Abstract Body

Superconducting magnetic bearings (SMBs) [1] consist of a rotor containing permanent magnets or magnetic materials and a stator containing bulk superconductors. Since the pinning effect levitates the rotor, there is no mechanical contact. Therefore, various research towards the industrial applications, e.g. flywheels [2], cryogenic pumps [3], and bearings for satellites [4], are in progress. We are particularly interested in low-heat dissipation cryogenic bearings for use as a part of the observational instruments for space-borne cosmology mission, called LiteBIRD.

SMB on a Cosmic Microwave Background (CMB) polarimeter enables continuous rotation (~ 1 Hz) for a large-diameter optical element with low heat dissipation (a few mW) in a limited cooling power environment. We have proposed an SMB structure consisting of a rotor composed of ring-shaped segmented permanent magnets and an iron yoke, and a stator composed of ring-shaped segmented bulk superconductors. The permanent magnets are magnetized radially. Due to the unideal magnetic field variation in both a rotor magnet and a stator superconductor, it is unavoidable to have electromagnetic losses even in a contact-less rotation system. We need to be able to model this loss properly and reduce the loss as required by the application. At the same time, it is essential to consider the bearing stiffness simultaneously as a part of the SMB performance.

We developed a FEM analysis model of the SMB and analyzed the levitation force and the bearing stiffness. We used a model that combines the H formulation and the A-V formulation. The analysis is performed using COMSOL Multiphysics, a general-purpose physics simulation software, and an n-value model for the superconductor's current-voltage characteristics. We also carried out an experiment using a prototype small experimental platform of axial type SMB and estimate the levitation force and the bearing stiffness to evaluate the validity of the numerical analysis. Fig. 1 shows a 3D rendering of the assembly of the experiment setup. The stator consists of eight cylindrical superconducting bulks arranged in a circular pattern. The rotor is an axially magnetized ring-shaped permanent magnet. We measure the levitation force with a load cell. We report the comparison of the levitation force between the experimental data and the simulation results, and discuss the potential improvement and the path towards the estimation of the energy loss.

References

[1] J. R Hull, Superconducting Science and Technology, Vol. 13, No. 2 (2000), pp. R1-R15.
[2] K. Demachi, et al., Physica C: Superconductivity, Vol. 426-431, Part 1 (2005), pp. 826-833.
[3] Q. Lin, et al., IEEE Transaction on Applied Superconductivity, Vol. 22, No. 3 (2012).
[4] E. Allys et al., Progress of Theoretical and Experimental Physics, Vol. 2023, Issue 4 (2023), 042F01.