IDEAL MHD STABILITY LIMITS OF LOW-ASPECT-RATIO TOKAMAK PLASMAS

The ideal magnetohydrodynamic (MHD) stability limits of low aspect rat io tokamak plasmas are computed numerically for plasmas with a range o f cylindrical safety factors q, normalized plasma pressures beta, elo ngations kappa and central safety factors q(0). Four distinct regimes are optimized, namely: (a) low-q plasmas with q(0) = 1.1 with and wit hout a stabilizing wall, (b) low-q plasmas with no wall and 1.1 < q(0 ) < 2, (c) high-beta, high bootstrap fraction plasmas at moderate kapp a requiring a wall and edge current drive and (d) high-beta, very high bootstrap fraction plasmas with moderate to high kappa requiring a st abilizing wall but little external current drive. A stable equilibrium is found at an aspect ratio of A = 1.4 and an elongation of kappa = 3 .0, with 99.3% of the current provided by the plasma pressure and beta = 45%. Special attention is paid to the issues of numerical convergen ce and the proper definition of bootstrap current fraction.