1Normal: the magnet is charged
The six coils carry a steady current of several hundred amps round a closed loop of superconducting wire, sitting in liquid helium at 4.2 K. There is no resistance, so no voltage and no heating, and the field is a steady 1.5 T. About 4 MJ of energy is stored in that field.
2A hot spot: a few centimetres of wire stop superconducting
Something small releases a puff of heat: a wire slips by a few micrometres under the magnetic force, or a patch of epoxy cracks. That warms a short piece of wire above its critical temperature (about 9 K, lower in a strong field). That piece suddenly has resistance, and the current has to squeeze through the copper around the niobium-titanium.
3The normal zone spreads and the field starts to collapse
Current through a resistance makes heat (I squared times R), and at hundreds of amps that is a lot. The heat warms the neighbouring wire above its critical temperature, so the resistive zone grows along the wire and into the next layers and coils, at metres per second. The coils heat up and the stored magnetic energy turns into heat, so the current and the field fall.
4The helium boils violently
The hot coils dump their heat into the liquid helium. Liquid helium needs very little heat to boil, and each litre turns into about 700 litres of gas. The gas has nowhere to go yet, so the pressure inside the helium vessel climbs within seconds while the liquid level falls.
5The rupture disc tears and cold gas rushes out
At roughly 2 bar the thin foil in the turret tears open. Helium gas, colder than -200 °C, roars through the vent pipe to the outside. The machine shakes and hisses loudly, and the vent makes a white cloud where moisture in the air condenses. The field keeps falling as the current decays in the warming coils.
6How a quench is detected, limited and prevented
- Detection: a healthy superconducting coil has no voltage across it, so any voltage across a section means part of the wire has become resistive. Electronics watch for it and react within milliseconds.
- Protection: heaters on the coils spread the heat through the whole winding quickly, so the energy is shared instead of burning one spot, and diodes and resistors carry the current safely.
- Safe venting: the rupture disc and a wide vent pipe lead the gas outside. The magnet room has an oxygen sensor and a door that opens outwards, because the gas can push out the air.
- Prevention: coils are wound tightly and soaked in epoxy so the wire cannot move, the cold head and helium level are monitored, and the field is raised slowly by trained staff.
- Emergency use: the red magnet stop button triggers a deliberate quench, for example if someone is pinned by a metal object.
- After a quench the magnet must be checked, refilled with helium and re-charged to field, which usually takes days and costs a lot. Modern magnets hold only a small sealed amount of helium.