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In powder-in-tube (PIT) Nb3Sn composites, the A15 phase forms between acentral tin-rich core and a coaxial Nb tube, thus causing the tin content andsuperconducting properties to vary with radius across the A15 layer. Since thisgeometry is also ideal for magnetic characterization of the superconductingproperties with the field parallel to the tube axis, a system of concentricshells with varying tin content was used to simulate the superconductingproperties, the overall severity of the Sn composition gradient being definedby an index N. Using well-known scaling relationships and property trendsdeveloped in an earlier experimental study, the critical current density foreach shell was calculated, and from this the magnetic moment of each shell wasfound. By summing these moments, experimentally measured properties such aspinning-force curves and Kramer plots could be simulated. We found thatdifferent tin profiles have only a minor effect on the shape of Kramer plots,but a pronounced effect on the irreversibility fields defined by theextrapolation of Kramer plots. In fact, these extrapolated values H_K are veryclose to a weighted average of the superconducting properties across the layerfor all N. The difference between H_K and the upper critical field commonlyseen in experiments is a direct consequence of the different ways measurementsprobe the simulated Sn gradients. Sn gradients were found to be significantlydeleterious to the critical current density Jc, since reductions to both theelementary pinning force and the flux pinning scaling field H_K compound thereduction in Jc. The simulations show that significant gains in Jc of Nb3Snstrands might be realized by circumventing strong compositional gradients oftin.Comment: 10 pages, 8 figures, 2 tables, submitted to J. Appl. Phy

Simulations of the effects of tin composition gradients on the superconducting properties of Nb3Sn conductors