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 3.6K and 38,800 at 0.2K when using a 3T magnetic field. The performance of the miniature ADR operating from a 3.6K bath is presented, demonstrating that a complete cycle (magnetisation, cooling to the bath and demagnetisation) can be accomplished in 82s. 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 3min at 0.2K with 13.14μW of parasitics, to 924min at 3K with 4.55μW of parasitics. The cooling power has been measured for operating temperatures in the range 0.25–3K by applying an additional heat load to the ADR via a heater, in order to reduce the hold time to 3min (i.e. approximately double the recycle time); the maximum cooling power of the miniature ADR (in addition to parasitic load) when operating at 250mK is 20μW, which increases to 45μW at 300mK and continues to increase linearly to nearly 1.1mW at 3K. To conclude, the predicted performance of a tandem continuous ADR utilising two of the miniature ADRs is presented.
Highlights
Adiabatic Demagnetisation Refrigerators (ADRs) use the process of magnetic cooling to reach millikelvin temperatures whereby cooling is achieved by reducing and controlling the entropy of a paramagnetic material; the alignment of the electronic dipole moments of the magnetic ions within the material is controlled by the use of a magnetic field
The ideal process of magnetic cooling involves: (1) a magnetic field being applied and the dipole moments aligning with the field reducing the entropy of the paramagnet; (2) the paramagnetic material being thermally isolated from its surroundings; (3) the magnetic field being reduced resulting
Operation of a single ADR is based on the ideal process of magnetic cooling described above, but real world limitations mean that magnetisation is not isothermal because of the limited thermal conduction of the heat switch, and demagnetisation is not fully adiabatic because of the parasitic and sample/detector heat loads onto the paramagnetic material
Summary
Adiabatic Demagnetisation Refrigerators (ADRs) use the process of magnetic cooling to reach millikelvin temperatures whereby cooling is achieved by reducing and controlling the entropy of a paramagnetic material; the alignment of the electronic dipole moments of the magnetic ions within the material is controlled by the use of a magnetic field. Operation of a single ADR is based on the ideal process of magnetic cooling described above, but real world limitations mean that magnetisation is not isothermal because of the limited thermal conduction of the heat switch, and demagnetisation is not fully adiabatic because of the parasitic and sample/detector heat loads onto the paramagnetic material. (2) a cooling period during which the paramagnetic is allowed to cool to the bath temperature via the heat switch; (3) isolation of the paramagnetic from the bath by opening the heat switch followed by demagnetisation At this point, the ADR can be either: (1) fully demagnetised and a heater used to provide a stable cold finger temperature with time (by decreasing the heater power with time); (2) partially demagnetised until the required operating temperature is reached and the remaining magnetic field 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; this can be achieved either manually or by the use of an automated control program.
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