Performance assessment of condensation pumping

Abstract

The main relationships which are of interest for describing the performance of condensation pumping are the variations of sticking coefficient and desorption rate for a particular gas species with gas impingement rate, total quantity of gas condensed, incident gas temperature and surface temperature. Since sticking coefficients near unity are associated with the process of condensation, the design of apparatus for studying the performance of condensation pumping must take account of gas streaming effects unless the condensing surface is extremely small in comparison to the size of the study chamber. These gas streaming effects may be taken account of in the Monte Carlo method of analysing molecular gas flows, and this technique has been used in the analysis of two experiments at Culham for studying the condensation of gases on to liquid helium cooled surfaces. The design of these two experiments is discussed with comments on the technique used to take account of gas desorption rates comparable to gas impingement rates, and on the method which has been developed for measuring small gas flow rates and calibrating ionization gauges. Experimental measurements of the condensation of hydrogen and deuterium have shown that for gas at room temperature the sticking coefficient remains essentially constant at about 0.9 for gas impingement rates, for hydrogen, over the range 4.6 × 10 12 to 1.4 × 10 18 molecules.cm −2.sec −1, total condensed gas loads over the range 5 × 10 15 to 7 × 10 21 molecules.cm −2, and surface temperatures over the range 2.2 to 3.9°K. Contrary to expectations it appears that the ultimate pressure attainable by condensation pumping of hydrogen on to an unshielded condensing surface is about 1 × 10 −9 torr.