Capacitive sensors for the polarization modulator unit of the Mid- and High- Frequency Telescopes of LiteBIRD space mission

The LiteBIRD mission is a next generation CMB space mission which aims at detecting CMB polarization produced by primordial gravitational waves, with a sensitivity for the tensor to scalar ratio delta r < 0.001. A polarization modulator unit (PMU) represents a critical and powerful component to avoid 1/f noise contributions and mitigate differential systematic uncertainties in orthogonal polarization sensitive detectors on both Mid- and High- Frequency Telescopes. Each PMU is based on a continuously transmissive rotating half-wave plate (HWP), held by a superconducting magnetic bearing operating in the 5K environment. The dynamical behaviour of the system during the rotation must be monitored, since it can introduce systematic effects during the measurements. In this contribution, we present the development of custom cryogenic capacitive sensors, used to monitor the spinning rotor. A set of these sensors allows to monitor the levitation height and wobbling of the rotor, with accuracy of similar to 5 mu m and similar to 0.1', respectively. A thermistor mounted on the rotor can be coupled with one of these capacitors, so that it can be properly biased and read.