High Power Refrigeration at Temperatures Around 2.0 K

Abstract

This chapter reviews the possible technologies for pumping on the liquid helium bath. It emphasizes on cryogenic dynamic compressors, their bearings and their wheels. It analyzes arrangement of the compressors and their behavior during transient situations. In order to achieve the required high compression ratios, pumping on the liquid helium bath can be made at room temperature, at cryogenic temperatures or partially at cryogenic then at room temperatures. The equipment to be cooled at temperatures around 2.0 K generate heat loads which are lower during periods when the system is not operating at nominal. During turn-down situations it is generally required that the pressure on the helium bath is kept constant, so the compression ratio is to be kept constant when the mass flow rate is lower. For volumetric machines, there is no problem. However, the situation is delicate with dynamic compressors. H. Quack proposes to allow the discharge pressure of the dynamic compressors to decrease with the mass flow, in combination with temperature stabilization at the inlet of the third stage compressor. The disadvantage is to have part of the room temperature system operating at sub-atmospheric pressure. CERN proposes to operate at a lower first stage suction pressure. There is still a significant amount of work to be done in this area.