Abstract
The design and performance of a fast thermal response miniature (24mm outer diameter by 30mm 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 2T superconducting magnets that can be ramped to full field in 30s. 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 3.6K bath is presented, demonstrating that a complete CPA cycle (magnetising, cooling to bath and demagnetising) can be accomplished in under 2.5min, with magnetisation and demagnetisation taking just 30s each. The cold finger base temperature of the prototype varies with demagnetisation speed as a consequence of eddy current heating; for a 30s demagnetisation, a base temperature of 161mK is obtained, whilst for a 5min demagnetisation, a base temperature of 149mK was measured (both from a 3.6K and 2T starting position). The measured hold times of the CPA pill at 200mK, 300mK, and 1K 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 200mK to 4K 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.
Highlights
The operation of a single Adiabatic Demagnetisation Refrigerator (ADR) is as follows: (1) an external magnetic field is applied to the salt pill which generates heat within the pill; (2) the heat generated during magnetisation is removed to the heat bath via the heat switch; (3) the salt pill is isolated from the heat bath by opening the
A fast thermal response prototype CPA pill has been designed, built and tested, proving that it can respond to a 30 s 0 to 2 T ramp in magnetic field
As part of the design process, the effects of thermal boundary resistance and eddy current heating were investigated and methods of minimising these were incorporated into the design; the thermal boundary resistance was minimised by having a sufficient contact area between the copper thermal bus and the CPA crystals and the eddy current heating was minimised by incorporating slits into the CPA pill end plate design
Summary
The operation ( known as the recycling process) of a single ADR is as follows: (1) an external magnetic field is applied to the salt pill which generates heat within the pill; (2) the heat generated during magnetisation is removed to the heat bath via the heat switch; (3) the salt pill is isolated from the heat bath by opening the. The salt pill is only partially demagnetised until a desired operating temperature (which must be greater than the base temperature) is reached. The remaining magnetic field is removed at a rate such that the cooling produced from demagnetisation counteracts the heat flows into the salt pill thereby maintaining a constant operating temperature or hold temperature (Th).
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