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Closed-Cycle Sorption Coolers and Tiltable Miniature Dilution Refrigerators for Astrophysics

May, Andrew James

[Thesis]. Manchester, UK: The University of Manchester; 2017.

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Abstract

The search for B-mode polarization anisotropies in the Cosmic Microwave Background is one of the primary preoccupations of modern cosmology. Detection would allow further constraining of cosmological parameters, as well as supporting the search for primordial gravitational waves and the testing of theories of inflation.There are several forthcoming experiments (so-called Stage-III) aiming to measure B-modes at a range of angular scales. Given the observing frequencies required and the predicted magnitude of the B-mode signal, this next generation of telescopes will be based around TES bolometer arrays, which require sub-Kelvin operating temperatures. Additionally, sections of the optics, filters and readouts for some of these telescopes will require cooling to 1 K. Moreover, these experiments will require longer hold times (or, ideally, continuous cooling), greater temperature stability and accuracy, and extended operation in remote locations such as the Atacama Desert and Antarctica.Several related cryogenic systems have been developed as part of the QUBIC, LSPE and POLARBEAR-2 collaborations, and are described herein.A double-stage 3He/4He sorption cooler has been extensively modelled, designed and manufactured. The 4He stage has been demonstrated down to 656 mK and operated up to 70 degrees away from the vertical. Cooling powers of >10 mW have been measured at 1 K. Hold times of over 6 hours have been achieved; however, further work is required in order to improve the film breaker to increase this hold time to the order of 24 hours. The 3He system is currently awaiting testing.A tiltable single-shot miniature dilution refrigerator has been developed, a prototype manufactured and preliminary testing carried out. Successful precooling to 400 mK has been achieved using a pair of 3He sorption coolers. Unfortunately however, the hold time of the precoolers used was not sufficient to allow dilution cooling to 100 mK as planned. As such, modifications are being made presently and a second experimental run is proposed to reach 100 mK.Finally, a design and preliminary thermodynamic model for a continuous tiltable miniature dilution refrigerator has been developed and is also presented. Detailed design and prototype manufacturing is forthcoming.