Benign saturation of ideal ballooning instability in a high-performance stellarator

The stellarator is one of the most promising concepts for commercial fusion reactors. Stellarator plasmas have been experimentally observed to be nonlinearly stable even when driven beyond linear MHD stability thresholds. This feature, if understood, could be exploited to expand the operation windows of present devices and improve future designs by relaxing the linear stability constraints. However, modeling MHD activities in stellarators has been difficult due to the lack of nonlinear codes that can effectively treat realistic stellarator geometry and transport timescales. Recently, we have extended the M3D-C1 code to allow non-axisymmetric domain geometry [Zhou et al NF 2021]. This new capability has been validated by simulations of the current-drive-induced sawtooth-like crashes in the W7-X stellarator, which show semi-quantitative agreements with experimental results [Zhou et al PoP 2023].

The high-performance operation of W7-X has profound implications on the viability of the stellarator concept, and its success critically depends on maintaining stability at high beta. To evaluate the impact of the MHD instabilities that may be triggered, we have undertaken simulations of W7-X plasmas above the designed beta-limit [Zhou et al PRL 2024]. Consistent with linear analyses, ideal ballooning instabilities occur as beta exceeds 5% in the standard configuration. Nonetheless, the modes saturate nonlinearly at relatively low levels without triggering large-scale crashes, even though confinement degradation worsens modestly as beta increases. This result suggests that the W7-X optimization for MHD stability is successful beyond expectation, and enhances the appeal of the stellarator approach to steady-state fusion reactors by suggesting the possibility to operate safely above the designed beta-limit. However, more significant profile change occurs when a low-order resonance is induced by a more peaked pressure profile, while interchange modes can cause a major pressure crash in an alternative, unoptimized W7-X configuration. Therefore, MHD stability should still be treated seriously in stellarator operation and design, for which nonlinear modeling using tools like M3D-C1 can play an instrumental role.