Characteristics of grassy ELMs and its impact on the divertor heat flux width

TL;DR

This study uses BOUT++ turbulence simulations to analyze grassy ELMs in EAST tokamak, revealing the roles of peeling-ballooning and drift-Alfvén instabilities in ELM dynamics and showing that turbulence broadens the divertor heat flux width beyond traditional models.

Contribution

It provides new insights into the turbulence-driven mechanisms of grassy ELMs and their impact on divertor heat flux width, highlighting the significance of drift-Alfvén turbulence.

Findings

Low-n peeling modes trigger ELM crashes.

Drift-Alfvén turbulence delays ELM onset and increases energy loss.

Divertor heat flux width is approximately twice larger than traditional estimates.

Abstract

BOUT++ turbulence simulations are conducted for a 60s steady-state long pulse high \{beta}p EAST grassy ELM discharge. BOUT++ linear simulations show that the unstable mode spectrum covers a range of toroidal mode numbers from low-n (n=10~15) peeling-ballooning modes (P-B) to high-n (n=40~80) drift-Alfv\'en instabilities. Nonlinear simulations show that the ELM crash is trigged by low-n peeling modes and fluctuation is generated at the peak pressure gradient position and radially spread outward into the Scrape-Off-Layer (SOL), even though the drift-Alfv\'en instabilities dominate the linear growth phase. However, drift-Alfv\'en turbulence delays the onset of the grassy ELM and enhances the energy loss with the fluctuation extending to pedestal top region. Simulations further show that if the peeling drive is removed, the fluctuation amplitude drops by an order of magnitude and the ELM crashes disappear. The divertor heat flux width is ~2 times larger than the estimates based on the HD model and the ITPA multi-tokamak scaling (or empirical Eich scaling) due to the strong radial turbulence transport.