Ideal magnetohydrodynamic stability limits of a high-β tokamak with superimposed helical fields

The stability of a high-β 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 l = 2 field is unfavorable, lowering the allowable Ohmic heating current and decreasing the maximum stable β. The addition of an l = 3 field, however, is favorable. Both the allowable Ohmic heating current and maximum stable β are increased as the l = 3 amplitude increases.