Life test and performance testing of a 4K cooler for space applications

At the Rutherford Appleton Laboratory we have developed a 4K closed cycle cooler suitable for space applications. This cooler is based around a 20K two-stage Stirling cycle cooler that is used to pre-cool a 4K Joule-Thomson (JT) system. For the purposes of these studies the 20K Stirling cooler was replaced by a commercial two stage Gifford McMahon refrigerator. The JT system relies on the expansion of helium gas through a fixed orifice. There is concern that this could block with contaminants over a period of time. The cooler is designed to try and overcome this problem in several ways; The gas is cleaned and purified before introduction into the cooler, there is a gas purification system based around a hot reactive getter and there are filters on each stage of the pre-cooler. Contamination was introduced directly into the working gas to assess the efficacy of the purification system. The results of these tests are presented together with a strategy for operation of the cooler in orbit. As the maximum flow through the orifice is a strong function of temperature the full cooling power is only attained at base temperature. This could be a problem if there is a constant temperature independent heat load. The flow through the orifice was measured as a function of temperature in order to aid the modeling of the system. Tests were made on the ability of the cooler to cool from the temperature of the pre-cooler with various applied heat loads.