Development of milli-Kelvin ADR technology for space missions

• Main Author: Weatherstone, S. A.
• Published: University College London (University of London) 2012
• Subjects: 500
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.625919

Cryogenics is increasingly required for space applications as new detector technologies emerge offering improved performance, as is exemplified by future X-ray astronomy missions. The IXO1 (International X-ray Observatory) mission will carry an X-ray Microcalorimeter Spectrometer (XMS), requiring a 50mK operating temperature. Two space-demonstrated technologies are capable of maintaining milli--Kelvin temperatures: Adiabatic Demagnetisation Refrigerators (ADRs) and open-cycle Dilution refrigerators. ADRs are simpler, more reliable, and have a longer lifetime than open-cycle dilution refrigerators which deplete their Helium stores after ~3 years. For long-life (>5 year) missions, ADRs are the best practical choice. MSSL have built an engineering model ADR for the XEUS (X-ray Evolving Universe Spectroscopy) mission. However, the ADR cannot achieve the required hold time and recycle time. This thesis shows how a magnetoresistive heat switch can improve performance such that the required hold time can be achieved. Tungsten Magnetoresistive heat switch technology is developed through experimental investigation. Heat switch performance is found to be a function of purity; the difference between an otherwise identical 99.992% and a 99.999% pure sample is very significant, with switching ratios of 437 and 1×104 respectively at ~4K. The ‘on’ state thermal conductivity is limited by sample size as it is reduced to the electron mean free path and below. A Tungsten magnetoresistive heat switch mounted in the XEUS ADR via bolted joints lined with 0.13mm thick Indium foil can improve its performance to meet the design requirements. MSSL are developing an ADR targeting the IXO XMS. The cooling chain required to support the ADR is presented. The design feasibility and compliance to requirements are verified by thermal and mechanical analyses, and it is shown that the ADR can be supported during holding and recycling for both warm and cold redundancy modes of higher temperature stage coolers in the chain. [1] NOTE: During the writing of this thesis, the IXO mission ceased to be a candidate of the L-class missions under consideration by ESA. However, due to the strong science case of IXO; ESA, Astrium and the scientific community are investigating to what extent a European-led mission could preserve the original science goals of IXO. This new study is ATHENA (Advanced Telescope for High ENergy Astrophysics).