Cryogenic pumping of helium, hydrogen, and a 90% hydrogen-10% helium mixture

An experimental program to measure the pumping speed of an Excalibur CVV2008 liquid helium cryosorption pump is described. Pumping speeds are reported for helium, hydrogen, and a 90% hydrogen -10% helium mixture in the pressure range 1×10−7-1×10−4 Torr. The helium pumping speed at 2×10−7 Torr was 3.2 l/s cm2, but decreased with loading to 1.6 l/s cm2 at 6×10−5 Torr. Attempts to operate at higher helium fluxes (Q≳8×10−5 sccm/s cm2) resulted in pressure and eventually thermal instabilities. The pumping speed of hydrogen at 5×10−7 Torr was 4.9 l/s cm2 and increased with pressure to 7.7 l/s cm2 at 5.5×10−6 Torr. For a 90% hydrogen-10% helium mixture, pumping speeds were inconsistent, varying between 1.4 and 5.6 l/s cm2. Residual gas analysis provided some insight into this behavior and these results are discussed.