Study of detachment and the processes involved in its dynamics in MAST-U and Magnum-PSI via radiation and emission analysis

The process of divertor detachment in tokamaks is associated with the reduction of power and particles to the targets. This is deemed necessary to limit damage and erosion of solid surfaces and preserve the target. The evolution of the radiation profile is studied in MAST-Upgrade, thanks to the novel Infra Red Video Bolometer (IRVB) diagnostic. The IRVB was optimised to observe the lower x-point and divertor and was successfully calibrated and verified post installation. The movement of the radiation along the divertor legs and inner separatrix was compared with other metrics of detachment. In MAST-U L-mode plasmas, progress of radiative detachment happens in the same sequence as in large aspect ratio tokamaks. Inner leg detachment is gradual, appearing to be contrary to expectations from theory, which is beneficial to detachment control. The IRVB, in combination with spectroscopic measurements, was used to infer that in MAST-U, during detachment of a super-x plasma, hydrogenic emission dominates radiation on the outer leg. This is in stark contrast with another carbon machine, TCV, where carbon dominates instead. ELM-like pulses have been reproduced on the linear machine Magnum-PSI at DIFFER. The target chamber neutral pressure was increased to simulate detachment of the steady state plasma. In some cases the ELM-like pulse energy was completely dissipated in the volume. This can potentially translate to tokamaks, if the ionisation front has sufficiently receded from the target. Despite significant radiative losses, most of the plasma energy losses are due to potential energy exchange. Both molecular assisted recombination and dissociation are important, with the latter being a more efficient path to dissociation than electron impact dissociation. Studying detachment in both machines it is determined that, when the plasma temperature drops below ~5eV, it is necessary to include molecular assisted reactions to accurately model the plasma’s power and particle balance.