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Adiabatic Demagnetization Refrigeration (ADR)

An adiabatic demagnetized refrigerator is made to cool things down. In order to do so, the machine goes through four thermal stages after the initial cool down. The machine itself is a coil of wire surrounding a salt pill. Inside the salt pill (made from salt ferric ammonium sulfate) are magnetic ions that align themselves with the magnetic field created by the coils. When the magnetic field releases, the ions de-align, take in energy, and cool down the system.

To begin, the system goes through a rapid cool-down by quickly decreasing the current in the coils and thereby decreasing the magnetic field. This makes the magnetic ions in the salt pill become unaligned so that they absorb the heat energy, keeping the entropy of the system stable but decreasing the temperature. Then, in order to keep the temperature constant for a long time, the current is put into a PID control system so that it can react to the changes coming in from outside the system. The temperature is constant, but the entropy of the system is increasing because the entropy is being added to the system from the outside. When the salt pill has absorbed all of the heat it can, the magnetic field is increased. Here, the opposite occurs from the rapid cool-down. Entropy remains constant, but because the magnetic field is increasing, the ions in the salt pill become aligned, increasing the temperature. Once the salt pill has become hotter than the heat sink, the heat switch is closed, connecting the liquid helium heat sink to the salt pill. Once the heat has been released, the heat switch is disconnected and the whole cycle begins again.