Recent Publications

Integrating Gyrokinetic Flux Predictions with Ideal MHD Stability Boundaries

J. McClenaghan, K. E. Thome, T. F. Neiser, J. Candy, F. D. Halpern, T. H. Osborne, S. Saarelma, the MAST-U team

Accurate prediction of pedestal height and width in tokamaks remains a critical issue as it strongly influences the predicted plasma performance of all future reactors. We present an integrated pedestal-stability workflow that combines equilibrium scans with magnetohydrodynamic (MHD) stability analysis using ELITE and GATO and gyrokinetic transport predictions using CGYRO/QLGYRO. The workflow reproduces the characteristic KBM first- and second-stability structure previously identified in gyrokinetic pedestal studies. Applied to spherical tokamaks (STs), the workflow shows good agreement with past studies when low-n peeling stability is included, emphasizing the importance of resolving low-n physics in ST pedestals. Extending the analysis to SPARC-like, high toroidal field plasmas, kinetic ballooning mode (KBM) and microtearing mode (MTM) heat fluxes are found to increase strongly with toroidal field, suggesting that access to the KBM second-stability region may become significantly more difficult in high toroidal field devices. Comparison with global ELITE finite-n analysis at SPARC parameters suggests that the H-mode pedestal lies in an intermediate regime bounded by local KBM second stability on one side and global finite-n ballooning instability on the other, consistent with the EPED picture once global effects are included

Effects of fusion-born alphas on MHD instabilities in burning plasmas with spin-polarized fuels

Yueqiang Liu

A new magnetohydrodynamic-kinetic hybrid model is developed with inclusion of an anisotropic (in the particle pitch) particle distribution for fusion-born alphas, for the purpose of modeling drift-kinetic effects of the latter on macroscopic instabilities in burning plasmas with spin-polarized (SP) deuterium-tritium (D-T) fuels. The new model is applied to numerically analyze both the internal kink (IK) and the resistive wall mode (RWM) stability, based on two ITER D-T scenarios. The adiabatic contribution of alphas generally destabilize the IK and the RWM, with the anisotropic model generally yielding a stronger destabilization compared to the isotropic counterpart. Partial stabilization of the IK mode is generally obtained with inclusion of the non-adiabatic resonance contributions (of various sorts) of alphas in the zero-orbit width approximation. Adding the finite-orbit width correction brings full stabilization of the mode. Destabilization of an IK-peeling branch is however computed, accompanied by a substantial modification to the mode eigenfunction by anisotropic alphas. Non-adiabatic contributions of alphas are also generally stabilizing for the RWM, independent of the assumed (isotropic versus anisotropic) particle distribution model. On the other hand, triggering of a high-frequency fishbone-like mode is found in the ITER 10 MA scenario with finite plasma flow, by the adiabatic contribution of alphas due to SP fuels.

Effects of tungsten radiative cooling on impurity, heat and momentum transport in DIII-D plasmas

Arsene Stephane Tema Biwole, Tomas Odstrcil, Xavier L Litaudon, Shengyu Shi, Darin R Ernst, Carl Friedrich Benedikt Zimmermann, Jeff B Lestz, Nathan T Howard, Pablo Rodriguez-Fernandez, Filipp Khabanov, et al.

A first-of-its-kind experiment was conducted in the DIII-D tokamak under WEST similarity constraints on plasma shape and core parameters. This work presents a detailed transport study comparing a reference regime dominated by intrinsic carbon radiation and a high-radiation regime resulting from controlled tungsten (W) injection using the Laser Blow-Off system, with a core tungsten concentration n W /n e ~ 3×10 −4 and a radiated-power fraction f rad > 0.5. The W-induced radiative cooling lowered the electron temperature, thereby decreasing T e /T i and stabilizing trapped-electron-mode (TEM) turbulence. This transition in turbulence regime reduced momentum and ion thermal diffusivities, yielding ion temperature peaking and a factor-of-two increase in toroidal rotation. At the outer plasma region, enhanced E×B shear and increased collisionality further suppressed ion-scale turbulence, causing a sharp drop in ion heat flux. Consequently, impurity transport, predominantly turbulent in the low-radiation regime, acquired a strong neoclassical inward W convection during radiative cooling, bootstrapping the cooling cycle. Despite f rad > 0.5, radiative collapse was not observed, likely owing to collisional ion-to-electron energy exchange acting as an electron-energy reservoir, together with 1/1 MHD activity modulating the radiated power through core impurity neoclassical T i -screening. These results support preparation for a tungsten wall change in DIII-D by elucidating tungsten-induced turbulence stabilization. They also provide key insights for interpreting plasma performance in WEST and are relevant to future reactors expected to operate with radiating tungsten-walled plasmas.

Neutron Irradiation Experiments to Support Fusion Blanket Development

Chase N. Taylor, Pattrick Calderoni, Xiao-Ying Yu, David X. Fischer, Thomas F. Fuerst, David Weisberg

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

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.

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.

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.

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

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.

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.

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.

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

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.

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

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.

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.

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.

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.

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.

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

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.

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.

First Experimental Evidence of Helicon Current Drive

J. B. Lestz, R. I. Pinsker, B. Van Compernolle, S. X. Tang, A. Dupuy, A. M. Garofalo, L. McAllister, C. P. Moeller, C. C. Petty, M. Porkolab, et al.

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

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

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.

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.

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

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.

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.

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.