Pumping optimization for a very high triangularity divertor in DIII-D

Applying first-flight neutral modelling to a new high triangularity divertor configuration (δ=0.9) finds that by optimizing the pump duct and plenum design, ~90% of the current outer leg pumping efficiency is maintained. DIII-D is planning to remove its upper inner cryogenic pump in order to accommodate plasma shapes with increased triangularity and volume. In the first phase of the iterative divertor installations, core confinement and efficient particle removal will be prioritized over divertor dissipation. Removal of the inner pump and relocation of the outer divertor target inboard away from the remaining cryopump plenum necessarily results in a reduction in pumping efficiency. However, estimates with first-flight neutral modelling find that with proper design, the pumping efficiency reduction at the outer leg can be minimized to ~10%. The model balances fast atomic influx from the target into the pump with molecular flow escaping back out of the plenum. Baffling structures for the outer divertor leg are designed to capture magnetic flux equivalent to four times the predicted heat flux width, with flexibility to move the strike point to maximize pumping.