The Adiabatic Demagnetization Refrigerator (ADR) is a cyclic cooling system. It alternates between two states.
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What sets the ADR apart from other refrigerators is the way it stores heat energy. Some refrigerators use a circulating gas. The gas absorbs heat at one point of its circuit, then flows to another point in the circuit where it dumps the heat. Some refrigerators use a liquid which evaporates as it absorbs heat, then condenses elsewhere in the circuit as it dumps heat.
The ADR stores heat in the disorder of magnetic moments of the molecules in a paramagnetic substance. In a paramagnetic substance, each molecule has a tiny magnetic moment. The magnetic moment arises from the angular momentum of the electrons in the molecule. Each electron has an orbital angular momemtum (resulting from its orbital state within the molecule) as well as an intrinsic spin angular momentum. In most types of molecules, the angular momenta of the various electrons cancel out to zero. In paramagnetic substances, however, each molecule has a certain nonzero electronic angular momentum.
Those electrons with their orbital and spin angular momentum are the microscopic equivalents of the coils of wire in an electromagnet. Thus, in a paramagnetic substance, each molecule acts as a tiny electromagnet.
LIke a compass needle, the magnetic moment of a paramagnetic molecule tends to align with an applied magnetic field. There are differences, however, between the way compass needles and paramagnetic molecules behave:
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This diagram shows the angles available for a magnetic moment associated with an
angular momentum of 5/2 (in units of Plank's constant divided by 2 Pi.) The
magneic field is assumed to be vertical. As the diagram shows, the magnetic
moment can point along the field, or directly opposite the field, or at small set
of directions in between. For those in-between directions, the diagram is a bit
oversimplified, since it implies that the magnetic moments are stationary. In
the standard model, however, the magnetic moments precess around the magnetic
field direction, as a child's toy top precesses around the vertical when as it
spins.
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With no applied magnetic field, the magnetic moments are randomly oriented, as
shown in this diagram of a small piece of a paramagnetic substance. As you can
easily see, the magnetic moments in this diagram have a high degree of disorder,
and thus high entropy.
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The amount of energy required to knock a molecular magnetic moment out of
alignment is proportional to the applied magnetic field. Thus, for a low applied
magnetic field, the energy of random thermal vibrations is enough to knock many
magnetic moments out of alignment, as shown in this diagram of a paramagnetic
substance with a weak applied field. The magnetic moments in this diagram are
more ordered than those in the first diagram. Thus, this group of magnetic
moments has lower entropy than those in the first diagram.
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With a strong enough applied field, virtually all the magnetic moments are forced
into alignment with the field, as shown in this diagram. As you can easily see,
the magnetic moments in this diagram have a high degree of order, and thus a low
value of entropy.
When the magnetic field starts at a high value, enough to align most of the magnetic moments, and then drops to a low value, many of the magnetic moments drop out of alignment with the field. As described above, the magnetic moments absorb thermal energy as they move out of alignment with the field. In absorbing the thermal energy, the magnetic moments cool the paramagnetic substance. In other words, as the field drops, the entropy of random thermal vibrations is transformed into the entropy of random magnetic moment alignment.
When the magnetic field increases again, the magnetic moments drop back into alignment with the field. As they drop into aligned states, they give up the energy they absorbed. The energy then appears as heat energy of the substance. In other words, as the field increases, the entropy of random magnetic moment alignment is transformed into the entropy of heat energy.
The ability of a paramagnetic substance to absorb heat energy and transform it into disorder of magnetic moments is the basic principle of the ADR. Now for some practical details.
The main parts of the ADR are:
To see how these parts work together, go on to the operating cycle page.
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