Pumping mechanisms for hydrogen and helium in an ion pump are quite different from the usual mechanisms by which other gases are pumped. Their sputtering yield is much lower and their diffusion higher than for heavier gases. Hence the pumping effect at the cathode plays an important role in pumping these two gases, being comparable with or even larger than the standard pumping at the anode. The effect of the presence of heavier gases on the pumping mechanisms for helium and hydrogen has been investigated for various pressures, concentrations, and gas species. The pumping speed for helium is shown to decrease as a function of heavier gas pressure; such a decrease is strongly dependent on helium pressure and on the amount of previously pumped gas. Pumping speed for hydrogen shows very different behavior: both considerable increases and decreases have been observed, depending on hydrogen pressure, heavier gas concentration and species, and temperature. Pumping speed for a getterable gas such as nitrogen, whose pumping mechanisms are well known, has been reported as a function of concentration of xenon for comparison. The various and sometimes contrasting phenomena responsible for these effects are analyzed, and their relative importance is pointed out.

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