Radiative cooling with thermal isolation shields can provide a reliable cooling system for instruments onboard satellites in orbit. We report the optimization study for the cryogenic architecture of the LiteBIRD satellite using radiative cooling. A trade study that changed the number of thermal shields and shield emissivity were conducted. The heat flow from 300 to 4.5 K, including active cooling by mechanical cryocoolers, was evaluated among the trade designs. We found that the design that consists of low-emissivity four-layer thermal shields is optimum in terms of thermal performance and system design. The optimum design achieved a heat load of 29.9 mW for the 4.5-K cooling stage, whereas the requirement was 30 mW with the assumed cryogenic system.
LiteBIRD is a candidate for JAXA’s strategic large mission to observe the cosmic microwave background (CMB) polarization over the full sky at large angular scales. It is planned to be launched in the 2020s with an H3 launch vehicle for three years of observations at a Sun-Earth Lagrangian point (L2). The concept design has been studied by researchers from Japan, U.S., Canada and Europe during the ISAS Phase-A1. Large scale measurements of the CMB B-mode polarization are known as the best probe to detect primordial gravitational waves. The goal of LiteBIRD is to measure the tensor-to-scalar ratio (r) with precision of r < 0:001. A 3-year full sky survey will be carried out with a low frequency (34 - 161 GHz) telescope (LFT) and a high frequency (89 - 448 GHz) telescope (HFT), which achieve a sensitivity of 2.5 μK-arcmin with an angular resolution 30 arcminutes around 100 GHz. The concept design of LiteBIRD system, payload module (PLM), cryo-structure, LFT and verification plan is described in this paper.
The Lite satellite for the studies of B-mode polarization and Inflation from the cosmic microwave background
(CMB) Radiation Detection (LiteBIRD) is a next generation CMB satellite dedicated to probing the inflationary
universe. It has two telescopes, Low Frequency Telescope (LFT) and High Frequency Telescope (HFT) to cover
wide observational bands from 34 GHz to 448 GHz. In this presentation, we report the optical design and
characterization of the LFT. We have used the CODE-V to obtain the LFT optical design based on a cross-
Dragonian telescope. It is an image-space telecentric system with an F number of 3.5 and 20 x 10 degrees2 field
of view. The main, near and far side lobes at far-field have been calculated by using a combination of HFSS and
GRASP 10. It is revealed that the LFT telescope has good main lobe properties to satisfy the requirements. On
the other hand, the side lobes are affected by the stray light that stems from the triple reflection and the direct
path from feed. In order to avoid the stray light, the way to block these paths is now under study.
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 α of the spacecraft. In this study, the geometrical models of the PLM that consist of the sunshield, three layers of Vgrooves, and 5 K shield were designed in the cases of α = 45° , 30° , and 5° . 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 α. 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.