Robustness of high-β W7-X plasmas against ideal ballooning instability*

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, and that interchange instabilities tend to be more detrimental than ideal ballooning in stellarators.