Comparison between MAST-U conventional and Super-X configurations through SOLPS-ITER modelling

TL;DR

This study uses SOLPS-ITER modelling to compare conventional and Super-X divertor configurations in MAST-U, showing the Super-X can significantly lower detachment thresholds mainly due to increased flux expansion, with neutral trapping being similar.

Contribution

It provides a detailed quantification of how the Super-X configuration enhances detachment access in MAST-U using advanced modelling, highlighting the role of flux expansion and neutral trapping.

Findings

Super-X reduces upstream detachment threshold by up to 1.6 times.

Neutral trapping is similar between configurations.

Strike-point angle variations have complex effects on detachment thresholds.

Abstract

MAST-U has recently started operating with a Super-X divertor, designed to increase total flux expansion and neutral trapping, both predicted through simple analytic models and SOLPS calculations to reduce the plasma and impurity density detachment thresholds. In this study, utilising the SOLPS-ITER code, we are quantifying the possible gain allowed by the MAST-U Super-X and neutral baffling geometry, in terms of access to detachment. We show that a significant reduction of the upstream density detachment threshold (up to a factor 1.6) could be achieved in MAST-U, for the Super-X, as opposed to conventional divertor geometry, mainly through an increased total flux expansion, neutral trapping being found very similar between the different configurations. We also show that variations of the strike-point angle are complex to interpret in such a tightly baffled geometry, and that a case in which the target normal points more towards the separatrix does not necessarily imply a lower detachment threshold. As in previous calculations for TCV, we quantify the role of neutral effects through developing and applying a quantitative definition of neutral trapping.