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Reactors will operate in the detached divertor regime, with the hot ionizing plasma away from the target, to avoid excessive heat fluxes to the walls. A reduced sensitivity of this detachment front location is advantageous as a passive stabilization measure against variations in the upstream parameters. During transients, it can avoid the hot plasma reaching the target, or the cold neutrals reaching the X-point and possibly degrading the core performance.
Alternative divertor configurations and divertor baffling can affect the detachment location sensitivity profile. They can be investigated on the MAST-U tokamak, either by varying the strike point location in proximity of the baffle or by comparing different magnetic geometries in strongly baffled conditions. While initial investigations focused on conditions with the detachment front on the outer legs within the baffled divertor chambers, recent extensions to the diagnostic coverage allow tracking the front location also outside them. This enables experimental studies on divertor geometries with varying levels of baffling, as well as variations in front sensitivity as the front exits the baffle.
Baffling is shown to have a strong effect on the detachment front sensitivity in L-mode discharges. Shifting the outer strike point away from the baffle entrance to a more open configuration results in a strong increase in the upstream density required for detachment and a much higher sensitivity of the front location, with the front immediately jumping to the X-point and then creating a MARFE. Comparable sensitivity is instead found in conventional and Super-X divertor configurations once the detachment front is outside of the baffle, indicative of the front location being insensitive to the divertor geometry downstream of it for comparable baffling profiles.
The role of fueling location on the front sensitivity is also investigated in fueling-driven detachment conditions, comparing valves in the main chamber, PFR, and in the divertor chamber. The detachment front location is found to agree better when compared at the same divertor neutral pressure instead of the upstream density, highlighting the role of the local neutral pressure as parameter driving the detachment evolution. Fuelling in the PFR is found to be efficient in pushing the detachment front towards the X-point, while fueling in the divertor results in a strongly reduced sensitivity once the front reaches the baffle entrance, in agreement with previous modeling [1].
[1] Roberto Maurizio et al 2025 Plasma Phys. Control. Fusion in press
https://doi.org/10.1088/13616587/ae24ab