Novel imaging techniques enable 2D physics exploration of the MAST-U Super-X divertor

The strongly baffled Super-X divertor has shown improved exhaust capabilities in MAST-U compared to more conventional divertor designs through additional plasma-neutral interactions. Novel visible imaging diagnostics and analysis, such as multi-wavelength and coherence imaging, enabled 2D divertor plasma parameter inferences. These results have been compared against reduced models and interpretive SOLPS simulations for scenarios with varying total flux expansion and depths of detachment, investigating the effects of plasma-neutral interactions in the evolution of the electron density profile.

Power and momentum losses significantly increase with increasing total flux expansion, decreasing the target electron density as the divertor leg is swept to larger major radius in detached conditions. The separatrix ne profile is observed to peak just downstream of the detachment front or at the target, depending on experimental conditions. This behavior is explained as a competition of ion sinks, lowering the electron density, and neutral drag of the plasma flow, increasing the electron density. In the interpretative SOLPS simulations the electron density is always peaked at the target, suggesting possible inaccuracies in the modeled competition of ion sinks and neutral drag. When the Super-X is further optimised with an X-point target geometry, preliminary evidence suggests a broadening of the electron density profile, resulting in increased plasma-neutral interactions.