Influence of the Position of a Small Hole on the Trapped Field Performance in a Holed Superconducting Bulk Magnet

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.