# Superconductors - some general questions.

1. Feb 2, 2013

### axawire

I will be trying to build an electric motor using superconductors and I have some general concept questions in regards to how the behavior might be different from ordinary wires.

1. The Meissner effect, expulsion of magnetic fields from the superconductor. If you construct a solenoid out of superconducting wire does this Meissner effect significantly change the field pattern compared to a regular wire solenoid?

2. If a solenoid is created out of superconducting wire is the current limited to the magnitude of the magnetic field it creates because if the magnetic field strength is too large it will return the superconductor to a non superconducting state?

3. If you have a superconducting wire with a current flowing through it and you expose it to a magnetic field does it still experience a force (obeying the same rules as a regular wire with current flow) or because of the Meissner effect the magnetic field never reaches the current carriers and so no force is experienced? Or is there only a fraction of the expected force experienced because the magnetic field does not penetrate through all the current carriers?

4. If you have two or more strips of superconducting tape stacked on top of each other with current flowing in the same direction through them and then you have a magnetic field strength probe and you probe the space above the stack, would the field be just the sum from each strip or would the Meissner effect in the strips closer to the top surface cause magnetic shielding of the lower strips and so the real magnetic field strength near the surface of the top strip would be less than the sum of the magnetic fields from each current carrying strip?

5. Is current flow in a superconductor only a surface effect? (i thought it was) Why did I see one company selling 3D wires? is that really isolated 2D stacks?

6. Is the only way to make a loop of superconductor run a continuous current is by induction?

Thanks for reading and thanks in advance for any responses.

2. Feb 3, 2013

### f95toli

No, the field pattern is the same; unless perhaps if you are talking about very small solenoids and you are in the near-field.

Yes, that is the basic idea and it will be correct in the ideal case. Although it gets a bit tricky if you are working with type II superconductors (which is always the case in real life) since flux motion becomes an issue. Moreover, there is always a risk that "hotspots" can form around defects etc. where the current exceeds the local Jc.

It behaves the same as for a normal wire.

I think this will depends on the geometry and the length scales involved (compared to the penetration depth of the superconductor.

No, DC current flow uniformly. For AC current you have something very similar to the skin effect, but with the typical depth set by the penetration depth. THis is one reason superconductors are bad conductors at very hight frequencies (say above 100 GHz or so).

No, you can inject current in a loop by first making a section normal (using a heat-swich), running a current through the solenoid as usual, and then "closing" the loop by turning off the heat in the switch (which puts the magnet into what is known as persistent current mode). This is how all superconducting magnets work.

3. Feb 3, 2013

### axawire

Thank you for the information, much appreciated.

4. Feb 8, 2013

### M Quack

As far as I know, all the superconducting materials used in magnets are type-II superconductors. This includes high-Tc tapes, NbTi, NbSn and the new materials like MgB2, pnictides and so on.

Above a certain, very small magnetic field Hc1, type-II superconductors do not show the Meissner effect. Instead the magnetic field penetrates the superconductor in the form of flux lines or vortices. The length scales involved are very small, so that in practice you can ignore it and treat the SC just like a normal conductor.

Typically, the difficult engineering problems are quench protection and mechanical, as well as making sure that the very long lengths of wire needed are superconducting along their whole length, without normalconducting defects.

5. Feb 9, 2013

### f95toli

This is not quite correct. Type-II superconductors show Meissner effect at a "global" scale even above Hc1, the fact that vortices form in some places does not change the fact that the field is expelled everywhere else. In fact, in demonstrations of the Meissner effect we typically use a YBCO (type-II, HTS) disc and show that a small magnet can levitate above
it. Now, there reason why the magnet is stable and does not easily move side-to-side is that the it is "stuck" in the field configuration it was in when the YBCO went superconducting: the flux penetrates the YBCO and holds the magnet in place.
Moreover, the flux lines are stuck to defects and impurities in the YBCO and does not want to move (vortices in cleans systems can be moved quite easily, but the discs used for these demonstrations are full of defects, GBs etc)

If you make the YBCO go superconducting first and THEN try to balance the magnet on top of it you will find that it is much more difficult.

6. Feb 10, 2013

### M Quack

I guess the question is what you call the Meissner effect.

If you call the Meissner effect the complete expulsion of magnetic flux from the superconductor, then the state between Hc1 and Hc2 is not the Meissner state.

If you consider the core of the vortex non-superconducting, then the remaining superconducting volume does show the Meissner effect because it is field-free.

On everything else I agree completly.

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