Monte Carlo analysis of pumping speed test dome performance for several vapour diffusion pump geometries

Abstract

The Monte Carlo technique of analysing free molecular gas flow has been used to study the performance o two designs of test dome in measuring the speed of several different designs of vapour diffusion pump. This study shows how the effective speed of each pump changes with the design of the test dome used, as a result of the change in the angular input distribution of gas molecules to the pump, and that the effective speed of the pump in the presence of the test dome may be as much as 25 per cent higher than the speed with a cosine input distribution to the pump mouth. The speed indicated by the gauge on the ISO recommended test dome corresponds within statistical error limits to the effective speed of the pump mouth in the presence of the test dome. For those who are interested in designing vacuum system tubulation, vapour traps, and even diffusion pumps themselves, to take optimum advantage of the pumping effect of a vapour jet it would be more useful to characterise pump performance by the effective capture coefficient of the top jet vapour stream, rather than by a speed at the pump mouth. This capture coefficient could be calculated by Monte Carlo analysis from normal pumping speed test dome observations, and a knowledge of the internal geometry of the pump.