Background: Adiabatic Demagnetization Refrigerator
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 their own tiny magnetic fields. The magnetic fields of a molecules in a solid can be made to align in the same direction if the material is placed in a strong enough external magnetic field. Molecules in this solid also vibrate randomly due to the inherant temperature of the material. These vibrations are referred to as "thermal motion", and are a form of heat in the material. In ADRs, this material is referred to as a "salt pill", since it is often a form of salt.
Suppose the salt pill is first placed in a strong magnetic field. Each of the molecules align with the external magnetic field, and the magnetic energy of an individual molecule is minimal. If the strenght of that field is decreased, then the thermal motion of a molecule may throw it out of alignment with the field. The thermal energy is thus transformed into magnetic energy. This transformation causes the material to cool down. After a while, nearly all the molecules are out of alignment with the external magnetic field. If the magnetic field is now increased, the molecules re-align with the field, giving up their thermal energy. The material thus heats up.
Where to put this heat ? The salt pill is usually attached to a cold reservoir, with a valve which controls the flow of heat. If the valve is open, heat can flow between the pill and the reservoir. If the valve is closed, the salt pill is isolated. When the external magnetic field is increased, causing the pill to warm up, the valve is open so the excess heat can flow to the reservoir. After all the molecules are aligned, the valve is closed and the strength of the magnetic field is decreased for the cycle to start again.
[notes - for reference only, not intended for final version]
- Molecules have magnetic fields
- These magnetic fields can be aligned by an external magnet
- Molecules also have heat via their random motions
- relation between heat energy and magnetic energy
- the cycle
- Turn on Magnetic field to align molecules
- Reduce Magnetic field, with heat switch to the heat sink closed.
- random thermal motions knock molecules out of alignment with magnetic field. this converts heat energy into magnetic energy, and heats the salt pill. This is the "adiabatic" part.
- Increase Magnetic Field & Open Heat switch to heat sink/reservoir
- molecules re-align with field, giving up magnetic energy into heat energy, which heats the salt pill.
- excess energy flows to the heat sink via the opened switch. Salt pill cools (?)
- magnetic field reduced, heat switch closed.
see [may include some of these links in final version]
- http://cryowwwebber.gsfc.nasa.gov/introduction/ADR_intro/ADR_intro.html
- http://cryowwwebber.gsfc.nasa.gov/ADR/ADR_primer/ADR_primer.html
- http://asd.gsfc.nasa.gov/docs/xray/astroe/ae/adr.html
