Design and Performance of a Liquid Helium Cryopump Operating in the Viscous Flow Regime

A liquid helium refrigerated cryopump with a 500 mm inlet flange was designed to operate in the viscous flow regime. A multitude of cryopanels in an accordian arrangement was used to provide a large cryopumping area to accomodate the high heat loads associated with the incident gas flux. Pumping speeds for deuterium gas were measured in the 1.3 × 10−4 to 53 Pa pressure range. In the molecular region the pumping speed was 14,600 L/s. As the pump inlet pressure increased into the transition and viscous flow regimes, the pump speed increased rapidly to a value of approximately 120,000 L/s at a pressure of 13 Pa. Measurements at higher pressures showed a decline in speed as the system becomes heat transfer limited. The observed variations in speed over the range of pressures are related to the transition from molecular to viscous flow, and the heat transfer characteristics of condensing deuterium and boiling helium.