Ideal Magnetohydrodynamic Stability Limits of a High Tokamak with Superimposed Helical Fields

The stability of a high 3 tokamak with a superimposed single helicity stellarator field is investigated in the context of the sharp boundary surface current model. The analysis treats external ideal magnetohydrodynamic modes including both pressure-driven ballooning effects and current-driven kink instabilities. The results show that the addition of an i = 2 field is unfavorable, lowering the allowable ohmic heating current and decreasing the maximum stable P. The addition of an I = 3 field however, is favorable. Both the allowable ohmic heating current and maximum stable P are increased as the i = 3 amplitude increases. * Permanent address: Atomic Energy Board, Pretoria, South Africa. Permanent address: University of Bergen, Bergen, Norway.