Strong magnetic field generation by superconducting bulk magnets using various types of refrigerators and considering an efficient magnetization

We are developing several types of superconducting bulk magnets with the goal of their industrial application, and we are studying to improve the magnetic field by pulsedfield magnetization (PFM). It is important to select a suitable refrigerator for cooling a bulk superconductor. This paper investigates magnetizing characteristics when using different refrigerators. The first is a Stirling refrigerator, in which the ultimate temperature was up to 50 K, and the cooling capacity was 10 W at 77 K. The other is a dual-stage GM-type refrigerator, in which the ultimate temperature was 13 K and the cooling capacity was 5 W at 20 K. When cooling and magnetizing tests were carried out using a GdBCO bulk 60 mm in diameter and 20 mm thick, a maximum trapped field of 3.0 T was achieved at 55 K, in the case of the Stirling refrigerator. Moreover, we also studied to improve the efficiency of magnetic flux trapping. Recent upsizing of the diameter and enhancement of the performance of bulk superconductors make PFM difficult, due to a strong magnetic shield. We propose a bulk material with small holes to supply magnetic flux into the bulk efficiently because superconductivity is intentionally lowered in the portion with holes. On the other hand, we are anxious about the flux flow in a high applied field. To investigate the effect of the proposed method, small holes were drilled in a bulk material, and a fundamental experiment was carried out. It was confirmed that the magnetic flux penetrated at a low applied field as compared with the holeless bulk material; moreover, the flux flow was suppressed in a high applied field at a low temperature. Furthermore, we aim to optimize the number, size, and position of small holes.