COMPUTATIONAL MODELING OF NEOCLASSICAL AND RESISTIVE MAGNETOHYDRODYNAMIC TEARING MODES IN TOKAMAKS

Numerical studies of the nonlinear evolution. of magnetohydrodynamic-t ype tearing modes in three-dimensional toroidal geometry with neoclass ical effects are presented. The inclusion of neoclassical physics intr oduces an additional free-energy source for the nonlinear formation of magnetic islands through the effects of a bootstrap current in Ohm's law. The neoclassical tearing mode is demonstrated to be destabilized in plasmas which are otherwise Delta' stable, albeit once an island wi dth threshold is exceeded. The plasma pressure dynamics and neoclassic al tearing growth is shown to be sensitive to the choice of the ratio of the parallel to perpendicular diffusivity (chi parallel to/chi perp endicular to). The study,is completed with a demonstration and theoret ical comparison of the threshold for single helicity neoclassical magn etohydrodynamic tearing modes, which is described based on parameter s cans of the local pressure gradient, the ratio of perpendicular to par allel pressure diffusivities chi perpendicular to/chi parallel to, and the magnitude of an initial seed magnetic perturbation. (C) 1996 Amer ican Institute of Physics.