The impact of 3D magnetic perturbations on the MHD stability of the pedestal plasma in tokamaks*

Experiments show that 3D magnetic perturbations (MPs) can mitigate or even eliminate edge-localized modes (ELMs) but without a rigorous theoretical model, extrapolation to future tokamaks is uncertain. ELITE has been used for two decades to assess pedestal MHD stability in axisymmetric plasmas, providing a prediction for ELM onset that agrees with observation. To efficiently understand the impact of MPs on pedestal stability a variational formalism has been developed [1]. Trial functions are constructed from the poloidal Fourier amplitudes of the displacement, calculated using ELITE for the toroidally averaged equilibrium. These are scaled relative to each other, fixing the scaling coefficients by minimizing the perturbed energy for the 3D equilibrium to derive the eigenfunction and growth rate. We extend that theory to: (1) real experimental tokamak equilibria, analyzed using a finite toroidal mode number, n, version of ELITE; (2) determine the stability threshold ([1] is restricted to unstable plasmas), and (3) explore the potential of magnetic compression to enhance stability. This will inform a robust stability code, ELITE-3D, as a new, efficient tool to assess pedestal stability in the presence of MPs.

[1] M S Anastopoulos Tzanis, et al, Nucl Fusion 60 (2020) 106003