Direct simulation of pumping characteristics for a model diffusion pump

Abstract

The pumping characteristics of a single/multi-stage diffusion pump were simulated using the Direct Simulation Monte Carlo (DSMC) method. The hard sphere model is used as a molecular model and the No Time Counter (NTC) scheme is employed as a collision sampling technique. Steady state flow characteristics such as velocity, temperature, density distributions of the pumped gas and oil vapor are calculated by simulating about a hundred thousand molecules. The main results are that the pumping speed of a single stage pump can be described based on the transmission probabilities which have nearly constant values irrespective of pressures at the pump inlet and outlet. Using these characteristics a new design technique of a multi-stage diffusion pump is proposed which is based on the transmission probabilities of pumped gas molecules in each stage, and it is shown that this design explains general features of pumping speed, compression ratio, ultimate pressure and tolerable forepressure.