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

Sawtooth-like core electron temperature crashes have been observed in W7-X experiments with electron cyclotron current drive applied. We present a nonlinear magnetohydrodynamic simulation of this phenomenon using the newly developed stellarator modeling capability of the M3D-C¹ code. The near-axis current drive gives rise to two ι=1 resonances in the rotational transform so that two consecutive (1,1) internal kink modes are seen in the simulation. A small-amplitude crash at the inner resonance takes place first, which may correspond to the sawtooth precursors observed in the experiments. A bigger crash at the outer resonance then follows, which shows semi-quantitative agreements with experimental measurements on metrics such as the crash amplitude and the inversion radius of the temperature change. These results validate the stellarator modeling capability of M3D-C¹ and illustrate the mechanism of the current-drive-induced sawtooth-like crashes in W7-X.