MHD simulations of small ELMs at low triangularity in ASDEX Upgrade

TL;DR

This paper uses non-linear MHD simulations to explore regimes in ASDEX Upgrade that avoid type-I ELMs, identifying resistive peeling-ballooning modes as key instabilities and examining the effects of plasma parameters on ELM suppression.

Contribution

It demonstrates the suppression of type-I ELMs through high separatrix density and low resistivity, highlighting the role of resistive peeling-ballooning modes and the importance of diamagnetic effects.

Findings

High separatrix density prevents type-I ELMs.

Resistive peeling-ballooning modes drive small ELMs.

Lowering resistivity reduces mode growth rates.

Abstract

The development of small- and no-ELM regimes for ITER is a high priority topic due to the risks associated to type-I ELMs. By considering non-linear extended MHD simulations of the ASDEX Upgrade tokamak with the JOREK code, we probe a regime that avoids type-I ELMs completely provided that the separatrix density is high enough. The dynamics of the pedestal in this regime are observed to be qualitatively similar to the so-called quasi-continuous exhaust (QCE) regime in several ways. Repetitive type-I ELMs are substituted by roughly constant levels of outwards transport caused by peeling-ballooning modes (with dominant ballooning characteristics) which are localised in the last 5\% of the confined region (in normalised poloidal flux). The simulated low triangularity plasma transitions to a type-I ELMy H-mode if the separatrix density is sufficiently reduced or if the input heating power is sufficiently increased. The stabilising factors that play a role in the suppression of the small ELMs are also investigated by analysing the simulations, and the importance of including diamagnetic effects in the simulations is highlighted. By considering a scan in the pedestal resistivity and by measuring the poloidal velocity of the modes (and comparing to theoretical estimates for ideal and resistive modes), we identify the underlying instabilities as resistive peeling-ballooning modes. Decreasing the resistivity below experimentally-relevant conditions (i.e., going towards ideal MHD), the peeling-ballooning modes that constrain the pedestal below the type-I ELM stability boundary display sharply decreasing growth rates.