Magnetic Field Operation and Shielding of a G-M Cryocooler

The operation conditions of Gifford-McMahon (G-M) cryocoolers are limited by the external magnetic field induction. A G-M coldhead with the cooling power of 1.5 W at 4.2 K (RDK-415 by SHI) has been tested in an axisymmetric inhomogeneous magnetic field created by a conduction cooled superconducting magnet, which is able to generate up to 10 T. The tests revealed that the second-stage cooling power decreases by 29% at 1.0 T and by 42% at 1.6 T. The coldhead demonstrated abnormal operation in the magnetic field higher than some threshold value. The threshold field depends on the second-stage temperature. The stacks of YBCO tapes for the second-stage region and an iron shell for the motor region were tested as a magnetic shield. The magnetic field induction fluctuation on the refrigerator surface caused by periodical movement of magnetic regenerator in the inhomogeneous magnetic field is reported.