NASA Goddard Cryogenics Group

The Adiabatic Demagnetization Refrigerator:
Temperature and Entropy Cycle

Operating Cycle 1: Brief Description

Here is a quick description of the ADR cycle. We'll go over it in more detail after the temperature-entropy diagram.

Rapid Cooldown
The magnetic field is dropped rapidly. The salt pill cools as the molecular magnetic moments drop out of alignment with the field, absorbing entropy.
Slow Cooling
Once the cooling reaches the desired temperature, the rapid decrease of the magnetic field stops. Heat slowly leaks in from outside. The magnetic field is slowly dropped, to allow the salt pill to absorb the heat without increasing in temperature.
When the salt pill has absorbed all the heat it can, the magnetic field is increased again. The salt pill warms as the magnetic moments align and dump their energy.
Dumping Heat
When the salt pill is hotter than the thermal sink, the heat switch is turned on. Heat flows from the salt pill into the thermal sink. Then the heat switch is turned off and the cycle begins again.

Operating Cycle 2: Temperature Entropy Diagram

I have a fairly long description of the following graph for those who'd like a guided tour. I wrote it originally for users with text browsers, then realized that its slightly different approach might be useful to anyone with any type browser. Feel free to read my graph description, or to read this page, or to try both. ADR temperature entropy graph

This diagram shows how the temperature and entropy of the salt pill vary through the cycle. The vertical axis of the graph is entropy divided by the universal gas constant, R, and is therefore unitless. The horizontal axis is temperature in degrees Kelvin (that is, absolute temperature.)

The green curve near the bottom shows the entropy of the pill at high field. The blue curve higher in the graph shows entropy at zero field. At any temperature, the zero-field entropy is higher than the high field entropy. The zero-field entropy includes both the entropy of random thermal motion (heat energy) as well as the entropy of the disordered magnetic moments. The high field entropy includes only the thermal randomness, since the magnetic moments are all lined up by the field.

The lines between the two curves show ADR cycles, an "ideal" cycle, which is shown as a rectangle, and a "non-ideal" cycle, which extends along a curved line to higher temperatures and which overlaps the full-field curve for some distance. The stages of the ideal cycle are labelled 1 through 4 on the graph. Here are descriptions of those stages:

1. Rapid Cooldown

2. Slow Cooling

3. Warmup

4. Dumping Heat