Simulation of divertor detachment in MAST-U’s Super-X Double Null configuration using SOLPS-ITER and comparison to experiment*

SOLPS-ITER simulations of MAST-U Super-X Double-Null H-mode discharges show that the divertor targets achieve detachment at a much lower upstream separatrix density than conventional shorter leg configurations. As the density increases and the cold detached front moves to the X-point, its speed—defined as the change in front location divided by the increase in upstream density—decreases by a factor of ~3-4 during the transition from the divertor chamber to the main plasma chamber. Comparison to experimental measurements of the ionization front, using D2 Fulcher emission as a proxy, shows that the simulations overestimate both the upstream density required to detach the targets and the variation in density needed to move the front to the divertor throat. The discrepancies are mostly resolved by refining model assumptions, implementing the same fueling scheme as in the experiment (divertor fueling), reducing injected power, and including particle drifts. In an FPP, achieving optimal divertor conditions and core performance requires placing the detached front between the divertor plates and the X-point, very challenging with short-legged magnetic configurations. Elongated divertor leg concepts can mitigate this challenge, and this work explores the Super-X as one such option.