Development of LiteBIRD’s cold readout sub-assembly

LiteBIRD is a JAXA-led spacecraft mission that has the objective of making a full-sky measurement of the CMB polarization B-modes to constrain the tensor-to-scalar ratio r with a sensitivity of σ(r) of 0.001. Its three telescopes will host a total of 4508 Transition-Edge Sensor (TES) bolometers at a temperature of 100 mK. The signal from the thousands of bolometers will be multiplexed using digital frequency-domain multiplexing (DfMux). In DfMux, each TES in a group of up to 68 is assigned to a frequency channel between 1.5 and 5.6 MHz by an inductor-capacitor (LC) chip, and the multiplexed signal is amplified by a Superconducting Quantum Interference Device (SQUID) Array Amplifier (SAA). Both SAA and LC chips will operate at sub-Kelvin temperatures and be mounted to the detector sub-assembly on the side opposite to the sky, in what is here defined as the cold readout sub-assembly. Components of the cold readout sub-assembly will need to withstand large mechanical stress during launch and achieve excellent electronic performance to meet the mission’s requirements. In this work we present the status of the design, prototyping and testing of cold readout sub-assembly components. This includes a measurement of the inductance of custom-made edge-coupled superconducting cables, which were found to be in the order of 0.5 nH/cm. Further, we demonstrate that a rigid-flex PCB could be used to thermally isolate SAAs while maintaining close to 5 nW of heat dissipation between the SAA and the LC temperature stages. Finally, we report on simulations of different magnetic shields that could be used to prevent pickup of environmental magnetic flux by the SAAs and the shields’ comparative effectiveness. The results presented here will be used to guide and predict the performance of the next iterations of the cold readout sub-assembly design for LiteBIRD.