Evaluation of Pulsed-Field Magnetization on a Superconducting Bulk Magnet System Using a 13 K Refrigerator

We developed a small-size superconducting bulk magnet system using a 13 K refrigerator. The industrial applications of bulk magnets demand the miniaturization of the magnet apparatus as well as the enhancement of the magnetic field. Downsizing of the apparatus can be achieved by restricting the magnetizing method to pulsed-field magnetization (PFM). PFM of a high-performance bulk, on the other hand, presents a difficult problem, i.e., large heat generation after the application of pulsed fields suppresses a trapped field. Therefore, a Gifford-McMahon cycle helium refrigerator with low ultimate temperature and large cooling capacity was adopted with the expectation to increase the J_c and quickly remove the heat. In a previous magnetizing test using a GdBa₂Cu₃O_7-x bulk material, a trapped field of 2.76 T was achieved. In this paper, we evaluate a trapped field when applying a magnetic field of constant amplitude with varying temperature, and we used a multi-pulse technique with a stepwise cooling (MPSC) method in which several pulsed fields were applied as a function of the amplitude and temperature to improve the trapped field. We successfully trapped a magnetic field of 3.02 T.