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How does an Adiabatic Demagnetization Refrigerator Work ?

An Adiabatic Demagnetization Refrigerator (ADR) works by using the properties of heat and the magnetic properties of certain molecules.

Some molecules have large internal magnetic fields, or "moments". Just like a tiny bar magnet, these molecules will align themselves with an external magnetic field. The random thermal motions of the molecules, on the other hand, tend to de-align them. The higher the temperature, the more they de-align. ADRs generally use certain types of salts for the molecules, because they have particularly large magnetic moments. The salt is contained in a cylinder, usually called a "salt pill". This salt pill is thermally connected to the object we want to cool (our X-ray detectors, for example)

Suppose the salt pill is first placed in a strong magnetic field. The molecules align with the external magnetic field, and the magnetic energy of each molecule is minimal. If the strength of that field is decreased, then the thermal motion of the molecules starts to twist them out of alignment with the field. This requires energy, which comes from the thermal motion of the molecules. The thermal energy is thus transformed into magnetic energy, cooling the salt pill down. As heat flows into the salt pill from the outside, the magnetic field is slowly reduced. This allows the molecules to twist further out of alignment, absorbing more heat. The rate at which the field is reduced can be regulated so as to keep the salt pill at a constant temperature as it absorbs heat. Conversely, increasing the magnetic field will convert magnetic energy back to thermal energy, raising the temperature of the salt pill.

Eventually the magnetic field is reduced to zero, and no more heat can be extracted by the salt pill. At this point, all that can be done is to increase the field, heating the salt pill. So far we have accomplished the feat of converting thermal energy to magnetic. Now we need to turn it back into heat and dispose of this heat somewhere.

If the magetic field is increased to a much higher value than it originally was, the temperature of the salt pill will increase. This will also raise the temperature of the detectors, rendering them temporarily inoperable. However, the salt pill temperature can actually be higher than the temperature of the surrounding dewar. (A dewar is a container, like a thermos, which holds a cold material, such as liquid Helium or liquid Nitrogen.) At that point it can be thermally connected to the dewar until it cools to the temperature of the dewar. Thus the heat originally absorbed by the salt pill is dumped to the dewar.

The salt pill is then disconnected from the dewar and the magnetic field is slowly reduced, beginning the cycle again. The temperature of the salt pill quickly reaches a level at which the detectors can operate, and the temperature is again regulated by adjusting the rate at which the magnetic field is reduced.

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