The superconducting synchronous rotating machines exhibit high torque at low speeds, making them suitable for gearless ship propulsion and wind turbines. Because they have multiple field poles, we want to efficiently magnetize the high-temperature superconducting (HTS) bulk material by pulsed field magnetization (PFM) [1]. However, as is well known, HTS bulk materials trap the magnetic field non-uniformly by PFM and have a low total magnetic flux with a low trapped magnetic flux density. So, improving the trapped magnetic field properties of HTS bulk materials is a key technology to the practical application of superconducting power devices.
The reason why HTS bulk material does not obtain a uniform and large trapped magnetic field distribution by PFM is that the temperature of the bulk material rises significantly as the flux jump occurs. Flux jumps should increase the trapped magnetic flux density because they activate flux motion, but more than that, they degrade the trapped magnetic field properties of the HTS bulk material. It is technically difficult to construct an electrical circuit that generates a voltage of several hundred volts or more and a current of several thousand amperes, which, operating at low losses, generates a strong magnetic field that exceeds the trapped flux density by several times. Therefore, it has been believed that the problem associated with PFM cannot be drastically improved. However, active control of power through advances in power electronics freely shapes the pulsed magnetic field waveform generated by the magnetizing coil with low loss. So-called waveform-controlled pulse magnetization (WCPM) controls the generation of flux jumps by applying a pulse field waveform with a shape different from the conventional LCR transient to the HTS bulk, thereby improving the trapped magnetic field properties [1]. Nevertheless, since the magnetic resistance of the magnetic circuit around the HTS bulk is reduced by the flux jumps generated, the range of conditions for achieving high field trapping in sequence-controlled WCPM is narrow, especially the lower the HTS bulk material is cooled to a low temperature, the more difficult it is to achieve. On the other hand, WCPM with feedback control, although technically challenging, can actively control the occurrence of flux jumps because the applied flux density can be adjusted to match the transient behavior of the magnetic flux in the HTS bulk material. With a negative feedback WCPM using as input the intrusion flux density measured at the surface of the HTS bulk material, we achieved a trapped flux density of about 4 T in a ø45 GdBCO bulk cooled to 40 K [2]. In this presentation, we will show these latest pulse magnetization technologies and their results [3].
[1] T. Ida et al., Materials preparation and magnetization of Gd-Ba-Cu-O bulk high-temperature superconductors, Supercond. Sci. Technol. 29 [5] (2016) 054005.
[2] T. Ida et al., Pulse field magnetization of GdBCO without rapid decrease in magnetic flux density, IEEE Trans. Appl. Supercond. 34 [3] (2024) Article#: 6800906.
[3] N. Kawasumi et al., Measurement of dynamic magnetic flux density distribution at the surface of HTS bulk, IEEE Trans. Appl. Supercond. 34 [3] (2024) Article#: 6800604.
The authors thank A. A. Caunes, H. Imamichi and N. Kawasumi for their help. This work was supported in part by JSPS KAKENHI under Grant 20K21044 (2020–2022) and Grant 21H01541.
Keywords: PFM, WCPM, GdBCO, Synchronous rotating machine