Hot spots on superconductors

A stabilized superconducting wire (single) containing a NbZr core coated with copper and insulated with formvar developed dark spots during the operation of a superconducting coil made from such a wire. The spots numbered in the thousands. The spots varied in appearance from dark stains, carbonized areas to blown volcanoes. The dark spots were carbonized areas where the formvar was pyrolyzed leaving a carbon residue, the temperatures were of the order of 700K. These were hot spots. The failure of the superconducting coil was a point failure and not a turn to turn failure. No sign of arcing was observed. The failure occurred at one of the hot spots due to a thermal fluctuation that may have been caused by flux jumping. Some speculations and comments are made as to the cause of hot spots. Some future work is recommended concerning the nature of imperfections at the hot spots and their relation to flux jumping.