Spectroscopic investigations of divertor detachment in TCV

TL;DR

This study investigates divertor detachment in TCV using a new spectroscopic system, revealing detailed Balmer line emission analysis and the role of recombination during density ramps, with findings differing from higher density tokamaks.

Contribution

Development of a new divertor spectroscopy system and advanced Balmer line analysis techniques for detailed detachment study in TCV.

Findings

Recombination dominates near the target during detachment.

Recombination peaks remain close to the target even at high densities.

Recombination's overall magnitude is small compared to ion current at detachment onset.

Abstract

The aim of this work is to provide an understanding of detachment at TCV with emphasis on analysis of the Balmer line emission. A new Divertor Spectroscopy System has been developed for this purpose. Further development of Balmer line analysis techniques has allowed detailed information to be extracted from the three-body recombination contribution to the n=7 Balmer line intensity. During density ramps, the plasma at the target detaches as inferred from a drop in ion current to the target. At the same time the Balmer $6\rightarrow2$ and $7\rightarrow2$ line emission near the target is dominated by recombination. As the core density increases further, the density and recombination rate are rising all along the outer leg to the x-point while remaining highest at the target. Even at the highest core densities accessed (Greenwald fraction 0.7) the peaks in recombination and density may have moved not more than a few cm poloidally away from the target which is different to other, higher density tokamaks, where both the peak in recombination and density continue to move towards the x-point as the core density is increased. The inferred magnitude of recombination is small compared to the target ion current at the time detachment (particle flux drop) starts at the target. However, recombination may be having more localized effects (to a flux tube) which we cannot discern at this time. Later, at the highest densities achieved, the total recombination does reach levels similar to the particle flux.