Performance of a fast response miniature Adiabatic Demagnetisation Refrigerator using a single crystal tungsten magnetoresistive heat switch. (December 2015)

• Record Type: Journal Article
• Authors: Bartlett, J.; Hardy, G.; Hepburn, I.D.
• Is Part Of: Cryogenics. Volume 72: Part 2(2015)
• Page Start: 111
• Page End: 121
• Publication Date: 2015-12
• Subjects: Magnetic cooling -- Tungsten heat switch -- Magnetoresistivity -- Continuous cooling -- Millikelvin cryocooler; Low temperature engineering -- Periodicals; Low temperature research -- Periodicals; 536.56
• DOI: 10.1016/j.cryogenics.2015.10.004
• Languages: English
• ISSNs: 0011-2275

Abstract

Highlights: Miniature fast response ADR with tungsten MR heat switch successfully demonstrated. 22 g CPA pill, total ADR mass of 1.48 kg, overall ADR dimensions 56 × 56 × 158 mm. Measured recycle time of 82 s and thermal response measured to be sub-second. Single millikelvin cryocooler (smKCC) design presented utilising two miniature ADRs. smKCC predicted cooling power of up to 17 μW, 58 μW and 2.1 mW at 0.25, 0.3 and 3 K. Abstract: The performance of a fast thermal response miniature Adiabatic Demagnetisation Refrigerator (ADR) is presented. The miniature ADR is comprised of a fast thermal response Chromium Potassium Alum (CPA) salt pill, two superconducting magnets and unconventionally, a single crystal tungsten magnetoresistive (MR) heat switch. The development of this ADR is a result of the ongoing development of a continuously operating millikelvin cryocooler (mKCC), which will use only magnetoresistive heat switches. The design and performance of the MR heat switch developed for the mKCC and used in the miniature ADR is presented in this paper; the heat switch has a measured Residual Resistivity Ratio of 32, 000 ± 3000 and an estimated switching ratio (on thermal conductivity divided by the off thermal conductivity) of 15, 200 at 3.6 K and 38, 800 at 0.2 K when using a 3 T magnetic field. The performance of the miniature ADR operating from a 3.6 K bath is presented, demonstrating that a complete cycle (magnetisation, cooling to the bath and demagnetisation) can be accomplished in 82 s. A magnet current step test, conducted when the ADR is cold and fully demagnetised, has shown the thermal response of the ADR to be sub-second. The measured hold times of the ADR with just parasitic heat load are given, ranging from 3 min at 0.2 K with 13.14 μW of parasitics, to 924 min at 3 K with 4.55 μW of parasitics. The cooling power has been measured for operating temperatures in the range 0.25–3 K by applying an additional heat load to the ADR via a heater, in order to reduce the hold time to 3 min (i.e. approximately double the recycle time); the maximum cooling power of the miniature ADR (in addition to parasitic load) when operating at 250 mK is 20 μW, which increases to 45 μW at 300 mK and continues to increase linearly to nearly 1.1 mW at 3 K. To conclude, the predicted performance of a tandem continuous ADR utilising two of the miniature ADRs is presented.