Hills, MJ; (2016) Thermal Transport in Tungsten and Applications to Miniaturised Adiabatic Demagnetisation Refrigerators. Doctoral thesis , UCL (University College London).

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Abstract

This thesis addresses the miniaturisation of Adiabatic Demagnetisation Refrigerators (ADRs) for cooling photon detectors that operate below 1 K. Such detectors offer vastly improved energy resolution and the potential to count individual photons, which make them attractive for both astronomical and ground-based applications. The primary focus of the thesis is on the heat switches used in ADRs. These components provide a strong thermal link to a pre-cooling bath during the magnetisation part of the ADR’s refrigeration cycle and – ideally – thermal isolation during the demagnetisation (cooling) part. In ADRs configured to provide continuous cooling, it is the heat switches which ultimately limit the efficiency of the ADR and the cooling power that can be obtained for a given size of refrigerator. Tungsten is an attractive material for constructing heat switches in ADRs because its thermal conductivity at liquid helium temperatures can be reduced by several orders of magnitude on application of a magnetic field. While the fields required are of the order of a few Tesla, they are very similar to those provided by superconducting magnets used in ADRs already. Moreover, the fact that tungsten switches are solid state devices, without moving parts or working fluids, makes them ideally suited to miniaturisation. Thermal transport in tungsten on the scales appropriate to miniature ADRs is investigated in both theoretical and experimental terms. Theoretical relationships between tungsten’s thermal conductivity and applied field are presented and verified by experimental studies on a variety of samples. The effects of different sample purity, crystal orientation and field orientation relative to the sample are also discussed. These findings are then incorporated into a thermal mathematical model which provides predictions of the performance of a miniature ADR and identifies key factors for optimising it. Finally, the future developments necessary to make a miniature ADR a reality are summarised.

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