First access to ELM-free negative triangularity at low aspect ratio

• Published in: Nuclear fusion Vol. 64; no. 12; pp. 124004 - 124011
• Main Authors: Nelson, A.O., Vincent, C., Anand, H., Lovell, J., Parisi, J.F., Wilson, H.S., Imada, K., Wehner, W.P., Kochan, M., Blackmore, S., McArdle, G., Guizzo, S., Rondini, L., Freiberger, S., Paz-Soldan, C.
• Format: Journal Article
• Language: English
• Published: IAEA IOP Publishing 01.12.2024
• Subjects: ELM-free; MAST-U; negative triangularity; tokamak
• ISSN: 0029-5515; 1741-4326
• DOI: 10.1088/1741-4326/ad89db

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 δ ≲ − 0.06 , an extended period of H-mode operation with Type-III ELMs is sustained at less negative δ 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 δ is decreased allows for similar normalized β in the ELM-free NT and H-mode positive triangularity shapes.