A common way to provide cooling below the normal boiling point of helium (4.2K) is to reduce the pressure above the bath of liquid helium, thus also reducing the bath’s equilibrium saturation temperature. For example, to produce a 1.8K bath of liquid helium, the saturation pressure has to be reduced to 1638 Pa (16 mbar). There are two primary approaches to producing and maintaining these sub-atmospheric pressures. Small laboratory-based systems tend to use room temperature vacuum pumps to pump away the helium vapor and reduce the bath pressure. Large-scale cryogenic systems instead accomplish this task by the use of cold compressors.

Cold compressors are turbomachines that pump off helium vapor at sub-atmospheric pressures and raise its pressure up to or close to atmospheric pressure while the gas is still at cryogenic temperatures. The use of cold compressors results in a number of advantages, including the near elimination of large diameter warm piping containing sub-atmospheric helium gas. Such pipes are bulky and can be a source of air leaks into the low pressure helium, resulting in contamination and subsequent blockage of the cryogenic system. In addition, by boosting the return gas up to atmospheric pressure while still cold, cold compressors simplify the recovery of refrigeration on the low pressure side of the cryogenic plant cycle.

Typically a single cold compressor cannot raise the pressure all the way from 16 mbar to atmospheric pressure, so a string of several cold compressors in series is employed. In some applications, such as the CEBAF accelerator at Jefferson Lab and the SNS machine at Oak Ridge, the entire pressure rise from sub-atmospheric to atmospheric is carried out via cold compressors. In others such as the LHC machine at CERN, the final pressure rise to atmospheric pressure is carried out by a room temperature compressor.

The development of reliable, efficient and commercially available cold compressors has been a significant factor in the large scale use of He II (superfluid helium) as a coolant. This is a result of extensive research and development funded and driven by accelerator and fusion laboratories and represents a significant technology transfer from these facilities. An early use of cold compressors occurred in the He II cryogenic system for the Tore Supra tokamak in Cadarache, France, in the 1980s. This was followed by extensive development at Jefferson Lab for the CEBAF and SNS machines and at Fermilab for the Tevatron Upgrade project. CERN also funded a significant cold compressor development project in support of the LHC machine. Cold compressors will play an important role in the cryogenic systems in a number of new accelerator projects, including the FRIB machine at Michigan State University, the 12 GeV Upgrade at Jefferson Lab, the XFEL machine at DESY and the European Spallation Source in Lund, Sweden.

Numerous technical papers describe the development and use of cold compressors. A description of the early applications at Tore Supra may be found in “Design of the Cryogenic System for the TORE SUPRA tokamak”, G. Claudet et al., Cryogenics Vol. 26 (1986). Later papers include: “Design, Fabrication, Commissioning and Testing of a 250 g/s 2K Helium Cold Compressor System,” V. Ganni et al., Adv. Cryo. Engr. Vol. 47A (2002); “Determination of the Optimal Operating Parameters for the Jefferson Lab’s Cryogenic Cold Compressor System,” J. D. Wilson et al., Adv. Cryo. Engr. Vol. 49A (2004); “Surge Recovery Techniques for the Tevatron Cold Compressors,” A. Martinez et al., “Performance Assessment of 35 Cold Hydrodynamic Compressors for the 1.8K Refrigeration Units of the LHC,” F. Millet et al. and “Experimental Results Obtained with the Air Liquide Cold Compressor System: CERN LHC and SNS Projects,” F. Delcayre et al., all in Adv. Cryo. Engr. Vol. 51B (2006). A recent example of the use of cold compressors in the cryogenic system for a fusion machine is found in “Performance of Upgraded Cooling System for the LHD Helical Coils,” S. Hamaguchi, et al., Adv. Cryo. Engr. Vol. 53B (2008).