Experimental investigation of steady state power balance in double null and single null H mode plasmas in MAST Upgrade
Accurate quantification of the power loss channels and overall power balance of the plasma is essential for the safe operation of current and future fusion devices. The relative magnitudes of core radiation, SOL radiation and divertor target heat loading are determining factors in both PFC design and plasma scenario design in the successful realization of next generation tokamaks. The choice of double null vs single and advanced divertor configurations will require accurate predictions of the power loading on PFCs, and these predictions in turn must be informed by experimental measurements in relevant plasma scenarios.
Power balance studies have previously been conducted on MAST-U [1] in Ohmic L mode plasmas with Ip=450 kA and <300 kW of power entering the SOL. We here present more recent studies of 750 kA H mode plasmas with up to 3.5 MW of neutral beam heating, 1.5 MW of which enters the SOL. Diagnostic coverage is greatly improved with IR thermography available for all 4 strike points (inner and outer, upper and lower divertors), upper and lower divertor radiated power measurements from foil bolometers, lower X point and divertor radiated power measurements from an infra-red video bolometer (IRVB) and Langmuir probes to assess additional particle flux on non-target surfaces such as the divertor nose. Estimates of the absorbed NBI power modelled with TRANSP, Ohmic heating power, core radiation measured by bolometry and the rate of change of stored energy are used to quantify the power entering the SOL.
Conventional and Super-X divertor configurations are presented, in both connected double null (DN) and upper (USN) and lower (LSN) single null geometries where the distance between separatrices ranges from 10% to 200% of the SOL heat flux width. Preliminary analysis of DN and LSN Super-X plasmas has revealed significant differences in the power distribution. In the connected double null the upper and lower inner strike points receive approximately balanced heat flux, as do the upper and lower outer strike points, whereas in LSN the inner divertors receive a 1:2.5 up/down split and the outer divertors a 1:3 up/down split. Radiative losses are enhanced at the lower inner leg in LSN compared to DN, and in the outer leg the lower divertor radiates more strongly than the upper divertor in LSN whereas the upper leg radiates more strongly in DN.
[1] Lovell et. al., “Power balance studies on MAST Upgrade”, PSI-25 (2022)