Development of a High Field Superconducting Magnet Cooled by a 2 K Cryocooler (2) — Characteristics of 2 K Cryocooler

This paper describes the experimental results of a small 2 K Gifford‐McMahon/Joule‐Thomson type (GM/JT) cryocooler, which was used to cool a high‐field superconducting magnet. We improved efficiency of the 2 K cryocooler by using a double Joule‐Thomson (JT) valve. We also adopted a high capacity compressor to obtain over 1 W cooling capacity at 1.8 K. To reduce the cooling down time, we adopted a bypass line between the 1st stage heat exchanger and JT valve outlet. The 2 K‐GM/JT cryocooler also has 4 K‐operation mode, which is used for long time stand‐by between 2 K‐operation. Every switching between 4 K‐mode and 2 K‐mode needs many valve operations. To control cooling capacity of GM/JT cryocooler requires difficult manipulation of JT valves. To overcome this shortcoming, we developed an automatically controlling unit, which demonstrated easy operation.