A study of the local variation of the critical current in Ag-alloy clad, round wire Bi2Sr2CaCu2O8+x multi-layer solenoids

Wind-and-react coils using alumino-silicate-braid-insulated Bi2Sr2CaCu2O8+x (Bi-2212) round wire consistently show reduced critical current densities (J(c)) compared to short samples heat treated under nominally identical conditions. This is a significant detriment to the design of Bi-2212 insert coils for high field magnets. Here we report on the superconducting properties of a series of wind-and-react test coils and short conductor samples systematically extracted from various sections of some of these wind-and-react coils. Overall we find remarkable uniformity of the superconducting properties throughout a majority of each coil, even though the J(c) of the coil is markedly below that of short samples processed at the same time. Analysis of the critical temperature (T-c) and the irreversibility field (H-irr) shows that these J(c) variations are solely related to changes in connectivity within the Bi-2212 filaments, rather than to any variations of oxygen uptake within the coil winding during heat treatment. We conclude that the reduced J(c) of the coils is related to a sample length dependence of J(c) rather than to any lack of full oxygenation of even a 20-layer coil. Raising the J(c) of the coils thus appears to be a problem that needs addressing chiefly at the strand rather than the coil level.