In-Situ Gas Conductance and Flow Measurements Through Alcator C-Mod Divertor Structures With and Without Plasma Present

A specialized arrangement of gas-puff capillaries and in-situ pressure gauges has been installed in Alcator C-Mod to allow gas conductances and flows around divertor structures to be measured with or without plasma present in the chamber. Results are presented from a series of dedicated experiments performed during the 1020725 run day (MP#313, " Instrumented Divertor Leakage Experiments "). Gas conductances and flows are compiled in this report with the aim of providing good benchmark points for numerical simulations of gas flow and " plasma plugging " physics in Alcator C-Mod. A strong plasma plugging effect is found at locations where the divertor structure is opened for diagnostics access (the so-called " open-port " locations), as evidenced by a factor of ~4 reduction in the local gas conductance with plasma present. The effect is most dramatic for locations in the upper chamber region where there is no neutral baffle structure; a factor of ~5 reduction in the local gas conductance is found there. Gas conductance through an open divertor flap is measured to be a factor of ~3 lower than that which has been commonly assumed using the area of the open flap. The conductance through the flap is found to be similar with or without plasma present, indicating that there is no " plasma plugging " effect occurring at this location. The gas flow rate through a flap that is adjacent to an " open-port " location is found to be a factor of 2 lower than at a location distant from open ports. The latter result suggests that significant toroidal variations in the pressures exist under the lower divertor modules.