Recent Publications

High-beta runaway transitions in a fluid model of electromagnetic ion-temperature-gradient turbulence

Y. Zhang, M. Barnes, A. A. Schekochihin, P. G. Ivanov, T. Adkins

Gyrokinetic simulations of tokamak turbulence indicate that fluctuation levels increase abruptly and dramatically when the plasma beta exceeds a certain critical value. This increase in fluctuation levels coincides with a transition from a state dominated by zonal flow to one in which turbulent eddies form radially-elongated `streamers'. Here we derive from gyrokinetics a minimal fluid model for electromagnetic ion-temperature-gradient (ITG) turbulence that captures the key features of this transition. Due to the relative simplicity of the model, we are able to conduct a detailed numerical study of the interplay between the turbulence and the zonal flow across a broad range of values of the plasma beta and the ITG. We find that the transition occurs when the Reynolds stress, which tends to strengthen zonal flows, is overwhelmed by the Maxwell and diamagnetic stresses, which tend to weaken them. Power-law scalings of the stress ratios with plasma beta and ITG are obtained, indicating a possible means by which the location of the transition could be predicted with minimal computational cost.

Resonant and Ponderomotive Pumping of Zonal Flows by Turbulence, Alfvén Eigenmodes, and Radiofrequency Waves

C. M. Gallaro, I. Y. Dodin

Analytical studies of zonal-flow (ZF) excitation by fluctuating fields, such as drift-wave turbulence and Alfvén eigenmodes (AEs) driven by energetic particles (EPs) in tokamaks, are usually carried out via intricate gyrokinetic calculations and simulations whose results are notoriously difficult to interpret. Here, we report a transparent result that is also not limited to any particular modes or frequency range. Using oscillation-center theory that captures both ponderomotive forces and quasilinear diffusion, we derive a compact formula for the ZF drive produced by any fluctuating field in terms of the field's canonical momentum and the dissipation power density. For ZFs generated by drift waves, our model subsumes the local relation between the zonal velocity and the drift-wave energy density that was previously derived ad hoc. The simplicity and generality of our result also opens a path toward optimization of ZF excitation with external waves and bridges the physics of AE-EP interactions with that of poloidal flows driven by radiofrequency waves in fusion plasmas.

The geometry of flux surfaces with quasi-poloidal symmetry

Rishin Madan, Wrick Sengupta, Elizabeth J. Paul, Mohammed Haque, Richard Nies, José Luis Velasco, Amitava Bhattacharjee

Quasi-poloidal (QP) magnetic fields have desirable properties for confining plasma: no radial drift of guiding centres (with positive implications for neoclassical transport), zero Pfirsch–Schlüter current and a lower level of damping for poloidal flows. Despite their attractive properties, QP fields are not amenable to the near-axis expansion, a major theoretical tool for understanding toroidal fields. In this paper we provide a novel framework for defining and understanding QP flux surfaces. This framework relies on a simplification that transforms the task of finding a QP flux surface from a three-dimensional problem to a two-dimensional (2D) problem. This simplification also applies to asymmetric magnetic mirrors with desirable properties. We sketch how this 2D problem can form the basis of an efficient optimisation problem for finding QP flux surfaces. We leverage this 2D problem for theoretical understanding: for instance, we identify a route to finding QP flux surfaces that are naturally flat mirrors (Velasco et al. 2023, Nucl. Fusion , 63, 126038). The reduced model is qualitatively checked against numerically optimised QP equilibria. These numerical solutions only satisfy QP approximately, but we predictably find that local discrepancies with the reduced model correspond to significant local QP errors, anomalous parallel currents and field lines deviating from geodesics.

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.

Revised error field assessment in KSTAR with profile diagnostics of resonant field penetration

Gwang-Geun Seo, Seongjun Han, Jayhyun Kim, Yeongsun Lee, SeongMoo Yang, June-Woo Juhn, Jong-Kyu Park

Error field (EF) correction against n=1 intrinsic errors has been revised in KSTAR in three respects. First, a local penetration timing criterion is defined from electron cyclotron emission (ECE) and charge exchange spectroscopy (CES) signatures near the reconstructed q = 2 surface. This timing determines the in-vessel control coil current used as the penetration threshold in the compass scan analysis. Second, the compass scans using the MID and OFFMID coils in 1.8T L-mode plasmas indicate an intrinsic EF level larger than the 2015 KSTAR reference under the conditions examined here. This implies that intrinsic error fields can vary after extended periods of device operations and hardware modifications. Third, the inferred EF direction is assessed by comparing the MID and OFFMID results in a GPEC overlap field basis, where the two configurations give nearly the same direction despite their different poloidal spectra. A rotating resonant magnetic perturbation (RMP) experiment provides an additional dynamic check of this direction, with penetration observed near the phase predicted from the static MID scan. The reversed-B t comparison constrains the possible source symmetry and is more compatible with a PF related contribution than with a TF dominated contribution, although the specific hardware source is not identified. These results provide an updated EF reference for KSTAR 3D-field operation and motivate periodic reassessment of intrinsic EF after extended device operation.

Geodesic extended modes in low magnetic shear tokamaks and stellarators

Richard Nies, Felix Ignacio Parra

Theories of ion-scale microinstabilities in tokamaks and stellarators typically assume that the passing electrons respond adiabatically due to their fast propagation speed. However, when the magnetic shear becomes sufficiently small, ion-scale modes can extend far along the magnetic field and the non-adiabatic response of passing electrons becomes important. We derive a theory of extended modes at low magnetic shear through a multiscale expansion of the gyrokinetic equation. The theory elucidates the physics of the geodesic extended mode, a new type of microinstability. The new mode couples the non-adiabatic physics of both electrons and ions, unlike extended modes at magnetic shear of order unity. The theory is validated against gyrokinetic simulations and the parameter dependences of the new mode are studied.

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.

Frequency-inference method for reduced modeling of energetic particle modes (EPM) utilizing resonant auto-optimization remnants of imperfect time-scale separation

Andreas Bierwage, Vinícius N. Duarte, Pablo Oyola, Kouji Shinohara, Roscoe B. White

Integrated codes simulating interactions between Alfvén waves and fast ions in tokamak plasmas use perturbative models for the relatively slow processes of instability growth, saturation, chirping and bursting, and transport. Faster processes by which an Alfvén mode's spatiotemporal structure forms are assumed to have been completed within the mode's oscillation period, $τ_0 \equiv 2π/ω_0$. This separation of time scales underlies the computational efficiency of perturbative models, where the Alfvén mode's time-dependence is reduced to that of a scalar signal $s(t) = A(t)\sin(-ω_0 t - φ(t))$ with variable amplitude $A(t)$ and phase $φ(t)$. For this, accurate input data in the form of a mode's spatial structure $δΦ({\mathbf x})$, damping rate $γ_{\rm d}$, and initial frequency $ω_0$ are required. For modes residing in dense or continuous spectra, $δΦ$ and $γ_{\rm d}$ could be estimated from the form of the continua and fast ion orbits, but it is difficult to guess the seed frequency $ω_0$. Here, we report results of numerical experiments showing that it is possible to find $ω_0$ using a prompt frequency shift that occurs during the first few $100$ time steps of a simulation. Restarts with the shifted frequency iteratively converge to a value of $ω_0$ that seems to maximize the resonant drive, suggesting an auto-optimization process. The need for iteration is attributed to the fact that the terms required for rapid frequency adjustments were truncated when deriving the perturbative model. Meanwhile, the fact that partial auto-optimization is possible at all is attributed to the fact that series truncation alone (without filter) does not strictly enforce slowness. Remnants of and cross-talk with faster dynamics still occur in numerical implementations. This entails potential for both uncertainty and utility.

AI-Accelerated Gyrokinetic Predictions of Turbulent Transport for Stellarator Design Optimization and Experimental Planning

R. Michael Churchill, Matt Landreman, Jong Youl Choi, Byoungchan Jang, Rory Conlin, Noah Mandell, Anima Anandkumar, Valentin Duruisseaux, Jeffrey Larson, Dario Panici, et al.

Previous work built AI-based surrogates for a nonlinear gyrokinetic simulation code with the goal of using them for fast, direct calculations of turbulent ion heat flux in stellarator design optimizations and scenario planning for experiments. These AI surrogates were trained on data from >200k nonlinear, adiabatic electron gyrokinetic simulations with the gyrokinetic flux-tube code GX, using a wide range of stellarator magnetic configurations ($\sim$23k), positions in the plasma, and gradient scale lengths. In this paper, we demonstrate the use of the AI-based turbulence surrogate in the optimization of stellarator magnetic equilibrium and to speed up stellarator transport solvers. Due to its speed ($\sim$ms), the AI-based surrogate enables previously unattainable optimization objectives, such as full radial profiles of ion turbulent heat flux, or directly optimizing to maximize the turbulent critical gradient at multiple locations across the plasma. These direct calculations provide a potentially more accurate optimization target and reduce reliance on ad-hoc heuristics that may not accurately capture the variation of turbulent transport with magnetic configuration. By including the AI-based surrogate for turbulent heat flux in a transport solver, we can quickly postprocess and confirm the improved ion temperature resulting from the optimized equilibrium. Finally, we demonstrate the use of AI agents with strong reasoning AI models to automate the outer loop, exploring many objective and hyperparameter configurations with this AI-based turbulence surrogate to discover improved turbulence optimized magnetic equilibria.

Poloidal asymmetries of main chamber neutral fueling in DIII-D ELMy H-mode plasmas

Laszlo Horvath, Florian Laggner, Raúl Gerrú, Filippo Scotti, Alessandro Bortolon, Eric Emdee, Shaun R Haskey, Ryan Hood, Jerry W Hughes, Tomas Odstrcil, et al.

Understanding poloidal asymmetries in main chamber neutral fueling is essential for characterizing the balance between particle sources and transport in the formation of the H-mode density pedestal. The primary goal of this work is to study changes in these fueling asymmetries and their relationship to divertor recycling across different magnetic configurations in DIII-D lower single-null type-I ELMy H-mode plasmas, using direct measurements of the pedestal ionization source obtained with the LLAMA (Lyman-alpha Measurement Apparatus) diagnostic. By combining these direct measurements of the neutral population and studying how it influences the pedestal density structure and overall pedestal performance, this work aims to provide new insights into the role of neutrals in H-mode pedestal physics. We find that the main chamber ionization on the high-field-side (HFS) strongly dominates when the ion ∇B drift is in the favorable direction. In contrast, reversing the drift direction to the unfavorable direction significantly reduces HFS fueling, that is only partially compensated by a moderate increase in low-field-side (LFS) fueling. The resulting changes in the edge particle source are accompanied by a decrease in pedestal electron density and an increase in pedestal temperature, such that the pedestal pressure remains approximately unchanged. Measurements of divertor recycling and divertor radiation indicate that these fueling asymmetries are also linked to the divertor regime. In particular, inner divertor detachment in the favorable drift configuration decouples main chamber fueling on the HFS from target recycling, while attached divertor conditions in unfavorable drift cases lead to a strong correlation between divertor recycling and main chamber fueling asymmetries. These results suggest that the ion ∇B drift direction, by modifying main chamber fueling, can influence the pedestal density structure and overall pedestal performance.

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.

Generalized Lawson-Cordey-Mills Accessibility of Fusion Ignition

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

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.

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.

Global gyrokinetic simulations of magnetic geometry and radial electric field effects on turbulent transport in Wendelstein 7-X

A. Bañón Navarro, L. Podavini, J.-P. Bähner, J. Smoniewski, S.K. Hansen, A. Di Siena, M. Porkolab, E.M. Edlund, A. von Stechow, S.A. Bozhenkov, et al.

We present a more detailed numerical analysis of the global GENE-3D simulations of Bähner et al (the W7-X Team) 2026 Nucl. Fusion 66 016007, focusing on turbulent heat and particle transport and the role of the equilibrium radial electric field across mirror ratio and rotational transform ( ι ) configuration scans in Wendelstein 7-X (W7-X). Ion and electron heat fluxes show only mild sensitivity to both configuration parameters. The particle flux, however, exhibits a pronounced dependence on the mirror ratio, with the low-mirror configuration driving a strong inward particle pinch and the high-mirror configuration producing a net outward flux. Density fluctuation levels at fixed plasma profiles do not reproduce the experimentally observed configuration dependence in the mirror scan, a discrepancy that can be understood in terms of the simulations not satisfying power balance. A synthetic phase contrast imaging (PCI) diagnostic applied to the simulations reproduces the experimental ordering of the wavenumber and frequency spectra in the ι scan but not in the mirror scan, consistent with the same interpretation. The equilibrium radial electric field modifies turbulent transport, with the largest effect on the electron heat flux in regions of strong E × B shear. A controlled parametric study, in which the E r profile is systematically varied in amplitude and radial location, confirms that transport suppression follows the classical shear decorrelation criterion, being strongest where the shear rate exceeds the linear growth rate over the widest radial extent.

A plasma-material interaction design basis for NSTX-U flash lithium evaporators

A. Maan, R. Majeski, C. López Pérez, D.P. Boyle, T. Le, R. Lunsford

Evolution of laser-driven magnetic fields from proton tomography

J. Griff-McMahon, V. Valenzuela-Villaseca, C. A. Walsh, S. Malko, B. McCluskey, K. Lezhnin, H. Landsberger, L. Berzak Hopkins, G. Fiksel, M. J. Rosenberg, et al.

Self-generated magnetic fields are commonly produced in high-power laser–plasma interactions. These fields can inhibit plasma heat-flow, which makes them important in inertial fusion and controlled laboratory astrophysics experiments. In this work, we characterize the time evolution of self-generated magnetic fields using multi-view proton tomography at two timings. Tomographic reconstructions of the magnetic field show a clear transition from fields located close to the target at early time to more extended coronal fields at later time. The tomographic inversion and mesh radiography also enable a direct measurement of the magnetic flux evolution. Comparisons with extended-magnetohydrodynamic simulations show only moderate agreement in field structure but good agreement in magnetic flux. This suggests that the field generation model is largely correct under these conditions, while the magnetic transport model requires additional development to reproduce the observed field structure.

Revisiting the MRX Electron Current Sheet Width with Semi-Collisional Kinetic Simulations at Hydrogen Mass Ratio

Sung Hyun Son, Adam Stanier, William Daughton, Jongsoo Yoo, Tongnyeol Rhee, Hantao Ji

For over eighteen years, the electron current sheet measured in the Magnetic Reconnection Experiment (MRX) has stood a factor of 2--5 wider than predicted by MRX-like kinetic simulations, and the measured electron force balance has not closed with the classical terms alone. As a consequence, the dominant nonideal terms responsible for breaking the frozen-in condition have remained unexplained. Here, two-dimensional kinetic simulations with binary Coulomb collisions are performed in a cylindrical geometry representative of MRX, at the realistic hydrogen mass ratio $m_i/m_e = 1836$ and at MRX-relevant collisionality. For the first time, these simulations reproduce the measured electron current sheet half-width. The simulated value, $δ_{BT} = 0.744 \pm 0.054$~cm or $6.26 \pm 0.45$ electron skin depths ($d_e$), lies within the experimental range of 5.5--7.5~$d_e$. The electron force balance closes through the classical channels alone: the pressure-tensor divergence supports 76\% of the nonideal electric field and collisional friction the remainder. The historical force-balance deficit reappears only when the simulated layer is sampled at the experimental 3~cm outflow resolution, suggesting that the deficit reflects probe resolution rather than anomalous dissipation. Beyond this reproduction, an analytic model of the layer width is developed that orders the meandering electrons by the coherence of their orbits against collisions. In this model, the Dreicer ratio $E_D/|E_y|$ selects the electrons whose current-carrying motion survives, and the resulting width prediction brackets the measured values across a wide collisionality scan. A discrepancy remains in the width normalized to the local electron gyroradius ($ρ_e$), whose measured value lies a factor of 3--6 above both the model and the simulations.

Characterization of ELM Pacing via Vertical Jogs on DIII-D

Kei Yasoda, Dario Giovanni Panici, Andrew Oakleigh Nelson, Florian M Laggner, SangKyeun Kim, Egemen Kolemen

Edge localized mode (ELM) pacing via vertical plasma oscillations—or "jogging"—has been successfully demonstrated on the DIII-D tokamak. Rapid vertical movement of the plasma toward the X-point effectively triggers ELMs. By vertically oscillating the plasma at 10 Hz, the ELM frequency increased from its natural rate of ∼5 Hz in similar DIII-D discharges to 10 Hz. Notably, downward jogs have been observed to trigger multiple ELMs in a single cycle. ELMs triggered at these higher-than-natural frequencies lead to smaller decreases in stored energy, dropping from 10% to below 1%. Consequently, the peak heat flux to the divertor is reduced by a factor of ∼2, alongside an observed reduction in carbon impurity concentration. During downward jogs in the lower single null (LSN) configuration, the X-point movement is slower and smaller than the top of the plasma, resulting in a reduced plasma cross-section and volume. To understand the mechanism of ELM triggering by jogging, a model of the edge toroidal current was developed and tested against DIII-D experimental data. Both the data and the model suggest that moving the plasma toward the X-point locally induces a net positive toroidal current in the edge region. ELITE stability analysis indicates that this induced current pushes the plasma state across the peeling side of the peeling-ballooning stability boundary into the unstable region, triggering the ELMs.