University of Ottawa NMR Facility Web Site

Please feel free to make suggestions for future posts by emailing Glenn Facey.



Friday, February 29, 2008

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.

2 comments:

Anonymous said...

Does this mean that if there is a fire in the MRI scan room the magnet will "automatically" quench? What temperature within the room would cause this to happen?

Glenn Facey said...

Anonymous.

Thank you for the questions. The answer to the first question is: not necessarily. The magnet would only quench if the fire caused the integrety of the magnet to be compromised which would cause a loss of the insulating vacuum releasing the cryogens at which point the superconducting wire would become resistive and the magnet would quench. This would of course happen in the event of a major fire but may not necessarily happen with a small fire.

The answer to the second question is more difficult and magnet dependant. I would guess that a quench would occur when the temperature in the room reached the point when the rubber O-rings sealing the vacuum degraded.

Glenn