In this paper the results of the investigation of a one-shot cooling technique, called adiabatic expansion of3He in superfluid4He, are reported. The expansion cooler basically consists of an expansion cell and a4He reservoir connected by a superleak. In the expansion cell nearly pure3He is gradually diluted to a saturated mixture by the injection of superfluid4He from the4He reservoir. The expansion of the3He produces cooling, which, in the ideal (isentropic) case can lower the temperature by a factor 4.56. In practice, the performance of this cooling method is limited by irreversibilities and heat leaks. In this paper several irreversible processes such as heat conduction, viscous effects, and supercritical4He flow, have been analyzed. Furthermore the effect of3He in a sinter layer in the expansion cell is discussed. The experiments have shown that the fountain pressure in the4 He reservoir is very suitable for driving the4He in and out of the expansion cell. During an expansion/extraction the4He chemical potential difference across the superleak is zero. The realised cooling factor, defined as the ratio of the initial temperature and the final temperature, is about 3.5 for initial temperatures between 20 mK to 190 mK. This value is lower than the ideal factor of 4.56 that can be obtained for isentropic expansions. The discrepancy is mainly due to the relatively large heat leak. The lowest temperature obtained in this investigation was 5.7 mK. The analyses have revealed no fundamental limitations for obtaining lower temperatures.
Voncken, A. P. J., & Waele, de, A. T. A. M. (1995). Adiabatic expansion of 3He in 4He at very low temperatures. Journal of Low Temperature Physics, 100(5-6), 463-499. https://doi.org/10.1007/BF00751521