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I understand that the maximum current that may be carried in a superconductor is limited by the magnetic field the superconductor is in, which is very often almost exactly the field generated by the current being carried.

In power distribution systems, it is conventional to carry current on suspended wires in one direction, and rely on earth ground to carry the return current. But in a superconducting power distribution system, all the current needs to flow in superconductors in both directions.

If outgoing current and return current are both carried in superconducting wires that are very close together, wouldn't the magnetic fields mostly cancel, enabling much greater current than would be possible in well-separated wires?

It seems that only minimal insulation would be needed between the outgoing and return circuits, because there would be negligible voltage difference between them. Thus, it seems like outgoing and return circuits could be co-mingled in a cable, each mostly surrounding the other, maximizing magnetic cancellation.

What is the limiting factor on current that may be carried in a system where the magnetic field is carefully minimized in this way?

Nathan Myers
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  • "It seems that only minimal insulation would be needed between the outgoing and return circuits, because there would be negligible voltage difference between them" You might want to rethink that. If I use two superconducting wires to connect a 12V battery to a lightbulb, what's the voltage difference between the wires? An electrical distribution system that delivers power to loads that do no work is kind of pointless. – DKNguyen Apr 22 '21 at 04:38
  • I'm also not sure what you are talking about when you say the magnetic field is the reason the critical current exists because what you are talking about is just magnetic cancellation to reduce inductance, but you may want to check out this: https://physics.stackexchange.com/questions/231357/why-do-superconductors-have-a-maximum-current-density – DKNguyen Apr 22 '21 at 04:46
  • @DKNguyen: I had guessed that with effectively unlimited current, there would be no need for an extreme voltage drop across the load. Kapton has a dielectric breakdown voltage of 300V/micron of thickness; three or four layers of 10-micron Kapton would hardly take up any room in a cable, even if wrapped around multiple strands. – Nathan Myers Apr 22 '21 at 05:45
  • We run regular transmission lines at very high voltages to minimize ohmic losses, but here there should be no such losses, so it seemed like there would be little need for a voltage difference large enough to cause any kind of material inconvenience. – Nathan Myers Apr 22 '21 at 05:48
  • Inductance is a phenomenon of varying current. I assumed that these cables would be carrying DC current, with negligible time variation, so enveloped in a nearly static magnetic field. The magnetic field around two equal and opposite adjacent linear currents in opposite directions must mostly cancel. – Nathan Myers Apr 22 '21 at 05:53
  • If the magnetic field experienced by the superconductor could be kept low enough, some other phenomenon might end up limiting current, such as charge carrier availability. But I don't find any information about that. – Nathan Myers Apr 22 '21 at 06:03
  • Oh, it wasn't clear that you were referring to running line voltages at maybe 120V instead of tens of kV. Regarding magnetic field cancellation, it still isn't clear to me why that has anything to do with the critical current. – DKNguyen Apr 22 '21 at 06:11
  • @DKNguyen -- When I read about superconduction, the "critical field" is always cited as an important property of a material; suggesting that where the field strength exceeds that level, the material will not superconduct. I took that as indicating that as current increases past the level needed to produce a critically-strong field, superconduction and current would collapse, and thus be the limiting current. But your reference suggests it is not so simple. – Nathan Myers Apr 22 '21 at 06:36
  • Ah, I found a reference so at least now I understand why you are relating the critical current to the magnetic field strength. – DKNguyen Apr 22 '21 at 06:49
  • https://arxiv.org/pdf/1501.07159 You might find section 2.1 interesting. It might give some clues as to why magnetic cancellation might not work. If the critical current density occurs inside at the core when the conductor "fills up" where the magnetic field cannot reach and stays locked that way. – DKNguyen Apr 22 '21 at 07:04
  • It's actually not conventional to transmit power that way. It's done sometimes, but not typically – user253751 Apr 22 '21 at 09:38

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