Maximsuper: A Computer Program to Assist in the Design of Multifilamentary Superconducting Composites

Typical Nb3Sn multifilamentary superconducting composites produced by the bronze diffusion process have repetitive fiber elements of niobium embedded in a bronze matrix with an Nb3Sn reaction zone serving as the interfacial layer between the two. Clusters of these units are surrounded by tin diffusion inhibitors, such as tantalum, and placed inside a second external copper matrix. The strain degradation of critical current density, Jc, in an inversion of this configuration has been analytically and experimentally investigated. The new geometry consists of bronze filament cores inside of niobium tubes with the usual Nb3Sn interface. Since the niobium acts as a diffusion barrier, copper can be directly utilized as the embedding matrix, eliminating the motivation for the tantalum addition. Analytic results were obtained from a computer program (MAXIMSUPER) that evaluates the three-dimensional strain fields on the Nb3Sn zones in a cylindrical repeating element of the composite, owing to both residual (thermal contraction) and applied axial loading. The code then incorporates the calculated Nb3Sn strain values into appropriate scaling laws to predict the critical current density and its degradation with increasing axial strain. Critical parameters, such as upper critical magnetic field (H c2) and critical temperature (T c), are thought to vary according to a two-level state model that expresses the Nb3Sn cubic-to-tetragonal-martensitic phase transformation as strain sensitive. Experimental investigations conducted with the Lawrence Livermore Laboratory (LLL) 12-T tensile-test facility on both the niobium-core and bronze-core geometries confirm the importance of the tangential strains (in addition to the axial strain) on the J c properties of the conductors. Particularly in the bronze-core geometry, the resulting higher radial and azimuthal strains yield lower J c values. Under applied axial loading, the increasing tangential strains exhibit contradistinc-tive behavior from that of the axial strain, which changes sign from compressive to tensile strains. Therefore, the zero intrinsic axial strain values that give the peak critical current coincide with nonnegligible tangential strains on the Nb3Sn layers. The magnitude of these tangential strains depends primarily on the composite geometry and configuration.