Edge localized modes and the pedestal: A model based on coupled peeling–ballooning modes

TLDR

The authors develop an MHD‑based model of the tokamak edge that describes how edge localized modes and the pedestal interact. The model incorporates large bootstrap currents from sharp pedestal pressure gradients, which lower edge magnetic shear to stabilize high‑n ballooning modes while simultaneously driving intermediate‑to‑low‑n peeling modes, and it is implemented with an efficient numerical code and simplified analytic models for pedestal limits and the ELM cycle. Coupled peeling–ballooning modes with 3 < n < 20 are found to limit the pedestal and trigger ELMs, and the model’s predictions agree with several experiments, with nonideal MHD effects noted.

Abstract

A model based on magnetohydrodynamic (MHD) stability of the tokamak plasma edge region is presented, which describes characteristics of edge localized modes (ELMs) and the pedestal. The model emphasizes the dual role played by large bootstrap currents driven by the sharp pressure gradients in the pedestal region. Pedestal currents reduce the edge magnetic shear, stabilizing high toroidal mode number (n) ballooning modes, while at the same time providing drive for intermediate to low n peeling modes. The result is that coupled peeling–ballooning modes at intermediate n (3&amp;lt;n&amp;lt;20) are often the limiting instability which constrains the pedestal and triggers ELMs. These modes are characterized in shaped tokamak equilibria using an efficient new numerical code, and simplified models are developed for pedestal limits and the ELM cycle. Results are compared to several experiments, and nonideal MHD effects are briefly discussed.