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We study the relationship between the levitation gap change and losses in a superconducting magnetic bearing (SMB) system. We develop a polarization modulator bearing for a cosmic microwave background (CMB) polarization experiment. The heat generated by the polarization modulator needs to be kept very low. Therefore, it is essential to model the effect of the levitation gap change of a rotor due to gravity on the rotational loss. We have constructed a prototype small experimental platform of an axial-flux SMB with an array of high-temperature superconductors, YBCO, and a permanent magnet ring. In this system, the weight of the rotor can be set. We measured the effect of the change in the levitation gap of the rotor due to gravity on the loss. When the levitation force was 5.5 N and 16.0 N, the losses generated in the rotor were 9.1 × 10−3 W and 1.6 × 10−2 W, respectively. It was confirmed that the loss increases with the increase in the levitation force of the SMB.

This paper describes the losses of the rotor of the prototype superconducting magnetic bearing (SMB) that is employed for a polarization modulator of a Cosmic Microwave Background polarization experiment. The stator is composed of 20 segmented YBCO bulks, and these multiple superconductors produce the inhomogeneous magnetic field particularly due to the gravitational displacement of the rotor magnet position from the initial field cooling. This inhomogeneity causes the power dissipation to the spinning rotor. We estimated the power dissipation originating from this inhomogeneous magnetic field by using a numerical approach. The superconductors were modelled as electrically conducting components in the numerical simulation to avoid the complexity in modelling superconductivity. The results show about 4 mW dissipated at the rotor.