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Heat dissipation of rotation mechanism of polarization modulator unit for LiteBIRD low-frequency telescope

LiteBIRD is a future space mission designed to observe the polarization of the cosmic microwave background (CMB) radiation. LiteBIRD employs polarization modulator units (PMUs) at telescope apertures to mitigate 1/f noise and systematic uncertainties. The PMU employed in the Low-Frequency Telescope (LFT) consists of a broadband achromatic half-wave plate (HWP) and a cryogenic rotation mechanism. A superconducting magnetic bearing, which is a rotor levitation type bearing, is used to eliminate physical friction. A contactless AC synchronous motor consisting of SmCo permanent magnets and copper coils is employed as the drive mechanism. One of the technical challenges for the PMU development is to reduce the heat dissipation generated by the rotation mechanism during cryogenic operation. We evaluated the heat dissipation owing to the eddy currents generated from the rotor in the rotation mechanism at room temperature. We performed a rotor spindown measurement using a breadboard model of the PMU. We established that eddy currents generated from the motor coil were dominant in the rotor at room temperature, and its estimated value was 3.91 ± 0.91 mW.