Characterization of 2D atomic and molecular emission processes in MAST-U super-X divertor during detachment
In this work a spatial dimension is added to the description of radiative detachment in MAST-U super-X divertor plasmas. This magnetic configuration is hypothesized to achieve improved heat load spreading at the divertor target and more particle-, heat- and momentum losses through plasma-neutral interactions than conventional exhaust solutions. Filtered camera imaging using the Multi-Wavelength Imaging (MWI) system facilitates simultaneous recording of six deuterium Balmer lines, an impurity carbon ion line, molecular deuterium Fulcher band emission and two neutral helium lines. Following absolute intensity and spatial calibration of the camera data, tomographic inversion techniques are used to produce 2D emissivity profiles in the poloidal plane. During a strike-point sweep, the on-target peak in deuterium Balmer emission follows the separatrix-wall intersetion from EFIT++ to within a 20 mm difference, suggesting good spatial accuracy of the inversion. Absolute emissivity values are evaluated by integrating the inverted profile over lines-of-sight of the spectroscopic Divertor Monitoring System, yielding errors of a few tens of percent. The spatially resolved nature of the MWI output sheds light on locked-mode induced strike-point splitting in MAST-U. Fulcher band emission front analysis suggests that electron temperature ratios between the two split branches of the outer leg varies from one magnetic geometry to the other. Generally speaking split branchesare found to enter the various stages of detachment at different moments in time, meaning 2D resolved emission analysis is of additional importance in these scenarios. Super-X fuelling scans are used to describe the various stages of detachment encountered in MAST-U. First signs of detachment appear in off-target movement of C-III impurity radiation, followed by emission fronts of electron impact excitation dominated deuterium Balmer lines and the molecular deuterium Fulcher band. The latter two were empirically found to follow the trailing edge of the ionization region in earlier work, and are observed to detach earlier in super-X than in conventional configurations. At higher densities, molecular activated recombination and dissociation elevate Balmer emission intensity. In this phase the emission legs are found to expand radially, likely due to the time scales involved in the chain of molecular reactions. When temperatures drop below ∼1 eV, the molecule assisted emission fades, and eventually atomic deuterium recombination emission appears at the target as inferred from Balmer line ratio evolution. The 2D imaging confirms that at the highest divertor fuelling rates this emission region moves away from the divertor target.