Development Status of a 2.5 K - 4 K Closed Cycle Cooler Suitable for Space Use

Temperatures below 2.5 K have been reached in a continuously operating closed cycle cooler that has been engineered for space applications. This cooler is a modification of a 4 K development model cryocooler that consists of a two-stage Stirling cycle precooler with a closed cycle Joule-Thomson (JT) stage. The Stirling cycle precooler and the compressors for the JT system incorporate all the long life features of the Oxford/RAL single stage coolers which are now in orbit and operating successfully. The precooler now incorporates larger pistons and a slightly modified regenerator; this has led to a lower precooler temperature and increased JT cooling power. A new heat exchanger between the lower and upper stages of the precooler has reduced the heat load on the precooler.

⁴He is used in the JT system for temperatures around 4 K and ³He is used below 3 K. Different JT expansion orifices are used to optimise the cooler at 4 K and 2.5 K. These give flow rates of about 3 mg/s and 1 mg/s respectively. For operation at the lower temperatures the reduced flow enables a lower pressure (and hence temperature) to be maintained on the low pressure side of the expansion orifice.

Under no-load conditions and with no gas in the JT system the precooler reaches a base temperature of 11.3 K. Using ⁴He the JT system achieved 4 K with a 10 mW heat load. With the smaller orifice and using ³He a temperature of 2.46 K was achieved with a heat load (at base temperature) of over 3 mW. The input power to the cooler was approximately 126 W. The temperature stability of the system at 4 K was better than 50 mK over a 4.5 hour period.