Toroidal effects on current driven modes in tokamaks

Abstract

The low beta stability of a toroidal circular cross-section tokamak is re-analysed. Although the structure of the unstable modes can be considerably modified by toroidal coupling, the overall stability picture is well represented by the ‘straight tokamak model’, with the major exception that the internal kink, although stabilized by toroidal effects, still has a free-boundary counterpart – the toroidal kink mode. The toroidal kink is unstable for q0 < qc ≤ 1 and is destabilized by toroidal coupling, having a growth rate γ that scales with inverse aspect ratio a/R as γ – (a/R)2[qs], where [qs] is the integer part of the edge safety factor qs.