Quench degradation limit of multifilamentary Ag/Bi₂Sr₂CaCu₂O_x round wires

Understanding safe operating limits of composite superconducting wires is important for the design of superconducting magnets. Here we report measurements of quench-induced critical current density J _c degradation in commercial Ag/Bi₂Sr₂CaCu₂O_x (Bi-2212) round wires using heater-induced quenches at 4.2 K in self magnetic field that reveal a general degradation behavior. J _c degradation strongly depends on the local hot spot temperature T _max, and is nearly independent of operating current, the temperature gradient along the conductor dT _max/dx, and the temperature rising rate dT _max/dt. Both J _c and n value (where n is an index of the sharpness of the superconductor-to-normal transition) exhibit small but irreversible degradation when T _max exceeds 400-450 K, and large degradation occurs when T _max exceeds 550 K. This behavior was consistently found for a series of Bi-2212 wires with widely variable wire architectures and porosity levels in the Bi-2212 filaments, including a wire processed using a standard partial melt process and in which Bi-2212 filaments are porous, an overpressure processed wire in which Bi-2212 filaments are nearly porosity-free and that has a J _c(4.2 K, self field) exceeding 8000 A mm^-2, and a wire that has nearly no filament to filament bridges after reaction. Microstructural observations of degraded wires reveal cracks in the Bi-2212 filaments perpendicular to the wire axis, indicating that the quench-induced I _c degradation is primarily driven by strain. These results further suggest that the quench degradation temperature limit depends on the strain state of Bi-2212 filaments and this dependence shall be carefully considered when engineering a high-field Bi-2212 magnet.