Comparison of detachment in Ohmic plasmas with positive and negative triangularity

TL;DR

This study compares detachment behavior in Ohmic plasmas with positive and negative triangularity in the TCV tokamak, revealing that negative triangularity makes detachment more difficult due to geometric and plasma confinement factors.

Contribution

It provides the first detailed comparison of detachment in Ohmic plasmas across a range of triangularity, highlighting the challenges of detachment in negative triangularity configurations.

Findings

Detachment is harder to achieve in negative triangularity plasmas.

Target heat flux widths are similar across triangularity, but spreading factors differ.

Negative triangularity plasmas have higher core density and lower neutral pressure, affecting detachment.

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: $\delta$) 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.