Local ideal magnetohydrodynamic stability of a quasihelically symmetric stellarator

The ideal magnetohydrodynamic stability with respect to localized ballooning modes and the Mercier criterion of a quasihelically symmetric stellarator configuration is investigated. Configurations with an enhanced magnetic mirror and a deeper magnetic well that can be achieved by energizing a set of auxiliary coils have also been analyzed. In all cases, the Mercier criterion imposes a slightly stricter β limit than the ballooning modes. Using relatively broad pressure profiles, the Mercier criterion yields β=0.4% for the standard configuration, β=0.3% for the high mirror configuration and β=1.3% for the deep well configuration. The comparable ballooning limits are β=0.7%, β=0.6%, and β=1.66% for the standard, high mirror, and deep well configurations, respectively. With a more peaked pressure profile, ballooning instabilities near the center of the plasma and Mercier modes near the edge of the plasma limit the performance of the standard configuration at β≂0.8%.