Stabilizing effects of edge current density on pedestal instabilities

Abstract

Resistive MHD computations using the NIMROD code have found strong dependence of the low-n edge localized instabilities on edge current density distribution. The computations confirm that the low-n edge localized modes can be driven unstable by increasing the edge current density. When the edge peaked current density is sufficiently large, the q profile develops a region where the magnetic shear becomes negative. In these cases, the low-n edge instabilities are partially or fully stabilized. The stabilizing effects of edge current density in regions with reversed magnetic shear appear to be consistent with analytical predictions on a necessary condition for the stability of peeling modes. Nonlinear simulations indicate that the stabilizing effects of edge current density on the low-n edge instabilities through reverse shear can persist throughout the nonlinear exponential growth phase. Near the end of this nonlinear phase, the filament size in radial direction can exceed the pedestal width, and disconnected blob-like substructures start to develop within the filaments. Relative pedestal energy loss from these radially extending filaments can reach above the average experimental level of 10%.