Island Formation for Finite Pressure Equilibria in Non-axisymmetric Tori Obtained by the HINT Code

Abstract

Confinement nature of an edge plasma is largely influenced by the structure of magnetic surfaces in the boundary region. This issue is of particular importance for non-axisymmetric tori. Three-dimensional (3D) MHD equilibrium codes, such as HINT and PIES, have been developed to answer the indeterminate question of whether or not a 3D finite-pressure equilibrium can really exist. One important discovery obtained is that magnetic surfaces are ergodized by the finite pressure effect in actual helical configurations, and the ergodization of surfaces in the core region often imposes severer limitation on the equilibrium β than the Shafranov shift. The breaking of surfaces often grows in the boundary region in particular. Also found by HINT is a remarkable property of self-healing' of magnetic islands as β increases. We have extended the HINT code to include the effects of the net toroidal current on the equilibrium in a fully consistent manner, especially the neoclassical currents such as the bootstrap current. In addition, it has been modified to treat the full torus geometry, and to construct a nonlinear simulation code in a full 3D geometry. In this paper, a new series of computations is shown, which became achievable by a modification of the code to have existence of coil currents in the computation region. Usefulness of the code has largely increased. Based on those HINT computations, importance of the 'global effect' as a physical mechanism of island formation is suggested, which is not described by the previous analytical predictions.