Affect on pumping-speed measurements due to variations of test dome design based on Monte Carlo analysis

Abstract

Test domes for measuring the pumping speed of cryopumps are defined in Pneurop standard PN5ASRCC/5 and the American Vacuum Society Recommended Practice for Cryopumps published in 1999. The test domes try to provide a measure of the speed of a pump as if the cryopump is mounted in the center of a large flat plate in a large chamber. This is referred to as the intrinsic speed. In this case, the flow distribution into the pump is diffuse while in the test dome the flow pattern has a stronger axial component. The location of the pressure gauge is intended to compensate for the different flow pattern. Certain parameters in the design of the test domes are not rigidly specified. A Monte Carlo analysis (MCA) has been carried out to analyze the differences in speed that will be measured for different allowable configurations of the test dome. The results show that the variations that are allowed in the recommended practice have little impact on differences in the pumping speed that will be measured. The analysis shows that the axial flow is greater and more uniform in the two-gauge dome than the single dome. The different flow patterns have no impact on pumps that have “porous” inlets (uniform capture probability from all angles) and the recommended location of the pressure gauge gives measured speeds that are very close to the intrinsic speed. Pumping speed measurements of a real cryopump are influenced by the flow pattern. The APD Cryogenics M8 cryopump was analyzed using MCA and found to have calculated speeds that are about 9% higher than the intrinsic speed in the single dome and 11% higher in the two-gauge dome. The probability of Ar molecules being captured by an M8 cryopump in either test dome is about 11% greater than in a large chamber.