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 beta limit than the ballooning modes. Using relatively broad pressure profiles, the Mercier criterion yields beta=0.4% for the standard configuration, beta=0.3% for the high mirror configuration and beta=1.3% for the deep well configuration. The comparable ballooning limits are beta=0.7%, beta=0.6%, and beta=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 beta similar or equal to 0.8%. (C) 1996 American Institute of Physics.