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arXiv · 2605.23002

First-Principles Explanation of the Drift Configuration Dependence of the Radial Electric Field and High-Confinement Access in Tokamaks

B. J. Frei·R. Bilato·O. Grover·W. Zholobenko·C. Angioni·M. Bergmann·P. Ulbl·F. Jenko·the ASDEX Upgrade Team
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Abstract

The origin of the difference in the high-confinement (H-mode) power threshold between favorable and unfavorable drift configurations in tokamaks, experimentally linked to a deeper radial electric field (Er) well in the former, remains unresolved. Using first-principles gyrokinetic simulations of edge and scrape-off-layer turbulence in ASDEX Upgrade, we show that turbulence-driven poloidal flows generate this deeper Er well in the favorable configuration through enhanced nonlinear turbulence-mean flow energy transfer. This transfer is significantly weaker in the unfavorable case, yielding a shallower Er well, while turbulence intensity is simultaneously higher. Within the turbulence-flow shear suppression paradigm, the combination of stronger shear and reduced turbulence facilitates H-mode access in the favorable configuration. These results provide the first validated, self-consistent full-f gyrokinetic explanation of how drift configuration controls the nonlinear dynamics of profiles, Er, flows, and turbulence, thereby setting the H-mode power threshold.

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B. J. Frei, R. Bilato, O. Grover, W. Zholobenko, C. Angioni, M. Bergmann, P. Ulbl, F. Jenko, the ASDEX Upgrade Team. 2026-05-21. First-Principles Explanation of the Drift Configuration Dependence of the Radial Electric Field and High-Confinement Access in Tokamaks. https://arxiv.org/abs/2605.23002

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