Catastrophic quenching

Superconductors make wonderful magnets and are used in a wide array of scientific and medical areas. These superconductors are not the "room temperature" type, but rather require temperatures close to 5 kelvins and thus are cooled with liquid helium (helium-4).

When the superconductor quenches, it suddenly looses its super conducting properties and becomes a resistor - that heats up rapidly. This in turn heats up the liquid helium and what was previously a liquid at 5 kelvins, is now a gas that takes up significantly more volume. "Bang!" is an accurate representation for what happens in the small superconductor (about 30 ampere running through it). The dimensions of the 'can' as it is called are about 5 feet tall and a diameter of about 2 feet. This contains a dewar of liquid nitrogen encasing a dewar of liquid helium and vacuum chamber, and the magnet itself.

Larger magnets (with about 60 or 70 ampere or larger) produce much more heat when they quench and become a resister instead of super conductor. Furthermore, they have significantly more liquid helium encasing them. When these large magnets quench, some believe that the explosive expansion of the helium gas at the core will blow away liquid helium into the air around it. As mentioned above, liquid helium is at 5 Kelvin and this could cause the air in the room to condense into a liquid and rain out.

While this has never been seen, very few want to see in person. For it to rain air, to happen two things must be the case:

• Liquid helium must be able to exist at room temperature and pressure for a period of time.
• Air must rapidly condense at near liquid helium temperature.

When refilling the helium in a superconductor, an elaborate process of evacuating chambers and forcing liquid helium in it to be certain that it is pure liquid helium in the dewar (impurities such as liquid oxygen cause many problems such as rapid oxidation). During this, the pipe carrying the liquid helium runs through the tank to the can. As this pipe cools down, a liquid condenses on it and drips off. This liquid is not water, but rather air. Furthermore, after the tank is topped off, the pipe is removed but still under pressure and liquid helium exists for a brief period of time in the air (it gets about 2 or 3 inches out of the pipe before evaporating).

So, could it rain air from a quench? Maybe... However, this should not keep one from getting in a medical NMR (where the subject being scanned is you). These super conducting magnets are vented so that while it will get cool (not cold) and foggy in the room, the liquid helium is vented in a safe manner. These super conducting magnets are actually forced to quench to verify that no danger to anyone around will occur.

As m_turner mentioned, catastrophic quenching is an important feature of MRI scanners. These scanners basically contain incredibly strong superconducting magnets (1.5 Tesla, for those of you who know about such things). As the safety video says,

The Magnet is Always On

and it doesn't shut off when you're not actively scanning. Thus, if you bring anything ferromagnetic into the scanning room, it's likely to go flying into the scanner. That's a minor problem if the scanner is empty and it's a small object like a paperclip; you can easily reach in and pull it off (though it'll take quite a tug). If, however, you do something even dumber--like bringing a wheelchair, oxygen bottle, or sledgehammer into the scanning chamber while a patient is lying in there--it too will go flying into the scanner at high speed aimed right at the patient's head. Now remember,

The Magnet Is Always On

and human beings aren't strong enough to yank a heavy object out of the scanner and off the patient (who is currently bleeding to death). In this case, you push the Big Red Switch located on the scanner or the operating console, which vents the coolant, wrecks the superconductivity, scrams the magnet, fills the room with vapor, and allows you to get the heavy metal object off the patient. It's a procedure about as serious as venting the warp core, in that you only do it when you're really screwed; it also has about a fifty-fifty chance of trashing the multi-million-dollar MRI scanner, so you don't want to hit it by accident (as my advisor once did before they installed the molly-guard). Although you never want to see someone quench an MRI scanner, it's still good to know that the safety feature's there.