Transition regime behavior of supersonic beam skimmers

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Abstract
The direct simulation Monte Carlo method is applied to typical skimmer flows associated with the production of molecular beams from a supersonic stream. Attention is concentrated on the skimmer interference arising from intermolecular collisions when the mean free path in the stream is of the same order as the diameter of the skimmer entrance. The effects of skimmer geometry, Knudsen number, molecular model, speed ratio, and surface temperature have been investigated. It is found that the onset of skimmer interference is best correlated by a modified Knudsen number based on the collision cross section at the stagnation temperature, rather than the cross section for the undisturbed stream. The interference becomes serious at modified Knudsen numbers of order unity. There are qualitative differences between the flows in slender and wide angle skimmers, the most striking effect being a very sudden breakdown of the flow in a slender skimmer.

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