Quenching the Magnet

July 28th, 2008 by Potato

We’re getting a new MRI unit here at work (actually, an MRI-PET-EEG hybrid imaging system). To make room for it, our old head-only scanner got the boot.

An MRI has a very strong static magnetic field (in the case of this scanner, 3 Tesla), about 60,000 times the strength of the Earth’s magnetic field (the magnetic field that lets you navigate with a compass). This field is on all the time (no matter how much TV and movie writers want to flip it on and off for plot gimicky reasons), due to a loop of current running through some superconductors (a superconducting electromagnet). To keep the superconductors superconducting, they are bathed in liquid Helium. To keep the liquid Helium from boiling off too quickly, it is in turn bathed in liquid Nitrogen (liquid Nitrogen is warmer than liquid Helium, but the difference is a lot smaller than the difference between liquid Helium and air, so the boil-off happens much slower, and liquid Nitrogen is much cheaper to boot, so it’s easier to keep topping up a large liquid Nitrogen buffer surrounding a small liquid Helium bath than to just keep toping up the smaller liquid Helium bath).

When something happens and we need to ramp down the field in the magnet, we “quench” it, venting the Helium and Nitrogen outside, letting the superconductors warm up, and then the magnetic field will come down as the current loop burns itself out (as heat due to the now non-zero resistance in the coil, and also by shunting the current away). Quenching is a very rare event, because it usually happens only when something bad happens and someone hits the emergency quench button (and we try really hard not to hit that button, as it can take a few weeks to get the MRI working again afterwards), or when a scanner has to be taken down to be moved. So our scanner move let us witness one of these rare events (though when I say “us” I don’t include myself, since I unfortunately forgot and didn’t show up to watch it in person, but I got the picture!). It’s just amazing to see that huge plume of condensation appear outside the vent — the now-boiled and thus gaseous Helium and Nitrogen are so cold that they condense not only the water vapour, but also the CO2 and Oxygen right out of the air.

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