The practical use of superconducting bulk in high-temperature superconducting (HTS) machines is limited by magnetization processes, usually field cooling or zero-field cooling, which are time-consuming. Pulse field magnetization (PFM), another process of magnetization allowing to trap a magnetic flux in the bulk momentarily, provides a radical solution to that challenge. The time saving by PFM comes at the expense of the amount of trapped magnetic field. A waveform control pulsed magnetization (WCPM), which utilizes a controlled pulsed magnetic field, can help increase the trapped magnetic flux density if the magnetic field penetrates the bulk substantially using a flux jump. However, since the flux jump sometimes greatly changes the magnetic and thermal state of the bulk, there is a limit to the improvement of the captured magnetic field characteristics by passive WCPM. To reflect the change of state of the bulk in the waveform control, we used a negative feedback control of the magnetic flux density on the surface of the bulk, using a Hall sensor. The Hall sensor detects the magnetic flux penetrating the bulk. The error between the feedback data and the targeted magnetic flux density is calculated each millisecond by the system and the applied magnetic field is controlled by modifying the pulsed current waveform flowing through the two vortex-type copper coils which sandwich the bulk. This setup around the bulk was designed to reproduce the internals of at axial-type HTS motor. The GdBaCuO bulk sample was cooled down to 60 K by a GM cryocooler. While searching for the ideal control conditions of the WCPM, the magnetization characteristics have been investigated. The flux jump, which assists the flux in penetrating the centre of the bulk, could be “slowed down” by the negative-feedback WCPM. This operation helped to decrease the heat generated by the moving flux inside the bulk and reduced the temperature rise, which contributed to increase the captured magnetic flux density. We observed that the amplitude and the apparition time of the flux jump vary depending on the region of the bulk. The control of the flux jump allowed us to control the trapped magnetic field characteristics. We would like to control more precisely the flux jump, to increase the trapped magnetic field density in the future.

Keywords: Pulsed Field Magnetization , Flux Jump, Negative Feedback, High-Temperature Superconductor