Comparison of detachment in Ohmic plasmas with positive and negative triangularity

• Main Authors: Février, O, Tsui, C. K, Durr-Legoupil-Nicoud, G, Theiler, C, Carpita, M, Coda, S, Colandrea, C, Duval, B. P, Gorno, S, Huett, E, Linehan, B, Perek, A, Porte, L, Reimerdes, H, Sauter, O, Tonello, E, Zurita, M, Bolzonella, T, Sciortino, F, Team, the TCV, Team, the EUROfusion Tokamak Exploitation
• Format: Journal Article
• Language: English
• Published: 18.10.2023
• Subjects: Physics - Plasma Physics
• DOI: 10.48550/arxiv.2310.11737

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: $\delta$) 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 $\delta\approx -0.3$ 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 ($\lambda_q$) 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, $\lambda_{int}$. 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.