A simple method to measure the temperature and levitation height of devices rotating at cryogenic temperatures

We describe a simple system to measure the temperature and levitation height of levitating cryogenic devices in rotation. Devices of this kind are the half-wave-plates rotating on superconducting magnetic bearings used in several cryogenic polarimeters for the cosmic microwave background. The temperature measurement is important to monitor the radiative background and potential systematic effects in the polarimeter. In our implementation, the temperature sensor is a thermistor, physically mounted on the rotating device. The sensor is biased with an AC current, which is transferred from the stationary electronics to the rotating device via capacitive coupling. The levitation height sensor is a network of capacitors, similar to the one used for the capacitive coupling of the thermistor. We describe the optimization of the readout system and its performance, which has been tested on a room-temperature prototype. We show that this system reaches an accuracy better than 3% for the measurement of the thermistor resistance and an accuracy of ∼10 μm for the measurement of its levitation height.