Cryopumping a large accelerator

A cryogenic vacuum pump, designed to lower the base pressure from 2 x 10-6 torr to 3 x 10-7 torr, was installed in the Bevatron in February 1972. The Bevatron, a particle accelerator at the Lawrence Berkeley Laboratory, contains approximately 11,000 ft3 of free volume and an excess of 100,000 ft2 of outgassing surfaces pumped by 24 freon-baffled 32-inch oil diffusion pumps. The gas loads consist of metallic and organic outgassing surfaces as well as air leaks from nonrepairable radiation-hardened aged gaskets and mechanical actuators. Nine cryopanels, fed by 20 K recirculating helium refrigerators and 80 K boiling liquid nitrogen, were installed. Monte Carlo computations predict noncondensable (at 77 K) pumping speed of 140,000 liters/sec and a condensable pumping speed of greater than 500,000 liters/sec. Final measurements are included. (U.S.)