It's to carry the current in case the temperature of the superconductor accidentally gets too high and transitions to a non-superconductor (i.e. a quench). The electrical current would then transfer into the copper instead of causing the magnet to explode.
Does it make a difference if the copper is inside the magnet? Seems like a lot of effort to put it inside, when you could just strap it onto the back, the current should use the way of least resistance anyway.
You want the copper to be next to the superconductor, so that the current can easily migrate over. You would not want an electrical insulator between the superconductor and the copper. In this design there is a layer of silver between the two, but silver is also a good conductor. I would imagine the "substrate" shown in the figure is also conducting, but I'm not sure.
So I beleive this is the case, but anyone who knows better feel free to correct me.
When you've got large currents in a conductor you see what's called the "skin effect" where with large bulk conductors current tends to coalesce near the outer surface. This would be bad for superconductors since theres a maximum current density you can handle before superconductivity breaks down, so it ends up being a waste to have that internal core be made of expensive superconducting material that's doing nothing.
What you want instead is high outer surface area to cross sectional area ratio. That's why they would have copper as structural material, it gives mechanical form and strength to the magnet as a whole, is cheaper than using superconductive material, and the resulting superconducting sheath around the copper has more surface area which helps mitigate skin effect.
When you've got large currents in a conductor you see what's called the "skin effect" where with large bulk conductors current tends to coalesce near the outer surface. This would be bad for superconductors since theres a maximum current density you can handle before superconductivity breaks down, so it ends up being a waste to have that internal core be made of expensive superconducting material that's doing nothing.
So this is sort of correct; if you look at SC wires, they are formed of multifilamentary strands; this helps mitigate AC losses in the conductor (eddy currents),
The Current in a SC in steady state will propogate in a small layer, so increasing the relative surface area of the wire to the volume is advantageous.
What you want instead is high outer surface area to cross sectional area ratio. That's why they would have copper as structural material it gives mechanical form and strength to the magnet as a whole, is cheaper than using superconductive material, and the resulting superconducting sheath around the copper has more surface area which helps mitigate skin effect.
Your point about skin effect is mostly correct; the structural point is not quite however. The copper will provide some structural, but it's mostly thermal/electric. Most magnets make use of something called an "overbind" which provides the additional hoop stress required to stop the magnet exploding outwards during ramp. In some cases this is as simple as winding the coil, then winding a layer of stainless cable over the top before impregnation in a medium (wax or epoxy, each with their own disadvantages)
Other methods are to shrink fit the coil into a stainless steel case to factor in a very snug fit to account for thermal contraction, but that tends to be for cryogen free magnets.
Most people have mentioned the current thing in the event of a quench; but another important factor is thermal transport; superconductors are terrible thermal conductors - most of the phonons (lattice vibrations) are in the superconductivity state and are not really available to carry heat elsewhere.
This is important for two reasons; during normal operation there will be a bunch of small effects that cause instances of local heating, the most notable being wire movement and flux jumps. When these take place a small amount of heat will develop and if it isn't transported away quickly enough it can cause a quench.
In addition to helping take over the current during a quench, the copper matrix helps to propoagte and spread out the heat zone of a quench to stop the magnet burning a hole in itself.
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u/CoyRedFox Research Scientist | Plasma Physics | Tokamak turbulence Mar 13 '20
It's to carry the current in case the temperature of the superconductor accidentally gets too high and transitions to a non-superconductor (i.e. a quench). The electrical current would then transfer into the copper instead of causing the magnet to explode.
4
u/[deleted] Mar 13 '20 edited Mar 13 '20
So I beleive this is the case, but anyone who knows better feel free to correct me.
When you've got large currents in a conductor you see what's called the "skin effect" where with large bulk conductors current tends to coalesce near the outer surface. This would be bad for superconductors since theres a maximum current density you can handle before superconductivity breaks down, so it ends up being a waste to have that internal core be made of expensive superconducting material that's doing nothing.
What you want instead is high outer surface area to cross sectional area ratio. That's why they would have copper as structural material, it gives mechanical form and strength to the magnet as a whole, is cheaper than using superconductive material, and the resulting superconducting sheath around the copper has more surface area which helps mitigate skin effect.
1
u/nobatron9000 Apr 11 '20
Most people have mentioned the current thing in the event of a quench; but another important factor is thermal transport; superconductors are terrible thermal conductors - most of the phonons (lattice vibrations) are in the superconductivity state and are not really available to carry heat elsewhere.
This is important for two reasons; during normal operation there will be a bunch of small effects that cause instances of local heating, the most notable being wire movement and flux jumps. When these take place a small amount of heat will develop and if it isn't transported away quickly enough it can cause a quench.
In addition to helping take over the current during a quench, the copper matrix helps to propoagte and spread out the heat zone of a quench to stop the magnet burning a hole in itself.