Development of a cryogenic remote sensing thermometer for CMB polarization experiment

We present about the remote sensing thermometer working at cryogenic temperature for a polarization modulator used in cosmic microwave background (CMB) polarization experiment. The polarization modulator employs a superconducting magnetic bearing (SMB) in order to realize a continuous rotation at cryogenic temperature with minimal heat dissipation. The cryogenic remote thermometer is a critical development item in the polarization modulator in order to measure cryogenic, contactless and spinning rotor. We propose a novel idea to take an advantage of a temperature dependent nature in NdFeB permanent magnet. The remnance of the NdFeB peaks about 100 K and decrease as its temperature becomes lower. A simple implementation of a cryogenic Hall sensor at a vicinity of the NdFeB enables to construct a cryogenic remote temperature sensor. We calibrated the thermometer using small NdFeB magnet and applied it to a ~ 400 mm SMB prototype system in a 4 K cryostat with GM cooler. The obtained temperature value is good agreement with the resistive thermometer and the accuracy is ± 0.4 K in the range from 4 K to 20 K. The results are good enough to use in the polarization modulator.