Comparison of Magnetizing Characteristics of Superconducting Bulk Magnet Cooled by Stirling and GM Refrigerators During Pulsed Field Magnetization

Abstract

We developed several type superconducting bulk magnets with the goal of their industrial application, and study to improve a magnetic field by pulsed field magnetization. It is important to select a suitable refrigerator to cool a bulk superconductor. This paper investigates magnetizing characteristics when using different refrigerators. One is a Stirling refrigerator, in which an ultimate temperature was up to 50 K and cooling capacity was 11 W at 77 K. Another is a dual-stage GM refrigerator with an air-cooled compressor, in which an ultimate temperature was 13 K and cooling capacity was 5 W at 20 K. The other is a dual-stage GM refrigerator with a water-cooled compressor, in which an ultimate temperature was 12 K and cooling capacity was 12 W at 20 K. When a GdBCO bulk with dimensions of 60 mm in diameter and 20 mm thick was cooled by the Stirling refrigerator, the lowest temperature was approximately 55 K. Then, in the other refrigerators, the temperature was adjusted to the same value by a temperature regulator. The bulk was cooled by each refrigerator and a single pulsed field was applied while varying the amplitude of 3.1 to 7.0 T. During magnetization, time variations of magnetic flux density on the bulk surface were monitored. After magnetization, the trapped field distributions were measured on the magnetic pole surface. These data show that even the bulk magnet using the Stirling refrigerator with relatively low cooling ability could trap a high magnetic field. Consequently, the bulk magnet with high cooling capacity could not necessarily generate a high magnetic field.