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The characteristic feature of this installation is the low temperature required: 1.8 K (i.e. 1.8 degrees above absolute zero) and the very low evaporating pressure of a little more than 16 mbar. In accordance with the operating specifications of the cavities to be cooled (superconducting accelerator elements of an electron accelerator), a high constant pressure of 0.1 mbar is required, also on load change.
The installation was realized through a combination of cold and warm compressors. In other words, the still cold helium vapour has first to be compressed to approx. 100 mbar and then warmed to the ambient temperature. Subsequently, the gas is brought to a little over ambient pressure in vacuum screw compressors connected in parallel. The advantage of the combination of two types of compression lies in the favourable partial load characteristic, which plays a key role in this application (stepwise development of the accelerator track and constant load change). In the area of system components, the ceramic ball bearings of the cold compressors deserve special mention; they are easy to handle and, as experience shows, extremely robust.
