Cryopump measurements relating to safety, pumping speed, and radiation outgassing

A test cryopump has been constructed to investigate operation in close proximity to a neutral beam, to determine static and dynamic pumping speeds, and to study outgassing such as might be produced by a pulse of nuclear radiation. No difficulty was encountered in operating the cryopump close to a deuterium neutral beam source suitable for a fusion-reactor injector. Static and dynamic pumping-speed measurements agreed well, but were somewhat lower than expectations, probably due to the unusual method chosen to supply liquid helium. Safety tests showed that hydrogen could not be ignited at any subatmospheric pressure resulting from a leak-up-to-air accident. The possible hazard of liquid-oxygen condensation in such accidents was not explored. Tests made with pulses of neutrons and gamma rays produced by a TRIGA reactor showed that the cryopump could be partially outgassed by radiation pulses of sufficient intensity. However, the effect is ascribed to the gamma-ray component of the TRIGA pulse which is about 103 times that expected from a fusion reactor such as TFTR