Cryogenics for English Majors

A Recipe for Liquid Helium

Let’s leave the MagLab for a little field trip to find out more about where our helium comes from.

Welcome to the amber waves of grain of Kansas. It is here and in several other Great Plains states that our quest for liquid helium begins. It’s one of a few spots in the world where helium gas can be found in relative abundance, if you dig in the right place.

Natural gas contains helium. Though the concentrations are small – usually less than 1 percent – it’s nonetheless much more plentiful there than in our atmosphere. Helium can be distilled out of natural gas, then shipped to the MagLab (not to mention countless party supply stores) in high-pressure tube trailers.

What do you need in order to turn that gas into a liquid? Why, a helium liquefier, of course. The Magnet Lab has three. Two of them are part of a closed system that recycles helium used by the world’s most powerful continuous field magnet, a 45 tesla hybrid that is part superconductive, part resistive electromagnet. (Tesla is a measure of magnetic field: 45 tesla is one million times the Earth’s magnetic field.) The magnet draws scientists from across the globe who use it for experiments, and a considerable amount of helium is needed to keep the superconducting part going around the clock. The magnet is not in use all the time, but is always kept at cryogenic temperatures (except for maintenance and repairs) because it takes more than a month to get from room temperature down to operating temperature.

The MagLab’s third helium liquefier services the rest of the facility, including a 21 tesla, 900 MHz superconducting magnet used by scientists for nuclear magnetic resonance.

The transformation from a room-temperature gas to a very cold liquid is less like a slippery slope downward and more like a hard climb up. It takes a fair amount of human effort to get those atoms to scale back on all their effort. Remember all those athletic gas atoms we discussed earlier? Well, the trick is to get them to slow down and relax. Because as they chill out, they – well, they chill out! Less work = less heat.

Our Magnet Lab specialists do this using a cyclical process of compression and expansion. First comes a one-two punch called isothermal compression: We cool the helium down with liquid nitrogen while at the same time compressing it.

The liquid nitrogen does its job through heat transfer – the same idea that’s behind cooling down a warm soda by sticking it in the fridge. At 77 K (-320 degrees Fahrenheit), the liquid nitrogen absorbs the helium’s heat, carrying it off in evaporation.

The gas is then allowed to expand into a larger area. Those helium atoms do work as they spread out; in so doing, they give off heat and cool down. This cycle is repeated several times as the helium gets colder than an Alaskan winter, frostier than Antarctica at its worst. Down, down, down goes the temperature, from about 300 K (room temperature) to, eventually, about 4 K, when the helium finally yields and turns from gas to precious drops.

In some liquefying systems (such as the Magnet Lab’s), helium is used to cool helium. In other words, the incoming helium – the stuff you’re trying to liquefy – is exposed to the outgoing helium, which has just finished its shift cooling the superconducting magnet. Though the outgoing helium is “warmed up” by now and gaseous, it is still chilly enough to draw heat from the incoming helium.

Very cool, huh? (Now that’s an understatement!)

So, why is it we need all this helium again?

Next Page Some Like it Cold

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