Study of detachment and the processes involved in its dynamics in MAST-U with the IRVB diagnostic*

In a tokamak with a conventional divertor geometry, increasing levels of detachment cause the peak in total radiation emissivity to move from near the divertor targets to around the X-point. The InfraRed Video Bolometer (IRVB) diagnostic recently validated in MAST-U[1] allows accurate mapping of plasma emissivity in the divertor and X-point region with higher spatial resolution than more established methods like resistive bolometers.

The IRVB was used in MAST-U to investigate the correlation between particle flux and radiation detachment in a spherical tokamak. During L-mode ohmic density ramps (I_p=600kA, ne_up=0.2-1.2x10¹⁹ #/m³) the radiation is observed to move off the target first on the inner leg up to the X-point while the particle flux is still increasing on the outer target. Then as the particle flux of the outer target rolls-over, radiation moves upstream off that target as well. The movement is gradual with ne_up on both legs, contrary to expectations from the analytic DLS model, which predicts an unstable transition on the inner leg[2,3].

During ohmic super-X discharges the IRVB, together with other diagnostics [4,5], was used to show that inside the super-X chamber, during detachment, most of the radiation is hydrogenic, and molecular effects have a significant contribution.

References: [1] F. Federici, et al. Review of Scientific Instruments, 94(3):033502, 2023

[2] B. Lipschultz, F. I. Parra, and I. H. Hutchinson. Nuclear Fusion, 56(5), 2016

[3] C. Cowley, B. Lipschultz, D. Moulton, and B. D. Dudson. Nuclear Fusion, 62(8), 2022

[4] T. A. Wijkamp, et al. Nuclear Fusion, 63(5):056003, 2023

[5] K Verhaegh, et al. Nuclear Fusion, (submitted), 2023