Quench Awareness Tutorial:

What is a quench?
A cryomagnet is basically just a closed loop of superconducting wire that allows an electric current to flow perpetually through it without any resistive losses, thus generating a stable, "permanent" magnetic field. The wires that are used to wind high field cryomagnets are only superconducting when cooled to very low temperatures. For this reason, cryomagnet coils are submerged in liquid helium at 4.2 degrees Kelvin.
Although there is no resistance in a cryomagnet coil when properly cooled and energized, there is a great deal of energy stored in the electric current. If, for any reason, a very small portion of the superconducting wire stops being a superconductor (goes "normal"), then the resistive heat generated in that section of wire will cause neighboring sections of wire to also go normal, resulting in a chain reaction that swiftly warms the entire coil and causes all of the stored energy to be dissipated as heat. This process takes only a few minutes, and can result in an impressive display of rapidly boiling liquid helium, similar to a geyser.

What can cause a quench?
Quenches are frequently due to improper liquid helium filling procedure, or failure to fill with helium at the required interval (a "dry" quench). Another common cause is the attraction of a large, ferromagnetic object, such as a chair, ladder or mop bucket, against the side of the magnet. While these are the most common causes, magnets sometimes quench for no apparent cause, sometimes even after years of uneventful service.

What are the consequences of a quench?
Fortunately, magnets manufactured after 1980 are well protected against quenches, and are rarely damaged by them. However, an elaborate and time consuming process is needed to bring them back to field. Typically, this is what must be done:

1. Fill the vacuum space with nitrogen gas and warm the magnet to room temperature.
2. Take the magnet apart
3. Reposition the magnet coil and concentric bore tubes inside the magnet
4. Replace the helium "getters"
5. Reseal the magnet with new 0-rings
6. Re-pump the vacuum
7. Fill the inner and outer dewars with liquid nitrogen to pre-cool the magnet
8. Remove the liquid nitrogen from the inner dewar and fill with liquid helium
9. Energize the magnet to the proper field (can take up to three days)
10. Adjust the magnet's homogeneity by putting current in the cryo-shims.


The entire procedure requires two weeks or more of on-site labor, in addition to parts and special equipment such as a pump, magnet power supply, and magnet lifting device. Typical cost is $18,000, and typical down time for the NMR system (including waiting for a magnet engineer to be available) can be four weeks or more. If significant time is required to obtain the needed funds, then the instrument down time can be much longer.


What can I do to protect against a quench?
1. Use care to exclude magnetic items from the area of the magnet.
2. Monitor the helium level, and fill regularly and well before the magnet runs out of helium.
3. Use proper equipment when filling the magnet, including a proper pressure regulator
and gage for the helium gas used to pressurize the liquid helium storage vessel.

What should I do if the magnet is about to quench?
If you feel that the magnet is about to quench, due to a sudden increase in the boil-off rate, or because the magnet is nearly out of liquid helium:
1. Fill the magnet with liquid helium if available.
2. Otherwise, DO NOTHING!!!


Many NMR systems are delivered from the manufacturer with a complete magnet "de-energization" kit. In our opinion, this is not a good idea, since a magnet can be easily damaged if an attempt is made to remove the field by someone without proper training. Even a skilled engineer will not attempt to remove the field unless the magnet is nearly full of liquid helium. It is very rare for a cryomagnet to be damaged if it is allowed to quench without interference. However, if it quenches during a hurriedly organized "'rescue attempt," the chances of damage to the magnet go up sharply.


What should I do after a quench?
If possible, fill the magnet with liquid helium within 24 hours after the quench. This will save considerable time and cost in restoring the magnet to field. If this is not possible within 24 hours, then simply leave it alone and let it warm up. Either way, don't worry too much. The chances of anything actually being damaged are very low.

Why purchase a QRA?
A quench recovery agreement protects you in two ways. First, against the unforeseen expenses associated with a quench. Second, against lost productivity associated with long delays in obtaining funding, issuing a purchase order, and waiting for a magnet engineer to be available. With an MRR QRA, just give us a call and you will have our top priority with no hassles and no red tape in the way.