Role of the pedestal current on the stability of non-ideal ballooning modes

Abstract

On the basis of a three-field flute-reduced magnetohydrodynamic model, which mainly describes the edge instabilities by shielding a major part of the J × B force in the flute reduction, we study the stability of ballooning modes in the edge pedestal, highlighting the role of an equilibrium parallel current gradient. This effect, which is designated as the current gradient driven (CGD) term in this paper, is shown to have an influence on the stability of finite-n pedestal ballooning modes due to the existence of a highly localized bootstrap current. An analysis in the ideal limit shows that the CGD term destabilizes the ballooning modes regardless of the sign of its gradient, especially near the stability boundaries. An inclusion of the finite Larmor radius (FLR) effect via ion diamagnetic flow and finite resistivity results in a coupling of the FLR effect and the current gradient. In this particular regime where the deviation from the ideal stability is considerable, this coupling effect is shown to dominate stability in intermediate n (20<n≤40) modes. Here, n is the toroidal mode number. Stability analyses using a series of model pedestal equilibria indicate that an increase in a bootstrap current can move the most unstable position from the pedestal middle to the bottom and the negative gradient of the bootstrap current at the pedestal bottom leads to further destabilization of intermediate n modes.