Evolution of radiation profiles in a strongly baffled divertor on MAST Upgrade

Plasma detachment in tokamaks is necessary in future tokamaks to sufficiently reduce the heat flux to the target. It involves interactions of the plasma with impurities and neutral particles, leading to significant losses of plasma power, momentum, and particles. An accurate mapping of plasma emissivity in the divertor and X-point region is essential to localise and infer the power losses influencing the detachment process. The recently validated InfraRed Video Bolometer (IRVB) diagnostic, in MAST-U (Federici et al., 2023), enables this mapping with higher spatial resolution than more established methods like resistive bolometers.

In previous preliminary work (Federici et al., 2023a), the detachment of the radiation from the target (radiative detachment) was characterised in L-mode (power entering the scrape-off layer, $P_{SOL}\sim$0.4 MW). With a conventional divertor the inner leg consistently detached ahead of the outer leg, and radiative detachment preceded particle flux detachment. This work presents results also from the third MAST-U experimental campaign, fuelled from the low field side instead of the high field side, including Ohmic and beam heated L-mode shots (with a power exiting the core up to $P_{SOL}\sim$1–1.5 MW).

The radiation peak moves upstream from the target at lower upstream densities than the ion target flux roll-over (typically considered the detachment onset), while radiation on the inner leg detaches before the outer one in high field side fuelled shots and about at the same time in low field side fuelled ones. The movement of the radiation is in partial agreement with the expectations from the DLS model (Myatra, 2021; Cowley et al., 2022; Lipschultz et al., 2016), predicting a sudden shift from the target to the X-point on the inner leg. The energy confinement is found to be related to detachment, but there seems to be some margin between the radiation peak caused by impurity radiation reaching the X-point and confinement being affected, a beneficial characteristic if it could be extrapolated to future reactors. For increasing $P_{SOL}$ the particle flux roll-over happens for similar upstream densities. Comparing the total radiated power with the $D_2$ Fulcher band emission evolution shows that in a conventional divertor a significant fraction of the radiated power is due to carbon radiation (outside of the divertor chamber).