Divertor detachment in MAST-U’s Super-X Double Null plasmas: SOLPS-ITER modeling and experiment

Experiments at the MAST-U tokamak, supported by simulations, investigate the exhaust benefits of the ‘Super-X Double-Null’ magnetic configuration, particularly whether its elongated, strongly wall-baffled divertor legs enable a broader detachment window than conventional, short-legged designs. This study presents SOLPS-ITER simulations of Super-X Double-Null H-mode plasmas achieving divertor detachment, benchmarked against 2D imaging diagnostics. Simulations of a well-diagnosed density ramp discharge show good agreement with experimental data early in the ramp, though as divertor fueling increases, the measured Fulcher emission peak shifts upstream more slowly than the modeled 10 eV front. Additional simulations assess the effects of input power and fueling location. Increasing power shifts the detachment trajectory to higher densities while preserving its slope, suggesting that real-time control of the front location remains similarly viable at higher power. Fueling location significantly influences the detachment trajectory (defined as the 10 eV front location as function of upstream separatrix density): the detached cold front advances rapidly toward the X-point with divertor fueling, and progressively more slowly when fueling from the high field side HFS midplane, LFS midplane, or plasma core. A preliminary explanation, based on the extended two-point model, is supported by observed increases in scrape-off layer (SOL) ion flows and heat convection as fueling shifts from the divertor to the main chamber. Finally, the role of wall baffling is determined. Without divertor baffles, detachment requires a higher upstream separatrix density, the detachment window narrows by a factor of three, and the 10 eV front advances toward the X-point more rapidly. This is attributed to an additional ionization source upstream of the X-point, present only in the unbaffled case and sustained by recycled neutrals escaping the divertor, which increases volumetric radiation and reduced parallel heat flux into the divertor, accelerating the detached front progression.