Nonlinear magnetohydrodynamic modeling of current-drive-induced sawtooth-like crashes in the W7-X stellarator

TL;DR

This paper models sawtooth-like crashes in the W7-X stellarator caused by electron cyclotron current drive using nonlinear MHD simulations, revealing a mechanism involving internal kink modes and validating the M3D-C1 code.

Contribution

It introduces a nonlinear MHD simulation approach for stellarators that captures current-drive-induced sawtooth crashes, advancing understanding and modeling capabilities.

Findings

Simulation reproduces key features of experimental crashes.

Inner and outer resonances trigger successive temperature crashes.

Results support the proposed mechanism for sawtooth-like events in W7-X.

Abstract

Sawtooth-like core electron temperature crashes have been observed in W7-X experiments with electron cyclotron current drive. We present nonlinear single-fluid magnetohydrodynamic simulations of this phenomenon using the newly developed stellarator modeling capability of the M3D-$C^1$ code. The near-axis current drive gives rise to two $\iota=1$ resonances in the equilibrium rotational transform profile so that two consecutive $(1,1)$ internal kink modes are seen in the simulations. A small-amplitude crash at the inner resonance occurs first, which may correspond to the sawtooth precursors observed in the experiments. A bigger crash at the outer resonance then flattens the core temperature profile, which shows semi-quantitative agreements with experimental measurements on certain metrics such as the crash amplitude and the inversion radius of the temperature change. These results illustrate a likely mechanism of the current-drive-induced sawtooth-like crashes in W7-X and, to some extent, validate the stellarator modeling capability of M3D-$C^1$.