First Access to ELM-free Negative Triangularity at Low Aspect Ratio

TL;DR

This paper reports the first achievement of negative triangularity plasma shaping on MAST-U, demonstrating ELM suppression at low aspect ratio and revealing unique stability and temperature behavior compared to conventional machines.

Contribution

It presents the first experimental realization of ELM-free negative triangularity operation at low aspect ratio, highlighting differences from conventional machines and analyzing stability and temperature effects.

Findings

ELMs are suppressed below δ < -0.06

Extended H-mode with Type-III ELMs is sustained at less negative δ

Core temperature rises as δ decreases, maintaining normalized beta

Abstract

A plasma scenario with negative triangularity (NT) shaping is achieved on MAST-U for the first time. While edge localized modes (ELMs) are eventually suppressed as the triangularity is decreased below $\delta$ < -0.06, an extended period of H-mode operation with Type-III ELMs is sustained at less negative $\delta$ even through access to the second stability region for ideal ballooning modes is closed. This documents a qualitative difference from the ELM-free access conditions documented in NT scenarios on conventional aspect ratio machines. The electron temperature at the pedestal top drops across the transition to ELM-free operation, but a steady rise in core temperature as $\delta$ is decreased allows for similar normalized beta in the ELM-free NT and H-mode positive triangularity shapes.