Abstract
A Joule-Thomson (J-T) expansion refrigerator with a Gifford-McMahon (G-M) precooler was installed in a dilution refrigerator for use in long-term experiments such as a nuclear demagnetization experiment without the necessity for 4He transfer. A helium circulation system was constructed in which the boiled-off gas was precooled by a G-M refrigerator with strong cooling power and liquefied by a J-T loop heat exchanger. The dilution refrigerator was operated for three weeks without liquid 4He transfer as a test run for the system. The only maintenance needed was supplying of liquid nitrogen to the trap and cleaning it every other day. A vibration isolation system reduced heat leakage caused by vibration of the G-M refrigerator. Nuclear demagnetization was possible with this system and the nuclear stage was cooled down to submillikelvin temperatures. This system is applicable to various pieces of low temperature apparatus as well as dilution refrigerators because little modification of the cryostat is needed.
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