Plasmoid-mediated reconnection during nonlinear peeling-ballooning edge-localized modes

Understanding the physics of edge instabilities, in particular as described by a self-consistent model of the nonlinear evolution, remains a challenging problem in magnetic fusion confinement. Here, using full extended MHD simulations in DIII-D discharges, we investigate the nonlinear evolution of peeling-ballooning modes and the associated physics of fast magnetic reconnection triggered by the formation of thin current sheets and their secondary instabilities in the late dynamical phase of the instabilities. We will present new results and a more complete picture of ELM dynamics by examining the sequential stages of the linear instability, the early and late nonlinear phases. Large-scale axisymmetric, as well as small-scale poloidal current sheets, are formed as the coherent P-B ELM filaments nonlinearly evolve. It is observed that, at high Lundquist numbers, these current sheets break during a reconnection burst, i.e. a secondary exponential growth of intermediate modes followed by relaxation due to the suppression of P-B drive. We find that as the linearly unstable intermediate-n ballooning modes and the nonlinearly driven peeling low-n modes grow and saturate, it is during a fast reconnection phase mediated by plasmoid instability, where nonlinear expulsion of finger-like currents (plasmoids) of finite amplitude ballooning modes occur. Work supported by DOE.