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The requirement for a helium refrigerator with high efficiency and compactness has been growing for cooling superconducting devices.We have developed a reciprocating magnetic refrigerator for liquefying helium from the temperature of 20K region which uses gadolinium-gallium-garnet (GGG) for the magnetic material. The working material (GGG single crystal, 30mm in diameter and 40mm in length) is placed at the end of the each piston, and those two pistons are driven by a pneumatic jack. When the working material is placed in a high-intensity magnetic field (4.5T), the temperature of the GGG rises to 15K. The inner surface of the cylinder is cooled by the auxiliary refrigerator (a G-M refrigerator), and heat is removed through the narrow gap (less than 50μm) filled with gaseous helium between the GGG and the cylinder. When the magnetic field given to the GGG is eliminated by moving the piston, temperature of the GGG goes below 4.2K. Here the refrigeration of liquid helium occurs by condensing the gaseous helium on the surface of the GGG.Technical emphasis was placed on the realization of a high heat exchange rate between the solid magnetic material and gaseous helium. A sufficient heat transfer rate was achieved after several component-level experiments. Sources of inefficiency to the refrigeration capacity has been also discussed.Finally the refrigeration power of 0.95W at 4.2K was achieved at 0.38Hz operation.

Development of reciprocating magnetic refrigerator.