Benign saturation of ideal ballooning instability in a high-performance stellarator
Abstract
To examine the robustness of the designed 5% β-limit for high-performance operation in the W7-X stellarator, we undertake nonlinear magnetohydrodynamic (MHD) simulations of pressure-driven instabilities using the M3D-C1 code. Consistent with linear analyses, ideal ballooning instabilities occur as β 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 as β increases. In contrast, in an alternative configuration with nearly zero magnetic shear, ideal interchange modes induce a pressure crash at β= 1\%. These results suggest that the standard W7-X configuration might have a soft β-limit that is fairly immune to major MHD events, which enhances the appeal of the stellarator approach to steady-state fusion reactors.
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