LOCAL IDEAL MAGNETOHYDRODYNAMIC STABILITY OF A QUASIHELICALLY SYMMETRICAL STELLARATOR

The ideal magnetohydrodynamic stability with respect to localized ball ooning modes and the Mercier criterion of a quasihelically symmetric s tellarator configuration is investigated. Configurations with an enhan ced magnetic mirror and a deeper magnetic well that can be achieved by energizing a set of auxiliary coils have also been analyzed. In all c ases, the Mercier criterion imposes a slightly stricter beta limit tha n the ballooning modes. Using relatively broad pressure profiles, the Mercier criterion yields beta=0.4% for the standard configuration, bet a=0.3% for the high mirror configuration and beta=1.3% for the deep we ll configuration. The comparable ballooning limits are beta=0.7%, beta =0.6%, and beta=1.66% for the standard, high mirror, and deep well con figurations, respectively. With a more peaked pressure profile, balloo ning instabilities near the center of the plasma and Mercier modes nea r the edge of the plasma limit the performance of the standard configu ration at beta similar or equal to 0.8%. (C) 1996 American Institute o f Physics.