First 2D electron density measurements using Coherence Imaging Spectroscopy in the MAST-U Super-X divertor

TL;DR

This paper introduces a novel non-linear inversion technique using Coherence Imaging Spectroscopy to measure 2D electron density and neutral temperature profiles in divertor plasmas, validated on MAST-U data.

Contribution

It presents the first application of Coherence Imaging Spectroscopy for 2D electron density measurements in the MAST-U divertor, with a new inversion method differentiating Doppler and Stark broadening.

Findings

Achieved mean absolute errors of 30% in simulated profiles

Inferred electron densities of 2-3 x 10^{19} m^{-3} in experiments

Results agree reasonably with other diagnostics

Abstract

2D profiles of electron density and neutral temperature are inferred from multi-delay Coherence Imaging Spectroscopy data of divertor plasmas using a non-linear inversion technique. The inference is based on imaging the spectral line-broadening of Balmer lines and can differentiate between the Doppler and Stark broadening components by measuring the fringe contrast at multiple interferometric delays simultaneously. The model has been applied to images generated from simulated density profiles to evaluate its performance. Typical mean absolute errors of 30 percent are achieved, which are consistent with Monte Carlo uncertainty propagation accounting for noise, uncertainties in the calibrations, and in the model inputs. The analysis has been tested on experimental data from the MAST-U Super-X divertor, where it infers typical electron densities of 2-3 $10^{19}$ m$^{-3}$ and neutral temperatures of 0-2 eV during beam-heated L-mode discharges. The results are shown to be in reasonable agreement with the other available diagnostics.