Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
7916050 | Cryogenics | 2015 | 12 Pages |
Abstract
The design and performance of a fast thermal response miniature (24Â mm outer diameter by 30Â 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 2Â T superconducting magnets that can be ramped to full field in 30Â s. 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 3.6Â K bath is presented, demonstrating that a complete CPA cycle (magnetising, cooling to bath and demagnetising) can be accomplished in under 2.5Â min, with magnetisation and demagnetisation taking just 30Â s each. The cold finger base temperature of the prototype varies with demagnetisation speed as a consequence of eddy current heating; for a 30Â s demagnetisation, a base temperature of 161Â mK is obtained, whilst for a 5Â min demagnetisation, a base temperature of 149Â mK was measured (both from a 3.6Â K and 2Â T starting position). The measured hold times of the CPA pill at 200Â mK, 300Â mK, and 1Â 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 200Â mK to 4Â 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.
Related Topics
Physical Sciences and Engineering
Materials Science
Electronic, Optical and Magnetic Materials
Authors
J. Bartlett, G. Hardy, I.D. Hepburn,