Nonlinear MHD simulation of core plasma collapse events in Wendelstein 7-X

Abstract

Three-dimensional nonlinear MHD simulations study the core collapse events observed in a stellarator experiment, Wendelstein 7-X. In the low magnetic shear configuration like the Wendelstein 7-X, the rotational transform profile is very sensitive to the toroidal current density. The 3D equilibrium with localized toroidal current density is studied. If the toroidal current density follows locally in the middle of the minor radius, the rotational transform is also changed locally. Sometimes, the magnetic topology changes due to appearing the magnetic island. A full three-dimensional nonlinear MHD code studies the nonlinear behaviors of the MHD instability. It was found that the following sequence. At first, the high-n ballooning-type mode structure appears in the plasma core, and then the mode linearly grows. The high-n ballooning modes nonlinearly couple and saturate. The mode structure changes to the low-n mode. The magnetic field structure becomes strongly stochastic into the plasma core due to the nonlinear coupling in that phase. Finally, the plasma pressure diffuses along the stochastic field lines, and then the core plasma pressure drops. This is a crucial result to interpret the core collapse event by strong nonlinear coupling.

Nonlinear MHD modeling of soft β limits in W7-AS

“…Several of the main workhorse nonlinear codes have either demonstrated recent nonlinear MHD extensions for stellarators, or are in the process of being extended for such studies [10][11][12]. In addition, the MIPS code has been used extensively for nonlinear MHD studies of stellarators [13]. While the solution of such detailed nonlinear initial value problems cannot be applied to stellarator optimization directly, it can help to inform and direct the development of a nonlinear analytic approach.…”

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

“…effects [15][16][17], and nonlinear simulations in simplified cylindrical geometry have shown that such internal kink modes can indeed result in core temperature crashes [18]. In contrast, nonlinear MHD simulations in W7-X geometry using rather different ι profiles with mid-radius ECCD have found that the nonlinear coupling of high-n ballooning modes can induce low-n modes to trigger core crashes by stochasticizing the magnetic field [19].…”