Initial observations in X-point target divertor discharges on MAST-U

TL;DR

This paper reports on initial high-power H-mode experiments with a novel X-point-target divertor on MAST-U, demonstrating improved plasma exhaust and transient behavior, which could inform future fusion reactor designs.

Contribution

It introduces a new X-point-target divertor configuration combining features of Super-X, showing enhanced exhaust and transient performance in high-power H-mode experiments.

Findings

Broader electron density profile near secondary X-point

Lower target electron temperatures and heat fluxes

Preliminary evidence of improved ELM buffering

Abstract

The first high-power (> 3 MW) H-mode experiments using a double-null X-point-target (XPT) divertor configuration have been performed on MAST-U. The XPT geometry is obtained by combining a large strike point radius, similar to the Super-X divertor (SXD), with an additional X-point near the separatrix in the baffled outer divertor chambers and leads to additional exhaust benefits over the SXD. The broader electron density profile near the secondary X-point leads to additional plasma-neutral interactions, evidenced by a broader hydrogenic emission profile, and resulting in larger power and ion sinks. The increase in plasma-neutral interactions also leads to lower target electron temperatures and heat fluxes. These benefits appear to extend to transients, and preliminary evidence of improved ELM buffering in the XPT is presented. These results showcase how multiple alternative divertor configuration strategies can be combined to improve momentum, power, and particle losses, which may be required for the challenging exhaust conditions of future reactors.