Nonlinear edge-localized modes as plasmoid-mediated reconnection bursts

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, we investigate the nonlinear evolution of Peeling-Ballooning Edge-Localized Modes (P-B ELMs) 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. In an earlier study, low-n edge non-axisymmetric reconnecting current-sheet instabilities, and the onset of plasmoid instabilities for given SOL current sheets, were examined [Ebrahimi PoP 24, 056119 2017]. Here, by including the critical effect of plasma pressure gradients in DIII-D discharges, we will present new results, and a more complete picture of ELM dynamics by examining the sequential stages of the linear instability, as well as the early and late nonlinear phases. Large-scale axisymmetric current sheets, as well as small-scale 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. [F. Ebrahimi and A. Bhattacharjee, Nucl. Fusion, under review] 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. Preliminary results on the stability of strong negative triangularity shaping plasmas using extended MHD model will also be presented.