University of Ottawa NMR Facility Web Site

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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.


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...


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.


Unknown said...

If i may ask, what happen if liquid helium reach minimum level that we still not refill it?

Is there possible to be quench without opening the helium turret?

Glenn Facey said...

Nizam I,
If the helium level goes below the minimum threshold, the magnet coil will begin to warm up. Once a part of the coil heats up beyond the critical superconducting temperature, the coil becomes resitive and the current will generate heat which will very quickly boil off the remaining liquid helium. This is a quench. The magnet can indeed quench without opening the helium turrets.


blog this bog out said...

Usually NMR labs are maintained in positive pressure but during an emergency quench can the lab in negative pressure?
Also to remove the helium and nitrogen can we have single duct system considering either helium or nitrogen will be released at a time?

Glenn Facey said...

I'm not clear on what you are asking.
The pressure in the lab is likely to be higher during a quench due to the evaporation of the liquid helium.
Nitrogen is not usually an immediate problem during a quench. Some labs with large high field magnets have quench tubes to vent the helium outside. Most labs with smaller magnets rely on the ventilation system or open doors to get rid of the helium. If the room is small, it is a good idea to leave the lab until the helium has dissipated to avoid asphyxiation. Some labs are equipped with oxygen sensors.