Physical Factors Influencing Accommodation Pumps

An experimental study has been made of a number of simple physical factors which were considered likely to influence the pump ratio of accommodation pumps. Single stage pumps operating between room and liquid nitrogen temperature were used throughout. For most experiments ${}^4\mathrm{He}$ gas was used, but pump ratios greater than unity were also measured with ${}^3\mathrm{He}$ , ${\mathrm{H}}_2$ , and ${\mathrm{N}}_2$ . For the smooth arm of the pump various surfaces were tested: Pyrex after a variety of treatments; various other types of glass; metal films. For the rough arm of the pump various modes of roughening were tested. Geometrical effects were tested by various configurations of pumps. The effect of thermal gradients was examined, as well as the effects of standard vacuum operations. Major conclusions were that vacuum heated Pyrex was the best smooth surface, that several methods of roughening surfaces were equivalent, that Ti was the only metal film tested which was of interest as a smooth surface, that geometrical effects were significant, but the effect of thermal gradients was minor.