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.