Quenching

MRI magnet quench: the movie

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• published: 04 Oct 2012
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After two days of testing various magnetic objects for the amount of force they exert when allowed into the vicinity of a 4 tesla (T) superconducting magnet, it was finally time to turn the old gal off. There are essentially two ways to do it: the long way - connecting the magnet to a special power supply and running it down - or the short way - by pushing the Big Red Button that is present in case of emergencies. We chose the latter. Why? Because we would never be using the magnet again and, well, because we wanted to see what would happen. After all, we are scientists. FAQ: 1. So what exactly is a quench? In a controlled quench - which this was because it was initiated using the magnet's built-in circuitry - all the electrical energy (tens of megajoules) is dumped as heat across diodes, causing all the liquid helium in the cryostat to expand and blow through a carbon "burst disk" located in a special duct atop the magnet. The circuit can be activated with an emergency button. It takes about 30 seconds for the field to decay to near zero. 2. Is it dangerous to the magnet to quench it? Yup. A quench is violent; vibration can damage the superconducting wire, for example. And air ice can get into the magnet turret via cryopumping if a new burst disk isn't installed in the magnet turret very soon after the quench. Air ice is as hard as steel, and really difficult to remove! 3. Is a quench dangerous to people? It could be. The helium gas vents at near supersonic speeds. Note the characteristic humming sound as it vents. (And you thought helium gas only made squeaky noises if you breathe it in and then talk like Mickey Mouse!) The helium gas is a shade above -269 C. Yes, C. It will freeze anything it touches. If you were to breathe in the gas your lungs would freeze and be massively damaged. That's why the quench duct vents atop the building, where the gas can dissipate into the atmosphere without coming into contact with anyone. 4. Can a quench go awry? Yup. If there is a blockage in the quench duct, or if something fails, e.g. because of the combination of ultra-cold temperatures (that tends to turn everything brittle, even steel) and vibration, then helium gas could end up somewhere it doesn't belong. Worst case would be helium pressure building up because of a blockage; that's akin to a bomb. Second worst would be helium gas blowing back into the magnet room because of a breach somewhere in the duct between the magnet and the roof. 5. Why did you quench it? The magnet was a decade old and no longer used for research. Weighing four tons and requiring expensive rigging to remove/ship, and with a scrap value only in the tens of thousands (there's not much of a market for old 4 T magnets), it was better for us to convert it into a "mock scanner," for practice functional MRI sessions. So the specialty equipment is still in use, just not as a scanner. 6. What happened to the scanner after the quench? We turned the old magnet cryostat into a "mock", or zero field, scanner for training functional MRI subjects. It's still working in this capacity and will be for a long time. We donated the old electronics and patient bed to the manufacturer to support the three or four existing 4 T magnets still out there. 7. Why didn't you donate the magnet, or sell it to someone who could use it as an MRI? The cryostat is steel on the outside, lots of Mylar insulation and other gubbins on the inside. The wire is a superconductor, Nb-Ti alloy. The whole thing weighs about 4 tons. It's a lot of money to crane/move these behemoths. The scrap value is far less than the cost to move it, and the cost to someone else who wanted to use it would be about $200,000 just to remove it from the current location. How much more it would cost to ship and re-install depends on the journey, but there aren't many organizations who would pay hundreds of thousands for something that is essentially obsolete. Luckily for us, we had need of a mock scanner. 8. Why did you waste all that helium? Isn't there a shortage? There isn't a shortage of helium per se - it's the second-most abundant element in the universe, after hydrogen. The shortage is specifically liquid helium, and the reasons for the shortage are entirely financial/political reasons. Helium is recovered as a by-product of some natural gas and oil operations, but there are other ways to capture it, except that they cost more money. 9. But couldn't you have captured the helium for reprocessing? There's no easy way to push liquid helium out of a supercon magnet; they aren't designed that way. And capturing all that gas would require a truly massive recovery vessel. The expense would be hundreds of times the value of the liquid helium. Some MRI facilities - if they have a lot of magnets near to each other - may have a recovery system "plumbed in," but the cost of such as system is prohibitive for a single MRI magnet.