Persistent normal regions in large conductors

Persistent normal regions, which continuously generate joule heat and reduce the stability of a conductor, can exist in the large built-up conductors being fabricated for large superconducting magnets. Many of these large conductors consist of a composite superconductor soldered to a high-conductivity substrate of copper or aluminum. For these conductors, delaminated lengths can exist which prevent full recovery and result in persistent normal zones. Delaminated lengths less than a certain critical length the conductor will always be able to recover. A simple analytic formula is derived for this length and compared with the results of a more accurate analysis. The critical length is found to be 2.1 cm for the conductor employed in superconducting magnet for the Component Development and Integration Facility (CDIF/SM) and 1.9 cm for the conductor employed in the General Dynamics Test Coil for the Large Coil Program. The length depends on the amount of normal metal in the composite and in the substrate, the bath temperature, the transition temperature of the superconductor, the operating current, and the critical current.