Description
Plasma exhaust is a major challenge for the realization of magnetic confinement fusion. The X-Point Target (XPT) divertor configuration, characterized by a long-legged divertor and a secondary X-point within the divertor volume, is a promising magnetic geometry for meeting power exhaust requirements in a fusion power plant. It offers several potential advantages, including radiation localization in the divertor, an increased plasma-wetted area, a broader plasma-neutral interaction volume, and reduced target electron temperature and heat flux for fixed upstream conditions [1,2]. Modelling the XPT experimental conditions observed in MAST-U provides model validation and a baseline for evaluating the configuration performances in future fusion power plant scenarios, including ARC.
The study presented here combines MAST-U experimental results with full-domain SOLEDGE3X-EIRENE edge/scrape-off-layer (SOL) simulations. A double-null (DN) XPT discharge is compared with a DN Super-X discharge under matched upstream conditions using modelling to examine transport, heat-flux distribution, target electron temperature, particle flows, and neutral accumulation. The simulations are tuned by benchmarking experimental quantities at the divertor targets and at the outer mid-plane. The spatial distribution of measured Fulcher emission is then qualitatively compared with the modelling using synthetic diagnostics.
The results are then qualitatively discussed in the context of previous SOLEDGE3X-EIRENE simulations of DN XPT configurations for SPARC. Those studies identified characteristic features of the XPT geometry, including target temperatures and heat fluxes lower than those of a conventional divertor configuration, low plasma and neutral density in the private flux region, and reversed parallel flows at the outer target [3]. The present MAST-U study investigates whether these qualitative features also emerge under experimentally accessible conditions.