Rarefied Background Flow in a Vacuum Chamber

Spacecraft propulsion systems, such as Hall thrusters, are designed and tested in large vacuum chambers. The pumping capacity of modern facilities makes it possible to maintain pressures as low as 3 10 4 10 Pa. In this study, several free molecular models are developed to analyze the background flow inside a vacuum chamber. These models lead to various sets of analytical results including velocity distribution functions for the background flow and formulas to compute the vacuum pump absorption coefficient. In the present study, the models are applied to analyze the rarefied background flow in a specific vacuum chamber. The results indicate that, with specific parameters, the background flow can have a significant nonzero mean velocity and cannot be considered to have a Maxwellian velocity distribution. Several modifications of the background flow treatment for numerical simulations are proposed with the aid of these analytical results.