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MgB 2 bulk magnets are one of the attractive bulk superconductors because they have homogeneous trapped field distribution, low material costs, lightweight, and so on. Pulse-field magnetization (PFM) also has some effective advantages in the point of view of compact setup, shorter magnetizing time, and an inexpensive way to magnetize the bulk samples. However, the trapped field by PFM is inferior to those by field cooling magnetization due to the heat generation caused by sudden changes of the magnetic flux motion. Especially, the temperature rise is remarkable for MgB 2 bulk magnets because of its low specific heat. In this study, we have carried out the PFM technique for five-stacked or three-stacked MgB 2 bulk samples to analyze the flux-invasion behavior. These samples were fabricated by ball-milling and hot-pressing. As a result, maximum trapped field of B T = 0.64 T was obtained at 14.6 K at the center of the topmost sample when the external magnetic field B ex of 0.90 T was applied. As B ex increases further, B T were dramatically decreased because of heat generation. Furthermore, flux jumps frequently took place in the applied field higher than 2.0 T.

We developed a desktop-type superconducting bulk magnet using a Stirling cryocooler with the aim of miniaturizing the system. This refrigerator was compact and highly energy efficient because a compressor was not needed outside; power consumption was approximately 160 W. Moreover, the length of the magnetic pole was decreased by restricting the activation method to pulsed field magnetization. A GdBCO bulk material 45 mm in diameter and 20 mm thick was attached to the cold stage of a refrigerator and was covered with a vacuum chamber. The sample was cooled from room temperature to 52.3 K in approximately 4.5 h. When a single pulsed field was applied in amplitudes varying from 0.8 to 7.0 T, the maximum trapped field was approximately 2.8 T at the center of the bulk surface in the applied field of 7.0 T. Moreover, the maximum magnetic field on the chamber's surface was approximately 1.3 T, and the total magnetic flux was 0.94 mWb. The operational status after magnetization was good, and the operating sound was small.

A large, high-J c bulk superconductor can trap a high magnetic field via the field cooling method, whereas a magnetic flux has difficulty penetrating the bulk due to the strong magnetic shield of pulsed-field magnetization (PFM). We have proposed a method of opening a small hole in order to supply the magnetic flux into the bulk easily by creating a weak part of the magnetic shield, and we have evaluated the trapped field performance when varying the size and number of holes. In our previous research, the holes were drilled at a growth sector region (GSR) based on the idea that the magnetic flux entered from a GSR with low J c and was trapped at a growth sector boundary (GSB) with high J c . On the other hand, when a hole is placed at a GSB, it can be expected to suppress the heat generation caused by pinning loss at the flux penetration and consequently to improve the trapped field. In this paper, trapped field performance is investigated in the holed bulks at a GSB and a GSR. The experimental results suggest that placing a hole at a GSB is advantageous in trapped field performance as compared with placing a hole at a GSR.