MHD stability in low-aspect-ratio tokamaks

Interest in tokamak configurations with high ..beta.. has led to renewed study of low-aspect-ratio tokamaks. We present a quantitative estimate of the MHD (..beta../sub crit/) predicted for such configurations, with emphasis on experimentally feasible cases. Ideal MHD stability calculations, extending previous results to the regime with aspect ratio less than 2.5, show that stability follows the Troyon semi-empirical scaling law. We find this scaling law to be a reliable theoretical guide in the search for high ..beta.. configurations at low aspect ratio. Ideally stable equilibria with (..beta..) approx.40% are found at an aspect ratio of 1.67. These calculations employ high-resolution equilibria to examine axisymmetric, low-n (n less than or equal to 3), and infinite-n ideal modes. Nonlinear resistive tearing mode calculations show that decreasing aspect ratio also leads to a reduction in the predicted saturation levels of m = 2/n = 1, m = 3/n = 2 islands and in their overlap region. The m = 1/n = 1 reconnection time has been found to increase as aspect ratio is reduced, leading to the possibility of sawtooth stabilization at low aspect ratio and high temperature.