Cryogenics for English Majors

Some Like it Cold

The lab’s cyrogenics department maintains a fleet of 44 portable dewars – specially insulated containers that keep the liquid helium c-c-c-cold. These dewars are ubiquitous at the MagLab: scientists and students gingerly wheel them around to their experiments like proud mommas pushing perambulators.

The helium is used for three main reasons.

We’ve already explained the most important use: running the magnets, particularly our 45 tesla hybrid magnet and the 900 MHz NMR magnet. The superconducting part of these instruments must have an ambient temperature of less than 2 K to keep running.

What happens, you may be wondering, if the temperature creeps higher? The dreaded quench. This is not the kind of quench that involves Gatorade or iced tea. It’s the consequence of the temperature rising above the minimum required to create superconductivity in the coils carrying the current that powers the magnet. If even a single coil among the 4 miles worth of coils in the magnet loses superconductivity because it gets too warm, the magnetic field will begin to break down in a series of unfortunate, domino-like events that works like this: When that segment stops superconducting, it begins conducting electricity normally instead – which means it gives off heat in the process. That heat will in turn increase the temperature of the adjacent coils, which in turn stop superconducting and begin to conduct electricity normally, generating more and more heat and eventually forcing the magnet to shut down completely, possibly damaging it.

The second use for helium at the lab is to cool down some of the experiments that go into the magnets. These ain’t no horseshoe magnets. They’re shaped more like a dowel with a small hole drilled through the center. That center is called the bore and that’s where the experiment goes, because smack dab in the middle of things is where the magnetic field is most intense.

The third and last use of helium here is as a fascinating subject of study in its own right. Researchers at the Magnet Lab conduct many experiments on this element at facilities that include the Liquid Helium Flow Facility, the Cryogenic Helium Experimental Facility and the Cryogenic Flow Visualization Apparatus. It is being studied to find ways to make better use of liquid helium as a superconductor cooler, and as a fascinating state of matter that scientists still only partially understand. Steven Van Sciver, a mechanical engineering professor and Magnet Lab scientist, has been investigating how superfluid helium can be used to separate extremely tiny particles. His work is of great interest to the pharmaceutical industry, as it could lead to improving drugs that are inhaled.

One fascinating aspect of helium that intrigues scientists is superfluidity. At 2.17 K, helium completely loses its viscosity. Left in a closed ring, it could flow without stopping: There is no friction, much as a superconducting current encounters no resistance. It gets even curiouser. This superfluid conducts heat perfectly. As a result, it actually rises up the sides of an open container and flows over the top. You can even make a fountain out of the stuff simply by heating it even slightly.

That kind of gravity-defying magic is almost as eerie as the sailors in Coleridge’s ballad who, though lifeless, manage to get up and usher their boat out of the icebergs and back to (relatively) tropical England. In fact, why not close our story with that very scene, as the poet evokes the same sense of mystery and awe often inspired by science.

They groaned, they stirred, they all uprose,
Nor spake, nor moved their eyes;
It had been strange, even in a dream,
To have seen those dead men rise.

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Thanks to the Magnet Academy's scientific advisers on this article. They were Dr. Steven Van Sciver, a professor of mechanical engineering with the Florida A&M University/Florida State University College of Engineering and an expert in cryogenics at the Magnet Lab, and Mr. John Pucci, Coordinator of Research Programs and Services for the Magnet Lab's Cryogenic Operations.