Development status of the cryogenic holder mechanism for LiteBIRD LFT PMU

LiteBIRD is an JAXA-led strategic L-class satellite mission designed to measure the primordial B modes of the cosmic microwave background radiation (CMB) to test cosmic inflation. The LiteBIRD Low-Frequency telescope employs a polarization modulator unit (PMU). The PMU is placed at the telescope aperture to modulate the incoming CMB polarization signal by using a continuous rotating half-wave plate to reduce the impact of 1/f noise and differential systematic effects. The current PMU design employs three cryogenic holder mechanisms that hold the rotor until the superconducting magnet bearing cools below its critical temperature after launch. They also serve a conductive path to the rotor when they are held. Minimizing the heat dissipation of this holder is one of the key development goals of the PMU due to the limited cooling power on the satellite system. In this paper, We report on the detailed design of the holder and the developed cryogenic stepping motor that actuates the holder. Also, we conducted the preliminary thermal characterization at around 7 K. The preliminary estimated total heat dissipation of the holder is 2.39 ± 0.09 mW when we activated it for 532 s.