The Cryopumping System for the Indiana University 200 WeV Cyclotron

The cryopumping system for the 200 MeV cyclotron being designed and built at Indiana University is described. The volume of the vacuum chamber is approximately 28,000 liters and the estimated net area is 3.5 × 106 cm2. Two flange mounted cryopump assemblies are used to reach a pressure in the low 10-6 Torr region. Each assembly consistg of a 20°K refrigerator which uses ambient temperature helium gas and a chevron radiation shield which uses helium gas in a closed cycle at a nominal 80°K from a cryogenerator. The pumping speed of each pump is in the region of 30,000 1/s for nitrogen at 1 × 10-5 Torr or less, plus 250,000 1/s for water vapor. Liquid nitrogen is not used for cooling the radiation shield because of explosive hazards. A cryogenic heat exchanger (part of the closed cycle) in a safe location can be used also with liquid nitrogen for cooling the 80°K helium circuit should the cryogenerator be out of use for more than its scheduled maintenance period. A method for cryopumping a beam line is also described.