque under the resonant magnetic perturbations (RMPs). TORQSNet is trained and validated using a numerical database of 3D equilibria computed using the MARS-F code for ASDEX Upgrade (AUG), DIII-D, MAST, and ITER. In oscillatory radial regions, torque density profiles are parameterized by Legendre polynomial expansion coefficients, which are predicted by the network and are used as interpretable features linked to resonant response characteristics in magnetohydrodynamic equilibria. Across all four devices and the investigated toroidal mode numbers, coefficients of determination of R 2 > 0 .946 are obtained for reconstructed NTV torque density profiles. Strong cross-device generalization is demonstrated, with models trained on a single device accurately predicting radial torque profiles in the other three devices. The combined influence of plasma rotation and resistivity is captured by the trained TORQSNet. Core torque is reduced while edge torque is increased, which supports optimization of RMP-based edge localized mode suppression by promoting pedestal penetration and mitigating core flow damping.

A coherent structure transport model for scrape-off layer turbulence

Zhichen Feng, James Myra, Junyi Cheng, Calder Haubrich, Yang Chen, Xinxing Ma, Darin R. Ernst, Scott Parker

University of Colorado Boulder, Lodestar Research Corporation, General Atomics, Massachusetts Institute of Technology

Understanding the locality of high-temperature plasma energy deposition on material surfaces in fusion reactors is critical for design. Here, we utilize the gyrokinetic electromagnetic turbulence, including X-points simulation, using ions as tracer particles, together with SOLPS-ITER solutions for the background equilibrium electric field, including drifts, to model the heat flux at the divertor plate and characterize the heat load width using realistic X-point geometry. We use a theory-based blobby transport model called the “Coherent Structure Transport” (CST) model to include the effect of plasma transport in the edge scrape-off layer. The CST model is extremely fast and can be used to quickly analyze any SOLPS-ITER solution. SOLPS-ITER provides the steady state, or equilibrium, on which we superimpose blobby turbulence characterized by blob size, amplitude,e and frequency. We obtain the 1/Bp scaling of the heat load exponential decay width λq, in agreement with the Eich empirical scaling and with the Goldston heuristic theory. When including blobby turbulence in combination with the SOLPS-ITER electric field, we find a secondary peak in the heat flux radial profile, outwardly displaced from the strike point radius, with a relative amplitude that increases with the initial blob density. We describe the CST model in detail and provide initial investigations of the scaling of λq and the secondary heat flux peak with blob size and amplitude.