Joule Thomson coolers
Joule-Thomson coolers provide cooling by isenthalpic expansion of a gas through a restriction. The efficiency of Joule-Thomson coolers is lower than that of Stirling and Pulse Tube coolers, so Joule-Thomson coolers are used to extend the cooling range of those coolers down to 4 K. The various Joule-Thomson coolers used in space can be classified by the compressor technology employed.
The same compressors used by the Stirling or Pulse Tube coolers, in combination with reed valves, are used to provide the high and low pressure side. This technology has been developed by RAL (UK) in the past. RAL is providing the 4 K Joule-Thomson cooler system for Planck.
Chemical sorption compressor Joule-Thomson cooler
(H2 sorption cooler)
This compressor uses the chemical sorption and de-sorption of hydrogen on metal hydrides to provide the low and high pressure side for a Joule-Thomson cooler. This technology has been developed by JPL in the past and is currently being used on the Planck spacecraft.
This compressor uses the physical sorption of gases on specific materials, for example activated charcoal. The advantages compared to the chemical sorption compressor are that compressors for all fluids can be build and that there should be no degradation over time.
However, depending on the working fluid, the compressor needs to operate at low temperatures. The University of Twente successfully developed a helium Joule-Thomson cooler, providing 5 mW of cooling capacity at 4 K and operating at around 50 K. This cooler is currently baselined for vibration free cooling on Darwin.
Last update: 20 June 2007