The effect of divertor baffling on Alcator C-Mod discharges

The effect of divertor baffling on Alcator C-Mod [Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] discharges has been investigated using a novel divertor bypass. The bypass allows the in situ variation of the mechanical conductance from the divertor plenum to the main chamber between or even during discharges. The results indicate a strong dependence on baffling for the compression of deuterium and recycling impurity gases in the divertor plenum in a variety of C-Mod discharges, indicating that tight baffling would be desirable in a future tokamak reactor. There is no apparent effect of the divertor baffling for the Ohmic, L-mode and EDA (Enhanced Dα) H-mode discharges in this study on main chamber neutral particle populations, scrape-off-layer flows, nonrecycling impurity screening, global energy confinement or H-mode threshold powers.