Comparison of detachment in Ohmic plasmas with positive and negative triangularity

• DOI: 10.48550/arxiv.2310.11737
• Publication Date: 2023-01-01
• Authors: Fevrier, O.; Tsui, C. K.; Durr-Legoupil-Nic...; Theiler, C.; Carpita, Massimo; Coda, S; Colandrea, Claudia; Duval, B. P.; Gorno, Sophie Dan...; Huett, Emanuel Paul; Linehan, B.; Perek, Artur; Porte, L; Reimerdes, H.; Sauter, O; Tonello, E.; Zurita, M.; Bolzonella, T.; Sciortino, F.; TCV Team; EUROfusion Tokama...
• Keywords: Plasma Physics (physics.plasm-ph); Divertor; Detachment; Negative Triangularity; FOS: Physical sciences; Tcv; Physics - Plasma Physics; Power Exhaust

In recent years, negative triangularity (NT) has emerged as a potential high-confinement L-mode reactor solution. In this work, detachment is investigated using core density ramps in lower single null Ohmic L-mode plasmas across a wide range of upper, lower, and average triangularity (the mean of upper and lower triangularity: $δ$) in the TCV tokamak. It is universally found that detachment is more difficult to access for NT shaping. The outer divertor leg of discharges with $δ\approx -0.3$ could not be cooled to below $5~\mathrm{eV}$ through core density ramps alone. The behavior of the upstream plasma and geometrical divertor effects (e.g. a reduced connection length with negative lower triangularity) do not fully explain the challenges in detaching NT plasmas. Langmuir probe measurements of the target heat flux widths ($λ_q$) were constant to within 30% across an upper triangularity scan, while the spreading factor $S$ was lower by up to 50% for NT, indicating a generally lower integral Scrape-Off Layer width, $λ_{int}$. The line-averaged core density was typically higher for NT discharges for a given fuelling rate, possibly linked to higher particle confinement in NT. Conversely, the divertor neutral pressure and integrated particle fluxes to the targets were typically lower for the same line-averaged density, indicating that NT configurations may be closer to the sheath-limited regime than their PT counterparts, which may explain why NT is more challenging to detach.