After magnetization, high-temperature superconducting (HTS) bulk materials can trap strong magnetic fields. Despite the relatively small size of HTS bulks, the peak trapped magnetic field can be stronger even than permanent magnets and electromagnets. Strong magnetic fields are very useful in industrial applications because they can increase the power output of rotating machines for generators and propulsion, or make them smaller. If the field poles of the rotating machine are replaced by magnetized HTS bulk from permanent magnets, the power output of the rotating machine can be increased due to the increased peak magnetic field. In that case, however, the size of HTS bulk is a limitation of the total flux of the machine. Increasing the total magnetic flux will increase its power output, without changing the size or weight. Thus, the use of large HTS bulks would be beneficial but their manufacture is challenging due to the limitations of the single-seed melt-growth process. Alternatively, multi-seeded melt-growth techniques are available. But multi-seeded bulks often have limitations in field trapping capabilities compared to single-seeded bulks. Instead of increasing the size of the HTS bulks, increasing their number is a possibility. Aggregating many bulks together can increase the total magnetic flux. Arranging several smaller bulks together is also useful to scale and shape large field poles. We have previously fabricated a prototype rotating machine with a larger total magnetic flux by integrating multiple HTS bulks in a two-dimensional array and obtained excellent power characteristics [1]. However, the method of magnetizing the bulk is an important issue, since a high magnetic field must be confined to the HTS bulk inside the fully assembled rotating machine before it can be used. The magnetization of an array of bulk is challenging. Until now, the experimental magnetization of arrays of bulks has been mainly done using field-cooling (FC) magnetization. This technique requires large superconducting magnets and a long magnetizing time. FC is consequently limited for practical use because it is structurally and costly impractical to allow for the magnetization of HTS bulks by the large superconducting magnet that individual rotating machines have inside. In contrast, pulsed-field magnetization (PFM) is the magnetization method that can solve the major problems caused by FC. Unlike FC, PFM, which is realized by small and inexpensive devices, relies on the application of a short magnetic field pulse. Due to the rapidity of PFM, with the magnetic field usually reaching several tesla and lasting several hundred milliseconds, copper magnetizing coils are suitable to apply the pulsed magnetic field. Copper coils should be cooled to withstand excessive pulsed currents, however, fundamentally don't need the same cryogenic system as superconducting magnets. So, the magnetizing equipment can be smaller and less than 1/10 of the price of FC equipment. Yet, a major problem subsists, which is the method of magnetization. The use of a PFM technique to magnetize multiple HTS bulks has been little studied, particularly in the case of HTS bulk arrays with non-alternate polarities. In this presentation, we show the experimental results of the magnetization of multiple GdBaCuO bulks using a PFM technique. Three square superconducting bulks have been arranged to reproduce a part of the internal design of our HTS motor [1]. The HTS bulks and coils are cooled to a temperature of 77 K by liquid nitrogen. We show the experimental results of the trapped magnetic flux density in the bulk array by PFM obtained using a prototype of magnetizing coil that we have fabricated for the bulk array. Since the magnetization sequence is important for the trapped magnetic field and the geometry of the coil and bulks, we discuss it based on our experimental results.

[1] M. Watasaki, M. Izumi, M. Miki, C. Bocquel, E. Shaanika, K. Yamaguchi, T. Ida, S. Englebretson, R. Chin, M. Morita and H. Teshima, “Stability model of bulk HTS field pole of a synchronous rotating machine under load conditions”, Supercond. Sci. Technol., vol. 34, no. 3, Art no. 035015, Feb. 2021.

Keywords: Pulsed-field magnetization, Multiple bulk superconductors, High-Temperature superconductor, Rotating machine