A portable dilution refrigerator

Refrigerating units of two dilution cryostats manufactured in the form of inserts for a commercial portable nitrogen-free cryostat for liquid helium are described. The He-3 circulation in both apparatuses is achieved by its evacuation via its condensation on a wall at a temperature of 0.35-0.5 K. which is cooled by the sorption-based evacuation of 3 He from a separate bath. One of the apparatuses operates periodically. A temperature below 0.1 K is maintained in it for several hours, after which the sorption pump needs to be regenerated for similar to 0.5 h. The other apparatus includes an additional bath with liquid 3He that maintains its circulation during the sorption-pump regeneration thanks to the large specific heat of this liquid. The minimum temperature reached in experiments was 0.035 K. The refrigerators operate at a circulation rate of similar to 2-10 mu mol/s and consume 1-2 1/day of liquid helium for maintaining their operation. The refrigerators are designed to cool photo-detectors or samples in experiments that do not require a high refrigerating capacity.