A closed cycle 3He–4He dilution refrigerator insensitive to gravity

The cooling power and the lifetime of an open cycle dilution refrigerator as developed for the Planck mission (100 nW at 100 mK during 30 months) are limited by the quantity of the helium isotopes carried on the satellite at launch, because the helium mixture obtained after the dilution process is rejected into space. Future space missions require to increase the cooling power and lifetime significantly (1 μW at 50 mK during 5 years).Therefore we are extending the open cycle dilution refrigerator with a helium isotope separator operating at 1 K to close the cycle. A first prototype to demonstrate the principle of the closed cycle dilution refrigerator has been tested and a cooling power of 1 μW at temperatures below 60 mK has been obtained. We present the apparatus and the experimental results and give some elements for its integration in a complete cooling chain. The advantages (continuous operation, absence of magnetic field, less weight) of a closed cycle dilution refrigerator with respect to an adiabatic demagnetization refrigerator are also discussed.