Sorption-cooled miniature dilution refrigerators for astrophysical applications

• Main Author: Teleberg, Gustav
• Published: Cardiff University 2006
• Subjects: 523.01
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.583954

The next generation of balloon-borne and ground-based mm/sub-mm astronomy experiments will require operating temperatures near or below 0.1 K. When these experiments are operated remotely on platforms or at sites with limited infrastructure and maintenance sup port, a compact and reliable dilution refrigerator becomes essential. We have investigated two different dilution refrigerators in order to evaluate which system is most suitable for these applications. We have carried out a feasibility study of the simplest of the two technologies, a single-shot dilution refrigerator. A thermal model for predicting its performance has been developed, and a first prototype which achieved temperatures of about 70 mK was built. We discuss advantages and disadvantages of a single-shot system and show how minor changes to the current design can make it useful for many astronomy applications. The second dilution refrigerator is based on the principle of condensation pumping. We have built and integrated such a refrigerator with a pulse-tube cooler in order to create a completely cryogen-free system. Temperatures below 50 mK have been achieved, and temperatures below 100 mK have been maintained for more than 10 hours with several micro-Watt of cooling power. Using two 3 He sorption coolers and gas-gap heat switches we have also demonstrated how this cooler can be operated in a continuous mode. The entire system is fully automatic in operation and can be controlled and monitored remotely through a standard http protocol. We show how existing thermal models can be used to predict the cooling power and lowest achievable temperatures of the refrigerator. Experimental results are analysed and used to estimate the condensation efficiency, the performance of the heat exchangers and the 3 He circulation rate.