The Role of Plasma-Molecule Reactions in Influencing detachment.

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Heat exhaust is a critical issue for future fusion reactors and it will need to be mitigated through divertor plasma detachment, in which the divertor plasma undergoes a reduction in momentum, power and particle flux en-route to the divertor target plates. Long-legged, strongly baffled divertor chambers intensify plasma-neutral interactions (MAST-U, TCV, STEP, SPARC, ARC) and facilitate detachment. In such divertors, plasma-molecular collisions, as well as plasma chemistry, can play an important role.This study presents a new insight into plasma-molecular kinetics through analysis of data from MAST-U and TCV, and comparison of this with simplified models, and simulations. High resolution D2 Fulcher band measurements (via divertor spectroscopy) on these machines allow the rovibrational distribution of the molecules to be obtained, providing information on plasma-molecule energy and momentum transfer and interactions. Detailed temporally and spatially resolved analyses of the rotational temperatures in MAST-U (4000-9000 K) [3] and TCV (2000-5000 K), and the vibrational distributions in both devices (v = 0, 1, 2, 3), show a connection between the rotational and vibrational distribution. Different divertor configurations (Super-X, Elongated and Conventional in MAST-U); baffling (with and without SILO baffles in TCV); and different fuelling locations and heating levels in both devices, have been studied. We find that rotational temperature increases during attached and detached conditions before the overall cooling of the divertor. A simplified model including energy transfer during elastic collisions between ions and molecules; molecule lifetime; and transport time, are consistent with these findings. Exhaust simulations are also consistent. Such elastic collisions in high neutral pressures, combined with these longer molecular lifetimes, lead to significant energy and momentum dissipation - i.e. deepening detachment. An isotopic study has also been carried out on TCV.References:[1] K. Verhaegh et al 2021 Nucl. Fusion 61 106014,[2] K. Verhaegh et al 2023 Nucl. Fusion 63 016014[3] N. Osborne et al 2024 Plasma Phys. Control. Fusion 66 025008