Non-ideal MHD analysis of pedestal scans of AUG and JET

Abstract The extended MHD code CASTOR3D is used to perform the MHD stability analysis of predictive pedestal scans of standard ASDEX Upgrade and JET-ILW scenarios. Comparing stability calculations considering, respectively, only ideal MHD, resistive MHD, ideal MHD with gyroviscosity and resistive MHD with gyroviscosity highlights the stabilizing and destabilizing mechanisms of the additional physics. Resistivity is found to be generally destabilizing, even to the lowest pedestal top pressure values considered. Stability calculations considering only gyroviscosity in addition to ideal MHD show a strong stabilizing effect in both devices and across a wide density range. The combination of resistivity and gyroviscosity shows a destabilizing trend with increasing plasma density, consistent with experimental observations. The destabilizing effect of resistivity in these scans has been attributed to increased degrees of freedom of the perturbation, particularly close to the separatrix, allowing the instability to minimize the stabilizing effect of shear Alfvén waves. Finally, the destabilizing effects of both an increasing relative shift between the density and temperature pedestals and increasing separatrix density have been attributed to a combination of the increased instability drive due to resistivity and a reduction of gyroviscous stabilization at high density, particularly close to the separatrix.