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Significance of Microtearing Turbulence in Turbulence-Reduced High-Density-Gradient Plasmas in Wendelstein 7-X

H. Cu-Castillo1,*, A. Bañón Navarro1, G. Merlo1, F. Reimold2, T. Romba2, O. Ford2, S. Bannmann2, L. Vanó2, M. Wappl2 et al.

J. Geiger2, A. Goodman2, A. Zocco2, F. Jenko1, and W7-X Team†

  • *Contact author: hugo.cu.castillo@ipp.mpg.de
  • †See Supplemental Material [1] for the W7-X Team.
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Phys. Rev. Lett. 137, 035102 – Published 14 July, 2026

DOI: https://doi.org/10.1103/nd3p-yb9t

Abstract

Gyrokinetic simulations of a turbulence-reduced Wendelstein 7-X discharge—characterized by a steep density gradient, moderate temperature gradients, and low plasma beta—show that microtearing mode (MTM) turbulence dominates transport. The simulated heat and particle fluxes agree with experimental measurements, leaving MTMs as the only mode consistent with the data. These conditions, the quasi-isodynamic and nearly max-J magnetic configuration of the Wendelstein 7-X stabilize ion temperature gradient modes and density-gradient-driven trapped-electron modes, while moderate collisionality and low magnetic shear further enable MTM growth. Further nonlinear scans of the density gradient reveal a significant reduction in turbulent transport at the experimentally observed threshold, which we identify as an ion temperature gradient to MTM-dominated turbulence transition. These findings provide a robust explanation for the turbulence suppression and deepen our understanding of low turbulent transport regimes in optimized stellarators.

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Phys. Rev. Lett. 137, 035102– Published 14 July, 2026
  • Received 20 October 2025
  • Revised 20 February 2026
  • Accepted 21 April 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

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