Mitigation of Demagnetization of Bulk Superconductors by Time-Varying External Magnetic Fields

Abstract

Large single-grain high-temperature superconducting bulks have significant potential to replace permanent magnets in various engineering applications. However, based on our previous research, the trapped field in a bulk superconductor can be attenuated or even erased when a bulk is subjected to a time-varying external magnetic field. Therefore, it is important to develop a method to protect bulks from demagnetization by improving the thermal conduction of the bulk and/or reducing ac losses. Improvement in the thermal conduction of bulks involves modification of the material fabrication process, which may have a detrimental effect on its superconducting properties. Employing shielding materials around a bulk helps to decrease the ac losses, but also provides a durable way to maintain the original material properties. In this paper, two shielding cases are proposed and evaluated numerically: ring-shaped shielding with a copper coil and surface shielding with a ferromagnetic material. Based on the numerical modeling results, the ring-shaped coil works well for externally applied ac fields of larger magnitude and higher frequency. However, the ferromagnetic material was preferable for surface shielding for relatively lower fields. Finally, an optimal shield design is presented.