The micro ADR was a novel, ultra compact ADR that I worked on during my Ph.D. It was developed as an additional cooling stage for the Oxford Instruments Heliox refrigerator, to extend the temperature range of the Heliox from 300mK down to 10mK. However, it was also designed to be compatible with our double ADR design, allowing the two systems to be linked to produce a triple ADR. The micro ADR could then be used either as a normal cooling stage to extend the temperatures range of the triple ADR, or as a thermal reservoir to allow continuous cooling. This latter scheme has been adopted in the Goddard Space Flight Centre's Advanced ADR design. The most important feature of the micro ADR was its compact size: it measured only ~20cm long and ~3cm in diameter. This allowed it to fit inside the Heliox refrigerator, and also provided valuable information for the design of future ADRs for use in space, where the mass, power consumption and volume of the device would all have to be minimised.
The micro ADR.
The micro ADR mounted on the Heliox refrigerator.
In order to minimise ite size, the micro ADR incorporated several novel design features, including an in-line mechanical heat switch. The paramagnetic refrigerant material used was cerium magnesium nitrate (CMN), allowing the micro ADR to reach base temperatures as low as ~10mK. In order to maximise thermal contact between the refrigerant and the cold stage, a novel thermal bus design was used, consisting of a set of fins connected to a central pillar. This allowed the entire thermal bus and cold stage to be made from a single piece of HCOF copper, maximising the thermal conductivity, in contrast to the more usual designs consisting of a number of copper wires welded to the cold stage.
The micro ADR heat switch. The coil former (right) contains an in-line plunger which engages with the jaws (left) and closes them onto a tongue on the top of the salt pill.
The micro ADR salt pill components prior to assembly and growth of the CMN refrigerant material. The CMN was grown directly onto the thermal bus, using seed crystals which can be seen in the photograph.
The assembled micro ADR salt pill. The cold stage is at the bottom of the image, and the heat switch tongue can be seen at the top of the salt pill.
During the design of the micro ADR I wrote an extensive library of ADR modelling software. This code was tested against experimental results from the MSSL laboratory ADR, and then used to predict the performance of both the micro and double ADRs. Some results from the micro ADR model are shown below, indicating that the micro ADR will achieve base temperatures in the region of ~10mK. The model also predicted a hold time of 19 hours at 10mK with a parisitic heat load of 10nW. For comparison, the Heliox system provides 20 hours of hold time at 300mK with 10nW of parisitic heating.
TAPS prediction of the dependence of the micro ADR base temperature on the magnetic field applied with a 0.3K bath temperature.
The micro ADR development program is continuing at the Mullard Space Science Laboratory. You can read about it in more detail, and see all the simulation results for this and other ADR systems, in my Ph. D. thesis.