Naked Science Forum
Non Life Sciences => Technology => Topic started by: evan_au on 02/06/2015 11:25:17
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Tonight I was talking to a radiologist, and she said that the only way to shut down the superconducting magnet on an MRI machine was to "quench" it. Apparently that damages the magnet, and it has to be replaced.
So how do you do a controlled shut down of an MRI machine, without damaging it?
What happens if there is a power failure? Or the cryogenic refrigerator breaks?
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Short power failures are not a big issue unless the coolant boils off too much; the whole thing is very well insulated.
At low field strengths the quench won't do much; but at high fields, such as if the refrigeration system fails, the quench will cause the helium to boil off in large amounts from the inductive energy stored in the magnetic field and this gets expensive and it can also damage the magnet and other equipment, and there's risk of asphyxiation to people nearby.
Basically when any part of a superconductor quenches, it's no longer a superconductor, and when there's a huge current flowing through it, then this creates lots of heat, which stops all the neighbouring parts superconducting, and it spreads like a chain reaction; and suddenly there's a whole bunch of heat, and helium gas; and the containment will vent.
For a controlled shutdown you have to gradually lower the magnetic field strength, and when it's gone, trigger the quench, and nothing major will happen, because there's no current.
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It's a serious consideration when designing a clinic - you have to allow for an accidental quench with loads of extremely cold gas hurtling out of the machine at near-sonic speed. It can be quite a problem in an inner city location. I use either high-temperature supercons where the helium coolant circulates as a gas, or room-temperature resistive magnets - and the problem with all these is getting rid of the heat from the chillers.
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you have to gradually lower the magnetic field strength
Since I presume that the number of turns in the coil is fixed during manufacture, this must mean slowly reducing the supercurrent flowing through the coils.
How do you do this? Is it the inverse of how you get the supercurrent flowing in the first place?
How long does this process take?
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Well in normal operation, there's a superconducting short circuit across the coils, and an external circuit they use for increasing the current.
To shutdown I believe they connect an external inverter across the short and then pull the short out, and progressively feed the power into the mains. Once they've degaussed it, they can quench.
From googling, it seems to take 15 minutes to ramp down; which is rather shorter than I expected.
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Thanks...