Mechanism for toroidal localization of edge ballooning instability in the presence of strong applied 3D magnetic perturbations

Recent observations on ASDEX-Upgrade have shown toroidally localized MHD activity in the presence of strong applied 3D magnetic perturbations [1]. In this work, we utilize VMEC to construct 3D MHD equilibria associated with ASDEX-Upgrade H-mode discharges with applied 3D magnetic perturbations. Subsequent analysis of the infinite-n ballooning stability of these equilibrium demonstrates strong agreement between theory and the experiment, with the ballooning instability having strong toroidal localization and dependence on the magnitude of the 3D edge displacement. Analysis shows localization of the ballooning mode to specific field-lines corresponding to locations in the pedestal region where there is a minima in the integrated local magnetic shear. This reduces the stabilizing field-line bending energy, causing the onset of the ballooning instability. 3D distortion of the flux surfaces cause significant change in the normal torsion, a key component of the local shear, and acts as the primary mechanism for ballooning destabilization on certain field-lines. [1] M. Willlensdorfer et al., Physics Review Letters, 119 (2017).