A Cryocondensation Pump for Particle Exhaust in the DIII-D Tokamak

For particle exhaust in the advanced divertor configuration of the D3-D tokamak, a cryopump with a pumping speed of approximately 50,000 I/s for D₂ is being designed. The location of the pump inside the tokamak presents several technical challenges, viz., proximity to the high temperature plasma, intense pulsed magnetic fields (2T), electromagnetic forces that result when the plasma current of up to 3 MA abruptly collapses due to MHD instabilities, the requirement for the pump to be compatible with the tokamak wall conditioning glow discharge procedure, severe space limitations in and around the tokamak, restrictions imposed on the allowed materials, and the high degree of reliability needed for the application. A concentric coaxial loop with forced convection flow of two-phase (primarily liquid) helium is selected as the cryocondensation surface. The helium loop will be surrounded by a liquid nitrogen cooled radiation shield, which will be protected from energetic particles emanating from the divertor stroke region. This configuration will permit pumping at pressures of several mTorr, for pulse durations of 5 s, and regeneration of the pump between tokamak discharges. This paper describes the details of the design and the operating scenario of the cryopump.