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The Adiabatic Demagnetization Refrigerator: A Cyclic Magnetic Cooler

The Adiabatic Demagnetization Refrigerator (ADR) is a cyclic cooling system. It alternates between two states.

simple ADR diagram D

Physical Background: Paramagnetic Materials

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:

arrows pointing every which way D 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.

arrows, about half pointing up D 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.

arrows, all pointing straight up D 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.

Parts of the ADR

The main parts of the ADR are:

Salt Pill
This is the block of paramagnetic substance. In many cases, this is a salt. For example, the XRS ADR uses Ferric Ammonium Alum (FAA, also called Ferric Ammonium Sulfate.) However, chemicals other than salts have been proposed, such as Gadolinium Gallium Garnate (GGG.)
Magnet
The magnet provides the magnetic field that controls the flow of entropy and energy into and out of the molecular magnetic moments.
Thermal Sink
The thermal sink is where the heat is dumped after it leaves the salt pill. In the XRS ADR, the thermal sink is a bath of liquid helium coolant. For other ADR's, the thermal sink could be some other cryogen, or even another cooler.
Heat Switch
The heat switch allows the salt pill to make or break contact with the thermal sink. When the switch is turned on, heat can flow. When it's turned off, heat cannot flow.

To see how these parts work together, go on to the operating cycle page.


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Curator: Brent Warner
NASA Official: Susan R. Breon
Last Updated: September 15, 2004