Small size helium refrigerating systems

Recently, for cooling the superconducting magnets used for medical nuclear magnetic resonance diagnostic system and for cooling Josephson elements, the development of the small helium refrigerating system of low vibration and maintenance free has been advanced. As the refrigerating system satisfying this requirement, a compact helium refrigerator using super-small, superhigh speed expansion turbines and diffusion joined laminate heat exchangers and a single stage screw compressor were developed. As the results of having carried out the performance test by the combination of these, the efficiency of the first turbine was 41 %, and that of the second turbine was 60 %. Besides, the refrigerating capacity was about 70 W (at 77 K) + 5 W (at 4.5 K). Thus the prospect of putting it in practical use was able to be obtained. The small helium refrigerator is composed of a compressor unit and a cold box containing two expansion turbines, five heat exchangers and Joule Thompson valves. These components are described, and the results of experiment are reported. (Kako, I.)