PARTICLE EXHAUST MODELING FOR THE COLLABORATIVE DIII-D ADVANCED DIVERTOR PROGRAM

A principal objective of the collaborative DIII-D Advanced Divertor Program (ADP) is to achieve density control in H-mode discharges with edge biasing and with continuous particle exhaust at a rate determined by the external fueling sources (typically 20 Torr. L/s). The divertor baffle-bias ring system has been optimized for pumping speeds almost-equal-to 50 000 L/s with the neutrals transport code DEGAS. With an entrance slot conductance of 50 000 L/s, a pumping speed of the same order is required to remove half of the almost-equal-to 40 Torr. L/s that enters the baffle chamber for typical H-mode discharges. Increasing the exhaust fraction with higher pumping speed is self-limiting, owing to the attendant reduction of the recycling flux. The effects of pumping on the plasma core, scrape-off layer (SOL), and divertor have been estimated with a model that self-consistently couples the transport in these regions. The required almost-equal-to 50 000 L/s pumping speed can be achieved with either titanium getter pumps or cryopumps. Evaluation of both systems has led to the conclusion that cryopumps will be more compatible with the environment of the DIII-D divertor.