Helium Exhaust Studies inH-Mode Discharges in the DIII-D Tokamak Using an Argon-Frosted Divertor Cryopump

The first experiments demonstrating exhaust of thermal helium in a diverted, H-mode deuterium plasma have been performed on the DIII-D tokamak. The helium, introduced via gas puffing, is observed to reach the plasma core, and then is readily removed form the plasma with a time constant of ~10--20 energy-confinzement times by an in-vessel cryopump conditioned with argon frosting. Detailed analysis of the helium profile evolution suggests that the exhaust rate is limited by the exhaust efficiency of the pump ( ~5%) and not by the intrinsic helium-transport properties of the plasma.