Design optimization of anechoic SKYLOAD dedicated for LiteBIRD MHFT on-ground testing

LiteBIRD is a future satellite mission designed to polarization-sensitive mapping of the Cosmic Microwave Background (CMB) anisotropies required to test the theory of cosmological inflation. The standard procedure of the on-ground characterization and calibration of the LiteBIRD Med- and High Frequency Telescopes (MHFT) requires the use a cryogenic reference unit, called also ‘SKYLOAD’, compatible with the vacuum environment.
In this paper, we present one possible baseline design of such SKYLOAD, proposed and currently under development by our research group. The SKYLOAD is composed from anechoic panels, which consist of a planar impedance substrate and a structured layer involving an anechoic cone array with a honeycomb arrangement. Both structured layer and planar impedance substrates should be made of a microwave absorbing polymer, compatible with vacuum cryogenic environment. The SKYLOAD is cooled down by liquid helium to 2 K. Accordingly, in our work, we address also key issues related to no-thermal cracking conditions, as well as the deterioration of the performance of the SKYLOAD assembled from anechoic panels due to their thermal shrinkage when cooling to cryogenic temperatures. Finally, an original manufacturing method of the proposed anechoic panels, involving polymer casting, is currently under consideration.