磁性蓄冷材を使用した小型冷凍機 磁性蓄冷材を用いたＧＭ予冷ＪＴ冷凍機による希釈冷凍機の蒸発ヘリウム再液化システムの性能向上

Abstract

A Joule-Thomson (JT) expansion refrigerator with a Gifford-McMahon (GM) precooler which had magnetic regenerator was installed in a dilution refrigerator for use in long-term experiments without the necessity of helium transfer. A helium circulation system was constructed in which the boiled-off gas was precooled by the GM refrigerator with strong cooling power and liquefied by a JT loop heat exchanger. The dilution refrigerator was operated for two months without liquid helium transfer. The cooling power of the JT refrigerator with magnetic regenerator material Er0·7Ho0·3Ni was about 3.5W and was about 17% larger than that with lead. Then, the performance and stability of the system was much improved with magnetic regenerator material compared with Pb because of the increase in cooling power. The system was able to have some margin for extra boil-off using magnetic regenerator material. The only maintenance needed was supplying of liquid nitrogen to the trap and cleaning it every four days. An automatic refilling system was made for liquid nitrogen trap. A vibration isolation system effectively reduced heat leakage caused by vibration of the GM refrigerator. Nuclear demagnetization was possible with this system and the nuclear stage was cooled down to sub mK temperature. This system is applicable to various low temperature apparatus as well as dilution refrigerator because of little modification of the cryostat.