V–I transition and n-value of multifilamentary LTS and HTS wires and cables

Abstract

Abstract For low Tc multifilamentary conductors like NbTi and Nb3Sn, the V–I transition to the normal state is typically quantified by the parameter, n, defined by (ρ/ρc)=(I/Ic)n. For NbTi, this parameterization has been very useful in the development of high Jc wires, where the n-value is regarded as an index of the filament quality. In copper-matrix wires with undistorted filaments, the n-value at 5 T is ∼40–60, and drops monotonically with increasing field. However, n can vary significantly in conductors with higher resistivity matrices and those with a low copper fraction. Usually high n-values are associated with unstable resistive behavior and premature quenching. The n-value in NbTi Rutherford cables, when compared to that in the wires is useful in evaluating cabling degradation of the critical current due to compaction at the edges of the cable. In Nb3Sn wires, n-value has been a less useful tool, since often the resistive transition shows small voltages ∼ a few μV prior to quenching. However, in “well behaved” wires, n is ∼30–40 at 12 T and also shows a monotonic behavior with field. Strain induced Ic degradation in these wires is usually associated with lower n-values. For high Tc multifilamentary wires and tapes, a similar power law often describes the resistive transition. At 4.2 K, Bi-2223 tapes as well as Bi-2212 wires exhibit n-values ∼15–20. In either case, n does not change appreciably with field. Rutherford cables of Bi-2212 wire show lower values of n than the virgin wire.