Neutral particle dynamics in the Alcator C-Mod tokamak

An experimental study of neutral particle dynamics has been carried out over a wide range of operating conditions in the Alcator C Modified (Alcator C-Mod) tokamak using special purpose pressure gauges. Both divertor and midplane pressures are found to depend primarily on the edge plasma regimes defined by toe scrape-off layer heat transport. While the maximum divertor neutral pressure (30-60 mtorr) is achieved at high core plasma densities ((n) over bar(3) = (2-4) x 10(20) m(-3)), corresponding to the detached divertor state, the maximum ratio of divertor to midplane pressure (similar to 70) is achieved in the high recycling transport regime. Variations in the divertor geometry have a weaker effect on the neutral pressures. For otherwise similar plasmas, the divertor pressure and compression are maximized when the strike point is located at the bottom of the vertical target plate. Modelling shows that the high pressure sustained during detached divertor operation (despite a considerable drop in the recycling source) can be explained by scattering of neutrals off the cold plasma plugging the divertor throat.