Polarization modulator unit harness thermal design for the mid- and high-frequency telescopes of the LiteBIRD space mission

Polarization modulator units (PMUs) represent a critical and powerful component in CMB polarization experiments to suppress the 1/f noise component and mitigate systematic uncertainties induced by detector gain drifts and beam asymmetries. The LiteBIRD mission (expected launch in the late 2020 s) will be equipped with 3 PMUs, one for each of the 3 telescopes, and aims at detecting the primordial gravitational waves with a sensitivity of δr< 0.001. Each PMU is based on a continuously rotating transmissive half-wave plate held by a superconducting magnetic bearing in the 5 K environment. To achieve and monitor the rotation a number of subsystems is needed: clamp and release system and motor coils for the rotation; optical encoder, capacitive, Hall and temperature sensors to monitor its dynamic stability. In this contribution, we present a preliminary thermal design of the harness configuration for the PMUs of the mid- and high- frequency telescopes. The design is based on both the stringent system constraint for the total thermal budget available for the PMUs (≲ 4 mW at 5 K) and on the requirements for different subsystem: coils currents (up to 10 mA), optical fibers for encoder readout, 25 MHz bias signal for temperature and levitation monitors.