Thermal design utilizing radiative cooling for the payload module of LiteBIRD

The conceptual thermal design of the payload module (PLM) of LiteBIRD utilizing radiative cooling is studied. The thermal environment and structure design of the PLM strongly depend on the precession angle alpha of the spacecraft. In this study, the geometrical models of the PLM that consist of the sunshield, three layers of V-grooves, and 5 K shield were designed in the cases of alpha = 45 degrees; 30 degrees, and 5 degrees. The mission instruments of LiteBIRD are cooled down below 5 K. Therefore, heat transfers down to the 5 K cryogenic part were estimated in each case of alpha. The radiative heat transfers were calculated by using geometrical models of the PLM. The conductive heat transfers and the active cooling with cryocoolers were considered. We also studied the case that the inner surface of the V-groove is coated by a high-emissivity material.