Evaluation of loss characteristics of superconducting magnetic bearings for LiteBIRD satellite by three-dimensional finite element method analysis

The polarization modulator of LiteBIRD satellite is required to be used by continuously rotating the half-wave retarder under a low temperature environment of about 10 K. Then, it becomes an important issue to reduce heat generation due to rotation loss. Therefore, a rotation mechanism in a cryogenic environment by a superconducting magnetic bearing (SMB) using the pinning effect of the bulk superconductor has been studied. In SMB, because of the pinning effect of superconductor, it is possible to rotate the half-wave retarder with stably floating, and it is possible to avoid frictional heat due to contact. The half-wave retarder is incorporated inside the rotating part of the superconducting magnetic bearing composed of the permanent magnet ring and the iron yoke. At this time, since the permanent magnet ring divides the parallel magnetized permanent magnets into pieces, a gap is formed at the joint of the permanent magnets. For this reason, the magnetic field distribution generated by the permanent magnet ring is not uniform. Although SMB can reduce friction as compared to mechanical bearings, the magnetic properties of the bulk superconductor change due to the magnetic field fluctuation caused by such ununiformity of the magnetic field, resulting in energy loss. In this study, heat generation in bulk superconductor in SMB is evaluated by three-dimensional finite element method. First, using JMAG Designer 17.0, the magnetic field distribution created by the rotor side is obtained by static magnetic field analysis. Next, using the COMSOL Multiphysics 5.3a, by applying the magnetic field distribution created by the rotor side obtained above and rotate the magnetic field, the loss generated in the bulk superconductor is investigated. The resulting loss value satisfied the required value.