Microtearing Turbulence and Its Role in High-Density-Gradient Plasmas in Wendelstein 7-X

TL;DR

This study uses gyrokinetic simulations to identify microtearing mode turbulence as the dominant transport mechanism in high-density-gradient plasmas in Wendelstein 7-X, aligning well with experimental data and influenced by specific plasma conditions.

Contribution

It demonstrates that microtearing modes dominate turbulence in certain W7-X plasmas, highlighting the role of magnetic configuration and collisionality in their emergence.

Findings

Microtearing modes dominate transport in high-density-gradient plasmas.

Simulated heat and particle fluxes agree with experimental measurements.

MTM onset is facilitated by moderate collisionality and low magnetic shear.

Abstract

Gyrokinetic simulations reveal that microtearing mode (MTM) turbulence dominates transport in a Wendelstein 7-X (W7-X) discharge characterized by large density gradients, moderate temperature gradients, and low plasma beta. This conclusion is supported by the close agreement between simulated and experimentally measured heat and particle fluxes. The emergence of MTMs is attributed to the absence of competing instabilities -- such as ion temperature gradient modes and density-gradient-driven trapped-electron modes -- under these plasma conditions, together with the stabilizing influence of the W7-X max-J magnetic configuration. Furthermore, moderate collisionality and low magnetic shear are found to facilitate MTM onset. These findings advance the understanding of high-density-gradient regimes, which are essential for achieving high-performance operation in W7-X plasmas.