Toroidal effects on current driven modes in tokamaks

The low β stability of a toroidal circular cross-section tokamak is reanalyzed. Although the structure of the unstable modes can be considerably modified by toroidal coupling, the overall stability picture is well represented by the so-called 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 qo<qc≤1 and is destabilized by toroidal coupling, having a growth rate γ that scales with inverse aspect ratio a/R like γ ∼(a/R)2[qs], where [qs] is the integer part of the edge safety factor qs. (author) 19 figs., 20 refs