MRI Scanner Superconducting Magnet Quench
Once a superconducting magnet is ramped up and fully magnetized, it literally takes no additional current or power to keep the magnet going. There's zero resistance -- that's the "superconducting" part -- so the current flowing in the magnet coils will run forever. That is, forever if the liquid helium cooling the magnet is kept cold enough, which is quite close to Absolute Zero. If the cooling system goes on the fritz, the magnet starts to develop resistance, which cause heat, which causes more resistance, and more heat, and so on until all the liquid helium gets hot enough to become a gas, which then erupts in a jet-engine-sounding event known as a quench. That's thousands of dollars worth of helium you see in the photo sequence above, going bye-bye.
The photo is of the GE MRI manufacturing plant where I used to work, and I myself saw a magnet quench much like this one. Unforgettable, much better than inhaling those balloons and talking like Donald Duck for 15 seconds. In a wide-open space like a manufacturing plant, there's really not much cause for alarm. But in a smaller space, the inert helium can displace all the oxygen and kill you. Several years ago a GE field engineer was killed by a slower, less spectacular helium leak at a customer's site. In a small, enclosed space it can sneak up on you pretty quick.
I found this photo here. You can go there for more information, but it was all Greek to me.