Dynamic resistance and loss analysis of a small HTS REBCO magnet carrying direct currents under perpendicular DC-biased and AC magnetic fields

High temperature superconducting homopolar inductor machine (HTS-HIM) is a promising choice for electric aircraft since it can operate at relatively high rotational speeds and has the simplest and most robust architecture. In HTS-HIM, the HTS REBCO magnets carrying DC current are exposed to the mixed magnetic field environments, including an AC component and a DC-biased component. When the HTS magnets carry direct currents under AC magnetic fields, the dynamic loss is produced due to the dynamic resistance effect. It will reduce the efficiency of the HTS-HIMs. Moreover, the critical current of the HTS REBCO magnet reduces under an extra DC-biased component, leading to the reduction of the current safety margin. This paper proposed a 2D axisymmetric HTS magnet model based on H-formulation, and the dynamic resistance and loss of the HTS REBCO magnet are investigated under different mixed magnetic fields. The results show that the dynamic resistance obviously increases due to the extra DC-biased field. Under this working condition, the outermost turns of the HTS magnet will generate higher losses compared to the normal working condition, which will incur higher temperature rise and even quench behaviour. Under a mixed magnetic field consisting of a small AC field and a high DC-biased field, the impact of the DC current on dynamic resistance is more significantly than the impact of the DC-biased field. When the DC current is high, the total loss will significantly increase due to the flux flow effect. Moreover, under small AC field and large position-linearly-varying DC-biased magnetic field, the dynamic resistance and total loss are small due to the decrease of the average magnitude of the mixed magnetic field. This work helps build a deeper understanding of the dynamic resistance and loss components of the HTS REBCO magnet under the new working conditions of the mixed background magnetic field, which can be an important reference for designing and optimizing the HTS REBCO magnets inside HTS-HIMs.