ELMs and constraints on the H-mode pedestal: peeling–ballooning stability calculation and comparison with experiment

TLDR

Pedestal constraints depend on density and temperature separately because collisionality affects the bootstrap current, rather than depending solely on pressure. The study reviews and tests the peeling–ballooning model for edge localized modes and pedestal limits. Using the ELITE MHD stability code, the authors calculate peeling–ballooning pedestal constraints, compare them with experimental data across varying density, and develop a parameter‑dependent technique that can be projected to future tokamak designs. The technique accurately predicts pedestal height as a function of density, triangularity, and plasma current, matching experiments, and extends to burning‑plasma tokamak scenarios.

Abstract

We review and test the peeling–ballooning model for edge localized modes (ELMs) and pedestal constraints, a model based upon theoretical analysis of magnetohydrodynamic (MHD) instabilities that can limit the pedestal height and drive ELMs. A highly efficient MHD stability code, ELITE, is used to calculate quantitative stability constraints on the pedestal, including constraints on the pedestal height. Because of the impact of collisionality on the bootstrap current, these pedestal constraints are dependent on the density and temperature separately, rather than simply on the pressure. ELITE stability calculations are directly compared with experimental data for a series of plasmas in which the density is varied and ELM characteristics change. In addition, a technique is developed whereby peeling–ballooning pedestal constraints are calculated as a function of key equilibrium parameters via ELITE calculations using series of model equilibria. This technique is used to successfully compare the expected pedestal height as a function of density, triangularity and plasma current with experimental data. Furthermore, the technique can be applied for parameter ranges beyond the purview of present experiments, and we present a brief projection of peeling–ballooning pedestal constraints for burning plasma tokamak designs.