Comparison of detachment in Ohmic plasmas with positive and negative triangularity
O. Février, C.K. Tsui, G. Durr-Legoupil-Nicoud, C. Theiler, M. Carpita, S. Coda, C. Colandrea, B.P. Duval, S. Gorno, E. Huett, B. Linehan, A. Perek, L. Porte, H. Reimerdes, O. Sauter, E. Tonello, M. Zurita, T Bolzonella, F. Sciortino, the TCV Team, the EUROfusion Tokamak Exploitation Team
Abstract
Abstract 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>δ</mml:mi> <mml:mo>≈</mml:mo> <mml:mo>−</mml:mo> <mml:mn>0.3</mml:mn> </mml:mrow> </mml:math> could not be cooled to below <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>5</mml:mn> <mml:mtext> </mml:mtext> <mml:mrow> <mml:mi>eV</mml:mi> </mml:mrow> </mml:mrow> </mml:math> 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.