Development of an adiabatic demagnetisation refrigerator for use in space

Cryogenic detectors under development by many groups worldwide for the UV, optical, near-infrared, and submillimetre wavelength bands will require cooling to temperatures in the 10 to 100mK range. If such detectors are to be used on future astronomical satellite missions a space qualified millikelvin refrigerator will be required. Of the techniques used to produce such temperatures on the ground, adiabatic demagnetisation refrigeration (ADR) is the most promising for adaptation to use in space. ADRs utilise the magnetocaloric effect, the reduction in temperature observed when an external magnetic field is applied isothermally to a paramagnetic material, and then removed adiabatically. This thesis describes the early stages in the development of a space-qualified ADR. In order to meet the strict requirements on mass, power consumption, and size imposed by a satellite platform, a highly optimised ADR will be required. A laboratory ADR was used to investigate various aspects of ADR design, concentrating on the salt pill, the component that contains the paramagnetic material. The areas investigated included new paramagnetic materials, salt pill support structures, salt pill thermal design, and temperature regulation. A dynamic thermal model of ADR operation was developed in order to understand fully the behaviour of the system during all the evaluation tests. Two new ADR systems were designed, and the performance of each was predicted using the thermal model. The first, the micro ADR, was developed as a test bed for miniaturised, low magnetic field ADR design as part of a collaboration with Oxford Instruments, and also served as an extra cooling stage for their commercial Heliox He3 refrigerator. The second, known as the double ADR, was a multi-stage device developed as a prototype for a future space-based, cryogen-free ADR. Details of the design and predicted performance of each system are presented.