Core/divertor/wall particle dynamics in the DIII-D tokamak

A wall model developed for the analysis of Tore Supra wall loading experiments has been applied to an experiment on DIII-D which demonstrated a substantial capacity for retention of deuterium gas in an all-graphite environment, and which showed the efficacy of the pumped divertor to deplete a gas-loaded wall. The Tore Supra model has been extended and applied to evaluate the particle exchange mechanisms between the core, divertor, and wall. Data-constrained plasma modeling is done for the discharges of the load/unload sequence. The poloidal distribution of the charge exchange flux profile to the divertor and outer wall is determined from the Eirene neutral transport code, to estimate the effective working areas for particle exchange and saturation. The deposition and saturation of the hydrogenic efflux in the aC:H layer and graphite is modeled with the 1-D WDIFFUSE code, applied to the regions fuelled by charge exchange flux to predict the instantaneous local wall recycling coefficient. A mechanism is proposed to explain the previous paradoxical result that exhaust quickly (approximately 3 s) balances the only particle input, due to beam fueling, whereas a long term net wall depletion is observed over approximately 10 discharges. The saturation and depletion of wall layers fuelled by energetic charge exchange particles provides such a mechanism.