What is a Magnet Quench?

Superconducting NMR magnets contain a large solenoid coil of superconducting wire in a closed loop. The wire is superconducting (i.e. passes current without resistance) only when cryogenically cooled by liquid helium. On installation, the coil is cooled below its critical point, a current is introduced by way of an external power supply until the specified magnetic field is reached. A superconducting switch is then closed forming a closed loop through which current perpetually flows without the need for an external power supply. Should any part of the wire increase in temperature beyond its critical point, the magnet will quench. During a quench, the wire becomes resistive and therefore generates heat. The magnetic field is lost. The heat boils off the liquid helium very quickly. Magnet quenches can be very dramatic.

To see a 900 MHz magnet quench, follow this link.