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冷凍機における蓄冷器内の温度分布および流量分布

Abstract

A two-stage GM cryocooler is commonly used in many cryogenic systems, such as a cryopump and an MRI. The lowest temperature for a conventional GM cryocooler, however, was limited to about 10K. Recently, magnetic regenerator materials, which have larger heat capacity below 10K than a conventional regenerator material of Pb, enable the GM refrigerator to achieve the liquid helium temperature level. In this temperature range, helium in the GM cryocooler cannot be regarded as an ideal gas. The purpose of this study is to investigate a regenerator, which is operated with non-ideal helium gas. The experimental results of the regenerator performance in a two-stage Gifford-McMahon (GM) cryocooler are described in this paper. Temperatures and pressure in the regenerator were measured and mass flow rate in the regenerator was calculated from these data. The temperature profile, temperature fluctuation and mass flow rate in the regenerator are compared between ideal gas operation and non-ideal gas operation. For ideal gas operation, the temperature profile was smooth and the temperature fluctuations at each positions are almost equal. For non-ideal gas operation, a steep temperature gradient and large temperature fluctuation, which almost covered the temperature difference of the regenerator were observed from the hot end to the middle of the regenerator. From the middle to the cold end, however, both the temperature gradient and the temperature fluctuation were very small. The mass flow rate in the regenerator for ideal gas operation decreased from the hot end to the cold end. For non-ideal operation, the mass flow rate increased. The difference between the regenerator for ideal gas operation and that for non-ideal gas operation was shown clearly.