A novel understanding of the role of plasma-molecular kinetics on divertor power exhaust

During detachment, a buffer of neutral atoms and molecules builds up between the target and the ionising plasma. Collisions between the plasma and the molecules play an important role in the detachment process. Studies of plasma-molecular kinetics indicate that the gas temperature is increased during detachment for a wide range of conditions on the MAST-U and TCV tokamaks. This is related to an increased $\mathrm{D}_2$ lifetime during detachment, leading to more plasma-molecule collisions that raise the molecular temperature. Such collisions subsequently result in significant power and momentum losses to the divertor plasma during detachment. Using a simplified inference, these losses are estimated using the rotational temperature, neutral pressure and ionisation front position. Significant power losses (about $10\%$ of $P_{SOL}$) and dominant momentum losses (majority of the upstream pressure) from plasma-molecule collisions are inferred experimentally in long-legged, strongly baffled, detached divertors (MAST-U Super-X divertor), consistent with SOLPS-ITER simulations. The vibrational distribution obtained is compared to a collisional-radiative model setup using the same rate data as SOLPS-ITER, indicating some qualitative agreements and disagreements, potentially highlighting model gaps with regard to the default rates used. These interpretations highlight the importance of plasma-molecular collisions, leading to power and momentum losses during detachment. Our analysis and reduced modelling of these processes provide further insights into detachment control observations, the workings of long-legged divertors and divertor power balance.