Nonlinear dynamics of pressure driven magnetic islands in low aspect ratio tokamaks

Abstract

An analytic theory of nonlinear pressure driven magnetic island evolution is presented which includes both neoclassical and curvature effects. This work differs from prior analytic treatments where large aspect ratio, small beta expansions were employed that are not appropriate for tight aspect ratio applications. In this work, this calculation is revisited by deriving and analyzing an island Grad–Shafarnov equation using an asymptotic expansion based solely on a small island width expansion. In conventional high-temperature tokamaks, the stabilizing curvature effects are weaker than the destabilizing neoclassical tearing mode instability drive in regions of monotonically increasing q-profile. At low aspect ratio, these two effects become comparable which suggests that tight aspect ratio configurations are less susceptible to the neoclassical tearing mode than are conventional tokamaks.