|
|
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.
For more information see
|
