Models for the pedestal temperature at the edge of H-mode tokamak plasmas

TLDR

The pedestal pressure gradient is limited by ballooning mode instability and expressed through magnetic shear and geometrical factors. The study develops predictive models for the pedestal temperature at the top of type‑1 ELMy H‑mode plasmas. Using theory‑motivated models for pedestal width and pressure gradient, experimental pedestal density, and bootstrap‑current‑adjusted magnetic shear calculations, the authors determine the pedestal temperature. Computed pedestal temperatures agree with over 500 measurements from four tokamaks, with uncertainties discussed and future improvement directions outlined.

Abstract

Predictive models are developed for the temperature at the top at the edge of type 1 ELMy (edge localized mode) H-mode (high-confinement mode) plasmas. Theory-motivated models are used for the pedestal width and pressure gradient, while the pedestal density is obtained from experimental data in this study. The pedestal pressure gradient is assumed to be limited by the ballooning mode instability and is expressed in terms of the magnetic shear and geometrical factors. The effect of the bootstrap current, which reduces the magnetic shear in the steep pressure gradient region at the edge of the H-mode plasma, is included in the determination of the magnetic shear. Approaches for calculating the magnetic shear, combined with proposed models for the pedestal width, are used to determine the pedestal temperature. The computed pedestal temperatures are compared with more than 500 measured pedestal temperatures for type 1 ELMy H-mode discharges in four tokamaks. Some of the uncertainties in these results are discussed, and directions for future work to improve edge pedestal scalings are described.