Comparing spontaneous and pellet-triggered ELMs via non-linear extended MHD simulations

TL;DR

This study uses non-linear extended MHD simulations to compare spontaneous and pellet-triggered ELMs in tokamak plasmas, revealing differences in mode structure, heat fluxes, and energy losses relevant for fusion reactor control.

Contribution

It provides a detailed simulation-based comparison of spontaneous and pellet-triggered ELMs, including effects of pellet timing and size on ELM dynamics and heat fluxes.

Findings

Pellet-triggered ELMs are initiated by a helical n=1 mode.

Triggered ELMs have reduced energy losses and narrower divertor wetted areas.

Earlier pellet injection during pedestal build-up decreases peak divertor energy fluency.

Abstract

Injecting frozen deuterium pellets into an ELMy H-mode plasma is a well established scheme for triggering edge localized modes (ELMs) before they naturally occur. Based on an ASDEX Upgrade H-mode plasma, this article presents a comparison of extended MHD simulations of spontaneous type-I ELMs and pellet-triggered ELMs allowing to study their non-linear dynamics in detail. In particular, pellet-triggered ELMs are simulated by injecting deuterium pellets into different time points during the pedestal build-up described in [A. Cathey et al. Nuclear Fusion 60, 124007 (2020)]. Realistic ExB and diamagnetic background plasma flows as well as the time dependent bootstrap current evolution are included during the build-up to capture the balance between stabilising and destabilising terms for the edge instabilities accurately. Dependencies on the pellet size and injection times are studied. The spatio-temporal structures of the modes and the resulting divertor heat fluxes are compared in detail between spontaneous and triggered ELMs. We observe that the premature excitation of ELMs by means of pellet injection is caused by a helical perturbation described by a toroidal mode number of n = 1. In accordance with experimental observations, the pellet-triggered ELMs show reduced thermal energy losses and narrower divertor wetted area with respect to spontaneous ELMs. The peak divertor energy fluency is seen to decrease when ELMs are triggered by pellets injected earlier during the pedestal build-up.