A Study of Cryopump Configurations in Free-Molecular-Flow Regions

Abstract

The cryopump has been established as a necessary component of vacuum systems in which very low densities and relatively high mass flows are required. Since the condensing efficiency of a cryopumping system depends more on the physical arrangement of thermal shields than on the properties of the gases striking the condensing surface, many cryoarray configurations have been developed. The performance of these arrays has been examined both theoretically and experimentally. Such examinations are usually conducted in the free-molecular-flow region, where two basic assumptions can be made: (1) the molecules interact only with the surfaces (not with each other), and (2) the molecules are reflected from the surface diffusely. Although an exact analysis requires knowledge of the accommodation coefficients, sticking coefficients, and capture coefficients, some arrays (such as the chevron type) are nearly independent of these parameters. These arrays, however, usually have a very low pumping efficiency; i.e., only a small percentage of the molecules entering the array is condensed.