Investigating the physics of detachment in alternative divertor configurations using imaging spectroscopy diagnostics

Alternative divertor configurations are being investigated as a solution to the exhaust challenge in nuclear fusion reactors.
In this work, imaging techniques are developed and used to characterize the physics of these configurations on the MAST Upgrade tokamak. A coherence imaging analysis technique is developed and characterized to infer 2D profiles of electron density in the divertor, while a neural network was trained on camera data of multiple spectral transitions to separate contributions to the deuterium Balmer alpha emission from different types of plasma-neutral interactions. Analysis of experimental discharges in L-mode and H-mode shows significant exhaust benefits in the Super-X and X-point target divertor configurations. The power and momentum losses increase as the divertor leg is swept to a larger major radius into a Super-X configuration, leading to lower electron density, peak particle flux, and peak heat flux at the target, and demonstrating the better exhaust performance of the Super-X as a divertor concept. The electron density is peaked at the target even in strongly detached conditions, which is attributed to large momentum losses induced via collisions with the molecules. Some discrepancies are observed with interpretative SOLPS simulations of the discharges, including the electron density at the target and the spreading in the private flux region due to drifts, which must be investigated further to increase confidence in the design of future reactors. Initial observations in the X-point target configuration show a further increase in performance compared to the Super-X, partially attributed to a broader electron density profile and the resulting additional plasma-neutral interaction volume available in the X-point target, leading to more hydrogenic emission and volumetric recombination, and lower peak heat fluxes to the target.
These results pave the way for a “mix-and-match” approach to divertor design in which multiple divertor strategies are combined to obtain a suitable solution to the specific reactor design under consideration.