Measurement of Dynamic Magnetic Flux Density Distribution at the Surface of HTS Bulk

Flux pinning in high-temperature superconducting (HTS) bulk is essential for power applications. Since HTS bulk integrated in the motor/generator requires pulse field magnetization (PFM) before operation, evaluating the time evolution of the trapped magnetic flux density distribution is important in the development of power applications. In general, scanning the Hall element over the HTS bulk surface provides a detailed measurement of the magnetic flux density distribution. However, the mechanical scanning speed is too slow relative to the elapsed time of PFM, to obtain information about the transient behavior of the penetration magnetic field. Therefore, we have placed Hall elements at several locations with characteristic crystal structures on the HTS bulk surface and measured the magnetic flux density in a short period of time. In this study, we have constructed a system that can measure the magnetic flux density distribution every approximately 1 ms, which we name a two-dimensional magnetic field sensor (2DMFS) and measured the distribution of the magnetic flux density that enters the bulk during PFM on the HTS bulk surface. The sample is a GdBCO bulk of 45 mm diameter and 19 mm thick, which we used in our axial-gap type HTS rotating machine. In this paper, we'll show the transient magnetic flux density distribution that penetrated the sample during PFM and contrast it with the conventional measurement method.

I. Introduction

High-temperature superconducting (HTS) materials can generate stronger magnetic fields than permanent magnets at cryogenic temperatures [1], [2]. Therefore, we perform pulse field magnetization (PFM) to use HTS bulk as a magnetic field in power equipment such as motors and generators [3], [4], [5], [6], [7], [8]. In general, trapped magnetic flux density in the HTS bulk by the PFM is smaller than that by field cooling (FC). Degradation of trapped magnetic field properties is due to heat generated by magnetic flux motion in the HTS bulk during the PFM. In order to improve the HTS bulk to trap magnetic field properties, it is necessary to evaluate the magnetic flux penetration process and trapping in the HTS bulk [9], [10].