Model for the decay of the magnetization of a bulk superconductor for different sources of magnetic fields with a transverse component

Bulk (RE)BCO samples can be used as efficient trapped-field magnets with surface flux densities of a few teslas. However, their magnetization may decay over time when subjected to time-varying parasitic crossed fields (applied perpendicular to the initially trapped flux density). In this work, we consider and compare the magnetization decays that occur under different types of crossed-field perturbations, mimicking realistic perturbations that are expected in synchronous motors. The model is based on a finite element approach carried in the GetDP environment. The superconductor is modelled with an isotropic E(J) power law and a field-independent current density. Under a magnetic field perturbation with a non-vanishing transverse component, the superconductor magnetization is modified due to a rearrangement of currents. We first investigate perturbations with different amplitudes that are applied perpendicular to the magnetization axis and have either a bipolar waveform with a zero DC component, or a unipolar waveform with a non-zero DC component. Then, these perturbations are compared to those obtained with an external field applied along the magnetization axis and being rotated away at different angles. The model predicts the evolution over time of the distributions of currents and the net magnetic moment in each case. It is demonstrated that the patterns of reorganized currents strongly depend on the waveform of the applied perturbation and an analysis of these patterns is provided.