The physics of plasma detachment in the novel MAST-Upgrade Super-X divertor

MAST-Upgrade is a new spherical tokamak with a tightly baffled, double-null Super-X divertor. Its first results show a ~ x2 reduction in the detachment threshold (line-averaged density) compared to a conventional divertor. In this presentation we show the first MAST-U Super-X detachment analysis obtained utilising visible spectroscopy to explain the macroscopic divertor plasma behaviour from its microscopic origin: plasma-atom/molecule interactions. Our measurements show that the tightly baffled divertor elevates the molecular density, resulting in an unprecedented impact of plasma-molecular interactions on plasma detachment. This work has implications for plasma-edge modelling and data analysis of tightly baffled divertor designs.

During detachment, first the ionisation source detaches from the target, leaving behind a region with elevated molecular densities. In that region, molecular ions are formed that interact with the plasma, resulting in strong hydrogen emission, ion sinks through molecular activated recombination (MAR) and neutral atom sources from molecular activated dissociation. The MAR Ion sinks are significant compared to the ion source and ion target flux and occur earlier in the detachment sequence than electron-ion recombination (EIR). EIR only has a similar magnitude to MAR when target electron temperatures of < 0.3 eV are reached.

Comparison of our experimental results against SOLPS-ITER modelling indicates that plasma-molecular interactions are strongly underestimated in modelling. However, once corrected rates for molecular charge exchange were implemented in Eirene, plasma-molecular interactions are stronger, resulting in a greatly improved agreement between experiment and simulation.