SENSITIVITY OF NEUTRAL THROUGHPUT TO GEOMETRY AND PLASMA POSITION IN THE TOKAMAK PHYSICS EXPERIMENT DIVERTOR REGION

Sufficient neutral atom and molecular throughput is essential for the steady-state operation of the proposed Tokamak Physics Experiment toka mak. To predict the throughput, the B2 edge-plasma fluid code and the DEGAS Monte Carlo neutral transport code were coupled globally. For th e day 1 low-power (17.5-MW) operation condition, the recycling coeffic ient for both codes matched at 0.985, implying that for every 1000 ion s striking the divertor plate, 15 are ultimately removed down the pump duct. The neutral molecular density was 2.52 +/- 0.15 x 10(19)/m(3), giving a throughput of 92.6 +/- 5.6 Torr . l/s. Varying the scrape-off length for the plasma extending into the gap between the baffle and t he plate from 0 to 2 cm decreased the throughput by a factor of >2. Mo ving the strike point away from the gap at first increases the through put by lessening the pumping efficiency of the plasma in the gap. As t he plasma is moved even farther away, the throughput drops due to a la ck of source term for neutrals entering the pumped region. Illustratin g the importance of moving the source term, moving the strike point aw ay from the gap but retaining the original plasma in the gap lowers th e throughput by a factor of 10. Altering the curvature of the baffle h as little effect on the neutral solution.