A study of the local variation of the critical current in Ag-alloy clad, round wire Bi₂Sr₂CaCu₂O_8+<i>x</i>multi-layer solenoids

Abstract

Wind-and-react coils using alumino-silicate-braid-insulated Bi2Sr2CaCu2O8+x (Bi-2212) round wire consistently show reduced critical current densities (Jc) 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 Jc of the coil is markedly below that of short samples processed at the same time. Analysis of the critical temperature (Tc) and the irreversibility field (Hirr) shows that these Jc 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 Jc of the coils is related to a sample length dependence of Jc rather than to any lack of full oxygenation of even a 20-layer coil. Raising the Jc of the coils thus appears to be a problem that needs addressing chiefly at the strand rather than the coil level.