Measurements of the Superfluid Joule–Thomson Refrigerator Using High Concentration 3He–4He Mixtures

The superfluid Joule–Thomson refrigerator (SJTR) uses a liquid superfluid 3He–4He mixture to provide cooling below 1 K. Performance measurements of the SJTR using 5% and 11% 3He concentration mixtures are reported. High concentration operation shows higher cooling powers at high temperature. Ultimate temperatures are seen to increase with increasing concentration due to a pinching of the temperature defect in the recuperative heat exchanger. This pinching effect is due to the variation of the heat capacity of the 3He–4He mixture with temperature and concentration and is discussed in detail and design changes are suggested to mitigate it.