MHD stability in X-point geometry: simulation of ELMs

A non-linear MHD code, named JOREK, is under development with the aim of studying the non-linear evolution of the MHD instabilities thought to be responsible for edge localized modes (ELMs): external kink (peeling) and medium-n ballooning modes. The full toroidal X-point geometry is taken into account including the separatrix, open and closed field lines. Analysis of the influence of the separatrix shows a strong stabilization of the ideal and resistive MHD external kink/peeling modes. One instability remains unstable in the presence of the X-point, characterized by a combination of a tearing and a peeling mode. The so-called peeling-tearing mode shows a much weaker dependence on the edge q. Non-linearly the n = 1 peeling-tearing mode saturates at a constant amplitude yielding a mostly kink-like perturbation of the boundary with an island-like structure close to the X-point. The nonlinear evolution of a medium-n ballooning mode shows the formation of density filaments. The density filaments are sheared off from the main plasma by an n = 0 flow non-linearly induced by the Maxwell stress. The amplitude of the ballooning mode is limited by this n = 0 flow and multiple (in time) density filaments can develop to bring the plasma below the stability boundary.