Stabilizing Effects of Edge Current Density on Peeling-Ballooning Instability

Resistive MHD computations using the NIMROD code have found strong dependence of the low-n edge instabilities on edge current density distribution. The low-n edge localized modes can be driven unstable by increasing the edge current density across the peeling-ballooning stability boundary. When the edge peak current density is sufficiently large, the safety factor q-profile develops a region where the magnetic shear becomes zero or negative. In these cases, the lown peeling dominant edge instabilities are partially or fully stabilized. The stabilizing effects of zero or reversed magnetic shear are consistent with analytical theory predictions on the necessary condition for the stability of the peeling mode. Nonlinear simulations indicate that the stabilizing effects of edge current density on the low-n edge instabilities through zero or reverse shear can persist throughout the nonlinear exponential growth phase. Near the end of this nonlinear phase, the filament size in radial direction can well exceed the pedestal width, and disconnected bloblike substructures start to develop within the filaments. Relative pedestal energy loss from these radially extending filaments can reach 20%. Both filament size and pedestal energy loss from the nonlinear low-n edge instabilities can be reduced and regulated by the equilibrium edge current density distribution.