Overview of negative triangularity experiments with strongshaping in ASDEX Upgrade

The Negative Triangularity (NT) is a regime where shaping appears to provide an edge-localized modes (ELMs) free scenario, while maintaining high confinement [1]. Complete ELM avoidance has been consistently achieved in strongly shaped plasmas on TCV [2] and DIII-D [3]. In contrast, early NT experiments in ASDEX upgrade resulted in H modes with weak ELMs, regardless of the ion ▽B drift configuration [5]. This outcome is attributed to the weak shaping with triangularity levels of δ_upp ≈ -0.33 and δ_low ≈ +0.01. In this work, we report on a series of the latest ASDEX Upgrade experiments with stronger shaping of δ_upp ≈ -0.5 and δ_low ≈ 0. An ideal MHD BALOO code calculation is in agreement with experimental observations and shows the closure of the second stability for those shapes, concomitant with the absence of ELMs. In addition, experience from TCV with similar plasma shapes suggested that these triangularity values would be adequate for ELM avoidance. This new shape served as a basis for various scans where experiments with increased density (via gas fueling and pellet injection), power levels up to 10 MW of pure Neutral Beam Injection (NBI) heating and combined with ECRH, and impurity seeding were conducted. There is evidence of strong power degradation; however, confinement improvement H₉₈ ≈ 1 was observed with nitrogen seeding. Confinement qualities and the stability boundaries of the plasma edge, in those various cases, are discussed.

[1] A Marinoni 2021 RMPP

[2] S Coda 2022 PCF

[3] M Austin 2019 PRL

[4] T Happel 2023 NF

[5] B Vanovac 2024 PPCF