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 3He 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 3He 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 ∼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 ∼2–10 µmol/s and consume 1–2 l/day of liquid helium for maintaining their operation. The refrigerators are designed to cool photodetectors or samples in experiments that do not require a high refrigerating capacity.
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