This is the sort of breathtaking (literally) experience that not many people have had... For some it's a really unfortunate occasion (ie: not intentional) but luckily for me, this was an intentional quench as the magnet was to be scrapped.
I say literally breathtaking as one of the risks is brain damage or death caused by Asphyxiation and Hypoxia. - depletion of oxygen in the room due to the Helium gas displacing it.
Here's a video of me "Quenching" an end-of-life Superconducting Magnet at Reading University (UK). Keep an eye on the spanners I stuck to it.
The Helium gas is clear, what we see as white clouds is actually moisture in the air condensing upon contact with the cold Helium gas. In this video, I'd opened up a port on the rear of the Helium manifold to allow the gas to escape, but the pressure was so high that gas is also seen to escape from the black pressure relief valve on top of the manifold.
A Superconductor, in this case a coil of superconducting wire forming an electromagnet / solenoid, relies upon being kept at a very cold temperature for it's resistance to stay at Zero Ω.
The Superconducting coil in a typical NMR magnet is kept in a cryogenic bath of Liquid Helium, which is at 4.2 Kelvin (-269°C). That's 4.2°C above Absolute Zero.
The Liquid Helium (LHe) relies on being kept within a vacuum chamber (a Dewar) to keep it away from room temperature air outside. If the helium tank was to be exposed to air, the air would freeze onto the side of the tank forming air-ice (water, plus frozen nitrogen), and the Liquefied Helium would boil away.
When Helium turns from a liquid to a gas, it occupies roughly x700 the volume (at atmospheric pressure), so 10 litres of Liquid Helium would turn into 7,000 litres of helium gas. This is obviously a problem, as it will cause a massive increase of pressure in the Helium tank, so magnets are designed with pressure relief valves, bursting disks etc..
So, If the vacuum chamber fails (ie: I purposefully allow a whisper of room-temperature helium gas into the vacuum chamber - it will carry that little bit of room-temperature heat energy to the helium tank, which warms up a small spot of the tank a touch, just enough to start a tiny bit of the Liquid Helium boiling - it's boiling point being 4.2 Kelvin.
The bubbles from this tiny bit of boiled Liquid Helium pass by the superconducting wire of the magnet, whereupon some of the wire ceases to be a superconductor, ie: it becomes resistive! The current flowing in the superconducting magnet coil now is passing through a slightly resistive part of the coil, which generates heat... The heat generated boils off more helium, which allows more of the magnet coils to become resistive, which boils off more Helium, which makes more wire become resistive (you can see where this is going) and the whole thing goes up in seconds!
In that very short time period, the current that may have been flowing round and round for 30 years or so, is dumped as a whole load of heat energy, which makes sure that not a single drop of Liquid Helium is left inside the tank.