Non-linear extended MHD simulations of type-I edge localised mode cycles in ASDEX Upgrade and their underlying triggering mechanism

TL;DR

This paper presents the first non-linear MHD simulations of type-I ELM cycles in ASDEX Upgrade, revealing a triggering mechanism involving magnetic field ergodicity that causes explosive ELM crashes lasting about 500 microseconds.

Contribution

It introduces a novel non-linear simulation approach that captures the explosive onset of ELMs and their underlying triggering mechanism, advancing predictive modeling for future tokamaks.

Findings

Simulated ELM crashes qualitatively match ASDEX Upgrade observations.

A proportional relationship between heating power and ELM frequency is observed.

The simulations provide insights into the explosive growth phase of ELMs.

Abstract

A triggering mechanism responsible for the explosive onset of edge localised modes (ELMs) in fusion plasmas is identified by performing, for the first time, non-linear magnetohydrodynamic simulations of repetitive type-I ELMs. Briefly prior to the ELM crash, destabilising and stabilising terms are affected at different timescales by an increasingly ergodic magnetic field caused by non-linear interactions between the axisymmetric background plasma and growing non-axisymmetric perturbations. The separation of timescales prompts the explosive, i.e. faster than exponential, growth of an ELM crash which lasts ${\sim}$ 500 ${\mu}$s. The duration and size of the simulated ELM crashes compare qualitatively well with type-I ELMs in ASDEX Upgrade. As expected for type-I ELMs, a direct proportionality between the heating power in the simulations and the ELM repetition frequency is obtained. The simulations presented here are a major step forward towards predictive modelling of ELMs and of the assessment of mitigation techniques in ITER and other future tokamaks.