3D MHD study of Helias and heliotron

Properties of magnetic islands induced by the finite pressure effect are numerically analyzed for three-dimensional magnetohydrostatic equilibria of Helias and l = 2 heliotron/torsatron types. For Helias, it is found that an island chain is generated on the 5/6 rational surface, when such a surface appears in the plasma region of a finite-β equilibrium. The island chain, however, is not so dangerous as to destroy the plasma confinement even if it appears in a vanishing small shear region. Moreover, it is definitely confirmed that the finite pressure effect sometimes exhibits a surprisingly good aspect, namely, that the vacuum islands are removed as β increases, which can be called 'self-healing' of islands. This property can be explained by the numerically discovered fact that the phases of islands induced by the finite pressure effect are always locked i.e. do not change regardless of β. The way islands appear at finite-β for heliotron/torsatron is significantly different from that for Helias. However, we find the self-healing of islands can also occur for the heliotron configuration. This phenomenon can partly explain the reason why the fragility of surfaces decreases when they are shifted inward by control of the external vertical field Bv, though the configuration develops a magnetic hill. Furthermore, we analyze the diversion properties of the magnetic field outside the last closed magnetic surface for finite-β equilibria, and find that divertor concepts which have been developed from the diversion properties of the corresponding vacuum fields can be maintained for finite-β equilibria for both configurations. (author)