The exploration of the distant and mysterious primordial universe is the focus of much astrophysical research into the complex dynamics of our universe. The satellite will be equipped with ultra-sensitive detectors that need to be cooled down to 0.1 K (absolute zero) using a single cryogenic chain for the entire assembly. Due to the accumulation of heat received from the sun, radiated energy and operation, the satellite's temperature finds its equilibrium at around 300 K. A specific cryogenic system must be implemented to cool the telescope and the detectors to their operating temperature. As a first step, radiative panels are used to provide an interface at 35 K. Then, a first refrigerator lowers the temperature of the instrument envelop to 4.8 K and a second system cools the first stage of the telescopes to 1.8 K.
Researchers at DSBT [collaboration] will provide the cooling system for the last two stages at 0.35 K and 0.1 K. To achieve this, scientists are using new magnetocaloric materials, such as YBGG (Ytterbium Gallium Garnet) or CPA (chromium potassium alum), which change temperature under the effect of variations in the magnetic field according to the principle of adiabatic demagnetization. Technological challenges include the compactness of components such as superconducting magnetic coils, and the development of gas-gap or superconducting thermal switches. In addition, the Irig researchers will provide low-temperature multi-stage links between the refrigerator and the telescopes, optimizing thermal and mechanical performance, since a single cryogenic chain requires greater distances to link the various instruments.
In addition, the researchers specially developed the refrigeration system, the last two stages of which have to be maintained at very low temperatures of 0.35 k and 0.1 K respectively.
For the 0.35 K stage, the characteristics of each of the components were checked, and they were then meticulously assembled to form a highly compact integrable unit that functions perfectly. Further measurements will be carried out to determine the various cooling capacities.
For the last stage at 0.1 K, a high-performance superconducting thermal switch was designed and installed. Its operation has been validated by tests showing that it achieves the cut-off factor required for the desired cryocooler efficiency.
This work on the overall thermal architecture is being carried out in parallel with technological developments, in order to propose the best possible couplings between the various sub-systems.
The satellite is scheduled for launch in 2032, following on from the Planck satellite in 2009. The budget is estimated at 500 M€, which is comparable to ESA M-class missions. All the teams contributing to the LiteBIRD project are due to deliver study reports estimating the feasibility of the project by the end of 2023. They will then deliver an engineering model of the refrigerator and thermal links in 2025, followed by flight models in 2029.
(c) JAXA /LiteBIRD
Collaboration: The project is led by the Japanese space agency JAXA (Japan Aerospace Exploration Agency), responsible for the satellite and the low-frequency telescope (LFT). The Medium and High Frequency Telescope (MHFT) is a European project, under the responsibility of CNES. Canada and the USA are involved in both projects for the detectors.