The relation between impurity neutral and impurity ion compression in the Alcator C-Mod divertor

The effect of divertor baffling on impurity neutral and impurity ion compression in ohmic discharges in the Alcator C-Mod tokamak is explored. Experiments are performed using a novel divertor bypass which allows in situ variations to the divertor baffling. The results indicate that divertor baffling in Alcator C-Mod plays an important role in impurity neutral compression, but little role in impurity ion compression. The apparent separation between neutral compression and ion compression is surprising, but is consistent with a model in which the neutral compression (both in deuterium and argon) is determined by a leakage flux through the baffle structure from the divertor region to the main chamber which is independent of the leakage conductance. Instead, the leakage fluxes appear to be determined by the divertor plasma conditions (including impurity and fuel ions), which are not influenced by the baffle structure.