Sub-Kelvin Cryogenics for Experimental Cosmology

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Abstract

Polarization anisotropies in the Cosmic Microwave Background contain a wealth of cosmological information. The forthcoming generation of observatories will make measurements across a range of frequencies and angular scales. These data will support a large number of science goals, including the search for direct evidence of cosmic inflation, measurements of neutrino properties, and the placing of further constraints on cosmological parameters. These observatories will be based around cryogenic receivers housing multiple cold optics stages at progressively lower temperatures. In order to reach the sensitivities required by their science goals, the receivers will have to support large optical throughput to arrays of transition edge sensor bolometers at sub-Kelvin operating temperatures. This thesis describes the development of a number of key subsystems for several of these forthcoming experiments: QUBIC, the Simons Array, and the Simons Observatory. QUBIC, the Q&U Bolometric Interferometer for Cosmology, is a European-led experiment which will deploy a Fizeau interferometer to directly target B-mode polarization anisotropies at degree angular scales. The beam combiner will be operated at 1 K and the resulting interference fringe patterns measured by two arrays of transition edge sensor bolometers at 350 mK. A high-capacity 1 K 4He sorption cooler has been developed, along with a novel superfluid film breaker to maximise the hold time of the system. Novel convective heat switches have been developed to precool the 1 K stage and provide isolation during the cooler recycling. A dual 3He/4He cooler has been provided to operate the detector stage at 350 mK. The Simons Array is a US- and Japanese-led continuation and extension of the successful POLARBEAR experimental program. It will consist of three POLARBEAR-type telescopes with upgraded cryogenic receivers, POLARBEAR -2A, -2B, and -2C,which will directly image the microwave sky. Each receiver will house the cold optics, focal plane detector array, and cold readout components. The focal plane in each case will be cooled to 270 mK by a multi-stage sorption cooler. Extensive efforts have been devoted to the development of a code, based on the calculation of Allan deviations, to characterise the stability and identify noise components in the focal plane temperature. The Simons Observatory is a US-led experiment that will field both a 6 m large aperture telescope and an array of 42 cm small aperture telescopes to probe a wide range of angular scales with very high sensitivity. The large aperture telescope will be coupled to a 2.4 m diameter receiver cryostat which will house thirteen optics tubes. The 1 K and 100 mK stages of the cryostat will be necessarily well-isolated from the higher temperature stages; in order to minimise the total cooling time of these stages, a precooling heat switch scheme has been devised, based on experimental measurements, and simulated. Furthermore, as both the large and small aperture telescopes will operate focal planes at 100 mK, a novel miniature dilution refrigerator has been developed for fast-turnaround operation of a detector wafer test cryostat.

Keyword(s)

Cryogenics; Experimental cosmology; Low-temperature physics; Miniature dilution refrigerator; Sorption cooler

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