Is there a measurement for how much power a superconductor of a given diameter can actually conduct? Of course we're all familiar with the "resistance = 0" concept but I don't think that means you could pump ten billion amps through a superconductor the diameter of a hair, or does it? What other forces limit high-density current flow besides simple resistance?
What limits it is that superconductors can only handle a certain density of magnetic flux; more than that will make it stop superconducting. (This property was used in the "cryotron", an electrical switch used in some early computers before transistors became common for the same function.) Any current-carrying wire produces a magnetic field; so, with a strong enough current, the magnetic flux will cause the superconductor to switch itself off. This can be limited by using a larger superconductor, so the flux density is lower.
1) what was never clear to me was if the resistivity of an infinitesimal volume element of superconductor is strictly a function of temperature, and the norm of the magnetic field, or also of the direction of current with respect to the direction of the magnetic field? near the critical current density, or even before, will the resistance depend on the direction of current?
2) while a general bulk high current superconductor is limited in currentcarrying capacity this way, most of the time one is not interested in sending current one way, but also completing the circuit and getting current back the opposite way. Why can they not make power distribution cables with indefinite currentcarrying capacity the following way: consider a "multilayer" coaxial cable where the conductors are superconductors, and to prevent a magnetic field from arising the layers have alternating current directions, so that in any layer the net magnetic field is below the critical magnetic flux? (this would only be for power distribution, since this can't be used to build stronger superconductive electromagnets). The total current would be the sum of all the even, or of all the odd coaxial layers (the 2 sums are identical since they complete a circuit...) in essence co-locate the 2 problematic magnetic fields such that they cancel... A superconductive "humbucker"
You can do this to enhance or change the magnetic field of the magnet. Core rope memory and such use this principle to read, write and refresh the contents of the cores. They send a pulse down one of the lines, read line sends one voltage, write line sends a higher one designed to flip the polarity if necessary, and then sense the reflected pulse on the third wire, called the sense wire.
Thank you for the explanation. I'm still confused. So if a wire is wrapped in a specially crafted magnet, then when sending an electric pulse through it the pulse will "bounce back"?
The term "critical current" is often used for the current that generates the maximum magnetic field a superconductor of a given size/geometry can sustain. In practice the actual maximum current before quench is much lower (orders of magnitudes in some cases, I've heard) due to imperfections in metallic lattices.
The limiting factor is the critical field of the superconductor. Superconductors tend to expel magnetic fields, but if the field is strong enough it will bring the material back to normal conduction. Since a current will generate a magnetic field, this is what sets the maximum current.
