Estimation of the Heat Dissipation and the Rotor Temperature of Superconducting Magnetic Bearing below 10 K

We present the thermal characteristics of a superconducting magnetic bearing (SMB) system that is designed for a polarization modulator of a cosmic microwave background polarization experiment. The SMB system consists of a ring-shaped magnet and an array of YBCO tiles. Both have an opening diameter of about 400 mm. The ring-shaped magnet is formed by an array of segmented magnets. The magnetic field inhomogeneity of this rotor magnet is originated from the magnetic field inhomogeneity of each magnet and the spatial gap between the segments. This inhomogeneity can contribute as a source of friction, and thus a source of heat in an operation at below 10 K. We estimate the heat dissipation from friction of the SMB by conducting the spin-down measurements, and we projected the heat dissipation of 2-20 mW from the hysteresis contribution depending on the levitation height. We discuss the consistency of the spin-down measurements to Bean's model between the magnetic field inhomogeneity and the hysteresis loss. We also discuss the potential temperature rise of the rotor magnet when it is levitating at the temperature at around 10 K. Thus, we address the means to estimate the rotor temperature by the physical contact to the rotor. The extrapolated temperature recovers the rotor temperature with the difference of less than 2 K without any correction.