May 8-10

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Abstracts

Author: Fatima Ebrahimi
Requested Type: Consider for Invited
Submitted: 2023-03-24 11:19:24

Co-authors: A. Bhattacharjee

Contact Info:
PPPL/Princeton University
100 Stellarator Road- Princeto
Princeton, NJ   08543
US

Abstract Text:
New results on the rise and nonlinear relaxation of Peeling-Ballooning Edge-Localized Modes (P-B ELMs) will be presented. 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 P-B 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. 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]. The cyclic nonlinear behavior of the low-n current-driven ELMs was explained via direct numerical simulations of the fluctuation-induced emf of a current-carrying filament. Here, by including the critical effect of plasma pressure gradients in DIII-D discharges, we will present 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 current sheets, as well as small-scale poloidally extending 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-type low-n modes grow and saturate, it is during a secondary sudden nonlinear growth of P-B modes that nonlinear finger-like structures of ballooning modes are expelled into the outer edge region (a short initial paper is available here: https://arxiv.org/abs/2110.09706).

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