Cryogenic Supply and Boil-Off Removal for a LHE Cryopump in a 4 Bar SF6 Insulated 1 MV Accelerator

Accelerated clusters of the hydrogen isotopes have been considered for plasma heating and fueling. With a 1 MV high power cluster injector, insulated by 4 bar SF₆, particle currents up to 100 A equiv. are presently under development. The cluster beam is produced at high voltage potential with a gas efficiency ≃5 %. To pump the corresponding gas load of about 250 mbarl/s a LHe cryopump is installed within the high voltage terminal. The paper describes the supply and boil-off system of the cryopump. The cryogenic storage at high voltage potential, rated for a many hours operation, is supplied by means of a 30 cm i.d. lock through the 4 bar SF₆. The boil-off gas is transferred to ground potential during high voltage operation by a voltage controlled polyamid tubing of 19 m in length. This allows a transfer rate of 1.5 l/s even at gas pressures down to 500 mbar. In addition an emergency transfer line can be used having, a transfer rate of 300 l/s, activated only with high voltage switched off. Experiments, where the heat load is buffered by the pressure rise of the cryogenic liquids are also described and discussed.