Neutral opaqueness and the H-mode pedestal in spherical tokamaks: edge sources in MAST-Upgrade

Neutral density and ionization source rate in the edge of fusion devices are examined using kinetic calculations of the neutral transport, and evaluated against experimental observations, including recent discharges in MAST-U. MAST-U is a spherical tokamak (R=0.8m, a=0.5m, B<=0.7T) with capability to resolve midplane pedestal profiles on both outboard and inboard sides, and which has a number of diagnostics providing information on emission from edge fueling neutrals. The slab-geometry kinetic neutral code KN1D code is used, allowing the velocity distribution of fueling neutrals to be tracked across the edge and pedestal region, with relative simplicity and speed of calculation. The degree to which neutral fueling may set the H-mode density pedestal has often been estimated by considering neutral opaqueness, which indicates the expected screening of neutrals by plasma, and which may be approximated by the product of plasma density and minor radius n x a. Experimental data and associated modeling provide the opportunity to validate this rule of thumb opaqueness against the inferred ratio of neutral penetration length to pedestal width. Simulations can further inform the prospects for increasing the opaqueness of MAST-U. This analysis will be compared to prior results in other devices including Alcator C-Mod (R=0.67m, a=0.22m, B<=8T), altogether spanning an order of magnitude in n x a, informing extrapolation to burning plasma devices at even higher values.