Mechanism of decay of trapped magnetic field in HTS bulk caused by application of AC magnetic field

In our previous work, it was observed that trapped magnetic field in an high-temperature superconductor (HTS) bulk was decayed and even erased by application of AC external field whose amplitude was much smaller than the peak value of the trapped magnetic field. Therefore, knowledge on the mechanism of the decay of the trapped magnetic field is important to design the machines and to develop a method to suppress the decay. This work studies a mechanism of the decay due to application of AC magnetic field by a numerical analysis and experiments. An analytical model is proposed to explain the mechanism based on the thermal effect due to AC losses in the bulk. In the model it is assumed that the AC loss characteristic follows the Bean model, which was experimentally demonstrated also in our previous work. We conducted experiments in which temperature rise of the bulk was measured by thermocouples and the decay of the trapped magnetic was observed by a hall probe. Results numerically calculated from the analytical model well agreed with those obtained from the experiments and we consider it was clarified that the decay of the trapped field was caused by the thermal effect due to AC loss. Once the mechanism has been clarified, methods to suppress the decay can be easily developed.