The effect of baffling on divertor leakage in Alcator C-Mod

The effect of divertor baffling on Alcator C-Mod has been investigated using a novel divertor bypass. The bypass allows in situ variations in the leakage conductance between the divertor plenum and the main chamber. The experimental results, supported by simple modeling, suggest that C-Mod operates in a regime where the leakage flux is independent of the leakage conductance. The relatively large leakage flux is driven by the high gas pressure in the C-Mod divertor and is `flux-limitedʹ by the atom creation rate at the target plate and the atom escape probability from the divertor plasma. These results, along with results from other tokamaks, suggest that baffling needs to be significantly tighter than had been previously thought to ensure low levels of divertor leakage and maximum divertor pressures.