Cool down processes of superconducting magnets combined with refrigeration systems are considered. Cool down time may be important in operation of superconducting magnets. It is shown that cool down time is calculated by a few parameters which are weight of magnets, heat inputs to magnets, efficiency of heat transfer between magnets and cooling fluid, and power of refrigeration systems. Graphical charts to estimate cool down time immediately are represented as a result of the calculation. Refrigeration systems are as follows: from ambient temperature to near 90°K cooling by helium gas cooled by liquid nitrogen, afterwards to near 20°K helium gas refrigerators are used, finally liquid helium is transferred to vessels of magnets.
Experimental results of pulse tube refrigerator, carried out to improve its characteristics, are reported. Pulse tube refrigerator is one of the new type refrigeration systems developed by W. E. Gifford, utilizes the phenomena of heat ejection on gas compression and absorption on expansion. In our experiment a temperature decrease of 156°K with a single pulse tube with helium gas was attained, and it was concluded that the minimum attainable temperature is a function of volume ratio, pulse rate, and pulse wave form. Furthermore, it was found that the performance of the pulse tube is improved by extending the heat transfer surface in the tube.