Initial Fulcher band observations from high resolution spectroscopy in the MAST-U divertor

TL;DR

This study used high-resolution Fulcher band spectroscopy in MAST-U divertors to analyze molecular temperatures and vibrational populations during plasma detachment, revealing temperature increases and non-Boltzmann vibrational distributions.

Contribution

It provides new insights into molecular behavior and temperature changes during divertor detachment using high-resolution spectroscopy in a fusion device.

Findings

Rotational temperature increases from ~6000 K to ~9000 K during detachment.

Vibrational populations deviate from Boltzmann distribution, especially in higher bands.

Qualitative agreement of temperature behavior observed between upper and lower divertors.

Abstract

High resolution Fulcher band spectroscopy was used in the MAST-U divertors during Super-X and elongated conventional divertor density ramps with $\text{D}_{2}$ fuelling from the mid-plane high-field side. In the Super-X case (density ramp from Greenwald fraction 0.12 to 0.24), the upper divertor showed ground state rotational temperatures of the $\text{D}_{2}$ molecules increasing from $\sim$6000 K, starting at the detachment onset, to $\sim$9000 K during deepening detachment. This was correlated with the movement of the Fulcher emission region, which is correlated with the ionisation source. The increase in rotational temperature did not occur near the divertor entrance, where the plasma was still ionising. Qualitative agreement was obtained between the lower and upper divertor. Similar rotational temperatures were obtained in the elongated divertor before the detachment onset, although the increase in rotational temperature during detachment was less clearly observed as less deep detachment was obtained. %In the elongated conventional divertor there was some qualitative agreement of this effect impeded by low signal. The measured vibrational distribution of the upper Fulcher state (first four bands) does not agree with a ground state Boltzmann distribution but shows a different characteristic with an elevated population especially in the $\nu = 2$ and $\nu = 3$ bands. The populations of the $\nu = 2$ and $\nu = 3$ band relative to the $\nu = 0$ band are roughly proportional to the $\textit{rotational}$ temperature.