Limiting β in a tokamak with a small aspect ratio

Calculations on the ideal MHD stability of the plasma in a tokamak with a small aspect ratio and an elongated cross section are discussed. The necessary condition for the stability of the n = 1 external mode, n = 1;q/sub s/≥2 (q>1), for these configurations is replaced by more stringent condition q/sub s/≥q/sub s//sup */>2. If the latter condition is violated, even a force-free equilibrium will be unstable. At q/sub s/>q/sub s//sup */, the limiting value of β for all ideal modes is close to the limiting value for small-scale ballooning modes