A solder bond requirement for large, built-up, high-performance conductors

Some of the large built-up conductors being fabriccated for large superconducting magnets are designed to operate above the maximum recovery current. Because the stability of these conductors is sensitive to the quality of the solder bond joining the composite superconductor to the high-conductivity substrate, a minimum bond requirement is necessary for an acceptable design. The present analysis finds that the superconductor is unstable and becomes abruptly resistive when there are temperature excursions into the current sharing region of a poorly bonded conductor. This abrupt transition, in turn, produces eddy current heating in the vicinity of the superconducting filaments and causes a sharp reduction in the minimum propagating zone (MPZ) energy. This sensitivity of the MPZ energy to the solder bond contact area is used to specify a minimum bond requirement. For the superconducting MHD magnet being built by General Electric for the Component Development Integration Facility (CDIF), the minimum bonded surface area is .68 cm²/cm which is 44% of the composite perimeter.