monkeyerror asked: A while back, you mentioned something about an adiabatic refrigerator. Could you please explain how it works? I'm kind of curious
At my job, I use something called an adiabatic demagnetization refrigerator (ADR) to cool experiments to just a tiny fraction of a degree above absolute zero.
ADRs work by taking advantage of the second law of thermodynamics, you literally push entropy around to cool the test stage.
An ADR consists of three parts:
- A salt pill. At the core of the ADR is a magnetic salt. All of the molecules in the salt have an alignment, like a tiny little compass needle. At rest they point in random directions, but when you run a current through them, they line up.
- A superconducting magnet. This is a coil of superconducting wire—usually Niobium—wrapped around the salt pill. When you run current through the wire, it produces a large magnetic field up through the center of the coil. This is used to control how much the molecules in the salt line up.
- A heat switch. There are a few different varieties of these, both active and passive. The switch is used to thermally isolate the coil from the bath temperature (the temperature you are cooling down from.) Without the switch the ADR isn’t adiabatic, and it won’t work.
Running an ADR consists of three basic steps:
- Ramp up. First we want to magnetize the salt. We gradually increase the current to the coil, lining up the molecules in the salt, reducing its entropy, dumping heat from the salt out into the surrounding material. This heats up the entire system.
- Thermalize. Once the coil has been ramped up to its maximum current, we wait. The switch is still on, so the system is still connected to the bath. We let the bath—a liquid Helium reservoir—cool the system back down to the initial temperature, while maintaining the entropy level in the salt using the coil. At the end of this step, the ADR will have cooled to the temperature it started at, and it will now be magnetized.
- Turn off switch and demagnetize. At this point we break the thermal connection, so that the ADR is thermally isolated from the bath. If we don’t do this, the bath will share its heat with the system, and we wouldn’t be able to cool farther. Once the switch is off, we demagnetize the coil, gradually, increasing the entropy in the salt, pulling heat out of the surrounding material. Once you have ramped the current all the way down to zero the system will be quite cold!