Electric Field and Current Characteristics of Bi2Sr2CaCu2O8+δ/Ag Superconduting Tapes during Flux Flow State

The electric field and current (E-I) characteristics of Bi2Sr2CaCu2O8+δ/Ag (Bi2212/Ag) superconducting tapes in a flux flow state have been investigated in order to clarify the quenching mechanism for oxide superconductors. The measured E-I curves were compared with those for a NbTi/CuNi superconducting wire. The n value, obtained from the slopes of log E-log I curves, for Bi2212/Ag decreased with the increase in electric field, while the n value for the NbTi/CuNi wire remained constant. This is explained by the fact that a part of the current transfers to the lowresistivity Ag substrate. The intrinsic n value for Bi2212 was found to be constant in the range of the electric field measured in this study. A mechanism of quenching for NbTi/CuNi superconductors in liquid helium causes an abrupt temperature rise induced by a transition from nucleate boiling to film boiling without a current transfer to the matrix. Oxide superconductors in liquid nitrogen are considered to be quenched by a mechanism similar to that for NbTi/CuNi superconductors in liquid helium, if the substrate resistivity or the critical current density is two orders larger than that of the present level.