Bartlett, J;
Hardy, G;
Hepburn, ID;
(2015)
Design and Performance of A Fast Thermal Response Miniature Chromium Potassium Alum (CPA) Salt Pill for use In A Millikelvin Cryocooler.
Cryogenics
, 65
(Janura)
pp. 26-37.
10.1016/j.cryogenics.2014.11.004.
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Abstract
The design and performance of a fast thermal response miniature (24 mm outer diameter by 30 mm long) Chromium Potassium Alum (CPA) salt pill is described. The need for a fast thermal response has been driven by the development of a continuously operating millikelvin cryocooler (mKCC) which uses 2 Tesla superconducting magnets that can be ramped to full field in 30 seconds. The consequence of magnetising and demagnetising the CPA pill in such a short time is that thermal boundary resistance and eddy current heating have a significant impact on the performance of the pill, which was investigated in detail using modelling. The complete design of a prototype CPA pill is described in this paper, including the methods used to minimise thermal boundary resistance and eddy current heating as well as the manufacturing and assembly processes. The performance of the prototype CPA pill operated from a 3.6 K bath is presented, demonstrating that a complete CPA cycle (magnetising, cooling to bath and demagnetising) can be accomplished in under 2.5 minutes, with magnetisation and demagnetisation taking just 30 seconds each. The cold finger base temperature of the prototype varies with demagnetisation speed as a consequence of eddy current heating; for a 30 second demagnetisation, a base temperature of 161 mK is obtained, whilst for a 5 minute demagnetisation, a base temperature of 149 mK was measured (both from a 3.6 K and 2 Tesla starting position). The measured hold times of the CPA pill at 200 mK, 300 mK, and 1 K are given, proving that the hold time far exceeds the recycle time and demonstrating the potential for continuous operation when two ADRs are used in a tandem configuration. The ease and speed at which the CPA pill temperature can be changed and controlled when stepping between operating temperatures in the range of 200 mK to 4 K using a servo control program is also shown, once again highlighting the excellent thermal response of the pill. All of the test results are in good agreement with the modelling used to design the CPA pill, giving good confidence in our ability to understand and estimate the effects of eddy current heating and thermal boundary resistance. To conclude, the design for the CPA pill to be used in the mKCC (which is heavily based on the design of the prototype) is presented.
| Type: | Article |
|---|---|
| Title: | Design and Performance of A Fast Thermal Response Miniature Chromium Potassium Alum (CPA) Salt Pill for use In A Millikelvin Cryocooler |
| Open access status: | An open access version is available from UCL Discovery |
| DOI: | 10.1016/j.cryogenics.2014.11.004 |
| Publisher version: | http://dx.doi.org/10.1016/j.cryogenics.2014.11.004 |
| Language: | English |
| Additional information: | This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. |
| Keywords: | Adiabatic Demagnetisation, magnetic cooling, Millikelvin, Eddy current heating, thermal boundary resistance |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Space and Climate Physics |
| URI: | https://discovery.ucl.ac.uk/id/eprint/1456623 |
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