Magnetohydrodynamic stability of negative central magnetic shear, high pressure (εβpol≫1) toroidal equilibria

Abstract

The magnetohydrodynamic (MHD) stability of negative central magnetic shear toroidal equilibria with q>2 everywhere and εβpol≫1 is investigated. Here, q is the safety factor of the equilibrium magnetic field in a torus with inverse aspect ratio ε, and βpol is the ratio of the plasma pressure to the pressure in the poloidal magnetic field. At small εβpol≪1, the elimination of the q=2 resonant surface in a negative shear equilibrium greatly improves resistive MHD stability as compared to equilibria with a monotonic q-profile containing a q=2 resonant surface. However, at large εβpol≫1, the reversal of the central magnetic shear and the elimination of the q=2 resonant surface does not improve MHD stability. The existence, or non-existence, of rational magnetic surfaces has no impact on MHD stability when εβpol≫1. Altering the current profile, and with it the q-profile, does not affect MHD stability when εβpol is no longer small. Stability is not improved by vertical elongation of the plasma in the poloidal plane. The utilization of an external vertical magnetic field to move the magnetic axis in major radius also does not improve MHD stability.