ANALYSIS OF THE COMPRESSION CHARACTERISTICS OF VAPOR JET INJECTED INTO HIGH-VACUUM

The working principle of diffusion pump, a kind of vacuum pumps, is to use the high momentum of vapor jet which is injected into high vacuum . Since flow regime inside the diffusion pump consists of continuum, t ransition and free molecular, a continuum approach is not valid to cal culate the flow field. In this paper, the compression characteristics of a vapor jet into high vacuum were simulated using the direct simula tion Monte Carlo (DSMC) method. The hard sphere model is used as a mol ecular model and the no time counter (NTC) scheme is employed as a col lision sampling technique. Steady state flow characteristics such as v elocity, temperature, density distributions of the pumped gas and oil vapor are calculated by simulating about hundreds of thousand molecule s. The main result is that the pumping speed of a single stage pump ca n be described based on the transmission probabilities which have near ly constant values irrespective of pressures at the inlet and outlet. Using these features, compression characteristics of diffusion pump ca n also be described by transmission probability concept as in the case of other high vacuum pump.