I was reading a brief news story about the accident at CERN’s LHC, which mentioned that a bus bar vaporized when an electromagnet lost its superconductivity. Superconductors are supposed to have zero resistance to current flow. What, if anything, limits the current flow in a superconductor? If you look at the electromagnet as an ideal inductor, shouldn’t it function as a limitless energy storage device? What stops someone from feeding more and more power into the device, increasing the energy stored in the magnetic field to arbitrarily high levels?
Critical current. Superconductors have a certain current value above which the material is no longer superconducting. The reason for this is that superconductors expel magnetic fields (Meissner effect) as long as the applied field is less than the critical magnetic field of the superconductor. This critical magnetic field is a function of temperature; it decreases as the temperature rises from 0K, dropping to 0 at the critical temperature of the superconductor.
Since current flowing in a conductor generates its own magnetic field, there is an associated critical current as well. Too much current = too much field = no more superconductivity. Simple explanation here.
We use big superconducting magnets in our lab. A magnet quench is a pretty damn impressive thing to see, preferably from a safe distance, and preferably someone else’s magnet. 