Cryocondensation pumping of thick films of hydrogen and deuterium

In order to obtain engineering data for design of large capacity cryopumps for hydrogen, deuterium and tritium, an apparatus was built where pumping speeds and sticking coefficients can be studied as a function of surface coverage for different leak rates. In addition, thermal conductivity and thermal absorption of these frozen gases can be inferred from the measurements of pumping speed equilibrium pressure and the helium boil-off rate. The following changes in the above parameters were observed when the surface coverage was increased from a few atomic layers to 3.8 x 1020 molecules cm-2 for H2 and 1.2 x 1021 molecules cm-2 for D2: the pumping speed decreased about 10 percent while the equilibrium pressure and helium boil-off were unchanged within the limits of our measuring accuracy; changing the gas leak rate from 0 to 2 x 10-3 Torr liters sec-1 cm-2 also decreases the pumping speed by approximately 10 percent. From the above measurements it is inferred that thermal absorption to frozen H2 and D2 is negligible. Based on the information obtained in our investigation we designed a cryopump having a pumping speed of 105 liters sec-1 for hydrogen