4. A cryo pump system developed for use in the high voltage centre terminal of a tandem Van de Graaff

Abstract

The Science Research Council are building a 30 MV tandem Van de Graaff accelerator at the Daresbury Laboratory in Cheshire. Negatively charges ions are injected into the Van de Graaff and then accelerated by the voltage gradient to the high voltage centre terminal where charge exchange takes place in a gas canal (or stripper) and a number of electrons are removed. These positive ions are again accelerated in the positive electric field gradient. The design of the vacuum system is to maintain an average pressure less than 1.3 × 10 −5 Pa and to achieve this a differential pumping system is used in the centre terminal to pump the gas flow from the stripper canal while maintaining the accelerator working pressure. This canal is supplied with nitrogen at a throughput of 2.8 × 10 −3 Pa m 3 s −1 , which requires a volume flow rate of 2.1 m 3 s −1 with no external exhaust to atmosphere. Cryo pump heads have been designed using helium closed cycle refrigerators with a storage time of 1600 h before saturation. The refrigerators are required to work in an ambient pressure of SF 6 at 7 bar maximum and to be very reliable.