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We do this trick with our 800 MHz Oxford Instruments-built magnet often to impress some folks: Remove the probe and plug bore of the magnet with a rubber cork. Take a donut-shaped disk made of aluminum, put it right under the magnet an let it hang there and slowly fall to the ground. My question is - can field induced by moving an external conducting object like that disk affect the superconductor and cause a quench? |
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It seems like this should be safe enough. When probes are put in and pulled out of magnets, they must get much stronger eddy currents in their housings that don't quench the magnets. Here's something fun you can try. (Perhaps I'll try it myself soon, too.) Split the ring and put an LED in the gap so it'll light up from the eddy current! Further, since it's a diode, it'll conduct if the ring is oriented one way, and not if it's oriented the other way, and that may be reflected in the rate it falls. Worth a try!
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My guess is that the effect would be minor. Of course the disk will do "work" against the current in the superconducting coil, but my feeling would be that it would be small. Is there a way of measuring a drop in the field being produced by the magnet, before and after doing the aluminium disk trick? It would give you an idea of the amount of energy being generated and you can figure out if that would boil the helium and cause a quench. Yeah that doesn't sound easy... S/ Boiling the liquid helium is not a problem; that happens constantly. It is an induced change of the coil current, or excessive physical force which could cause a problem. - Jerry Hirschinger (Sep 28 '10 at 13:27) |
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In your specific case and with probe changes, there is no danger because the eddy currents do not create a stray field expansive enough to cross the magnet wire and induce anything. The physical force applied internally to the coil is less than the mass of the falling object. Mostly it just changes the homogeneity. However, the generic answer is yes. If any extra current induced into the SC wire causes the total current to exceed the critical current for superconductivity, the wire will quench. If you forcefully and rapidly move the ring, then the eddy stray field will reach farther. At some point, the eddy stray field will reach far enough to cross a coil, even if only a cryoshim coil, and induce a current change. Whether there is a quench depends on how close to the critical current the wire is already operating. Induced current change is not the only danger from conductors in motion within the magnet field. There is also the physical force. Whatever force is applied to pull the ring away is coupled magnetically to the main coil, so an equal force is pulling directly on the magnet coil, former, and internal mounting structure. |
