Measurements of the Superfluid Joule-Thomson Refrigerator Using High Concentration He-He Mixtures

The superfluid Joule-Thomson refrigerator (SJTR) uses a liquid superfluid He-He mixture to provide cooling below 1 K. Performance measurements of the SJTR using 5% and 11% He 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 He-He mixture with temperature and concentration and is discussed in detail and design changes are suggested to mitigate it.