Control of wall particle inventory with divertor pumping on DIII-D

Reduction of the net wall particle inventory has been achieved in the DIII-D tokamak by the use of a divertor cryopump for the particle exhaust. A sequence of 12 discharges was conducted without helium glow discharge cleaning (HeGDC) and without active cryopump exhaust, resulting in a net wall loading of 1250 torr·L (8.8×10²² atoms) by the end of the sequence. The cryopump was activated for the subsequent set of 10 discharges. At the end of this set of discharges, the net wall loading was reduced to its initial value or less, i.e. the wall loading state for the reference discharges with HeGDC. Plasma performance did not suffer without the HeGDC in that the stored energy during the ELMy phase was restored to the reference discharge level during the discharges with the active cryopump. Hence, with continuous particle exhaust (provided, for example, by a pumped divertor), next generation, long pulse devices, such as ITER, will not require interdischarge HeGDC for particle control, which would require turning off superconducting coils between discharges.