Development of the FDM readout for the LSPE/SWIPE experiment

The Large Scale Polarization Explorer (LSPE) is the combination of a ground-based experiment and a balloon-borne mission designed to measure or to set an upper limit on the B-mode polarization pattern of the cosmic microwave background (CMB), providing a compelling evidence in support of cosmological inflationary theories. The Short Wavelength Instrument for the Polarization Explorer (SWIPE), its balloon-borne instrument, will be launched from Longyearbyen (Svalbard Island, 78° N) for a long-duration flight during the winter night, operating in a space-like environment at an altitude of 40 km. SWIPE is a Stokes polarimeter that will measure the CMB polarization exploiting three hundred large-area spider-web transition-edge sensors (TES), coupled to the optical stages of the instrument through multi-mode horns. The readout electronics of SWIPE is based on a frequency-domain multiplexing (FDM) system, composed of a “warm” section at roughly -90 °C, the temperature of the stratosphere during the polar night, and of a “cold” section hosted on the 300 mK and the 1.6 K stages of the instrument. This thesis focuses on the prototyping (design, development, and test) of FDM electronics for SWIPE, as well as a contribution to the characterization of the TES bolometers fabricated at INFN Genova. Though the design of LSPE/SWIPE is frozen, its construction and development are still in progress: therefore, the work presented here represents a snapshot of the R&D that we started from scratch in the laboratories of INFN Pisa, from the set-up of the cryogenic test facility to the fabrication and measurement of superconducting devices at sub-kelvin temperatures.