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In the past 5 years there has been a remarkable increase in layer critical current density, J c , for Nb 3 Sn. At ∼5000 A mm −2 (12 T, 4.2 K), the average layer critical current density is now double that of the best ITER central solenoid model coil strand. The improvements in critical current density are a result of increased Sn content in the superconducting A15 phase and better compositional homogeneity, and perhaps other reasons not yet understood. The first indication that such large increases in critical current density of the Nb 3 Sn layer were possible was in the powder-in-tube strands produced by Shape Metal Innovation. The design of these strands allows us to accurately measure the composition gradient as well as the gradient in critical temperature across the A15 layer, and we report these measurements here for a wide range of heat treatments. Inductive T c measurements were used to measure the radial gradient of T c , while specific heat measurements revealed the position-insensitive inhomogeneity of the A15 layer. Composition gradients were measured by energy dispersive x-ray spectroscopy in a field emission scanning electron microscope. The three characterizations show that the composition and T c gradient is quite shallow near the central Sn source and only becomes steep adjacent to the Nb sheath of each filament, a result that is beneficial to maximizing the fraction of the layer with high T c . This strong nonlinearity of the T c and composition gradient means that excellent properties are obtained, in spite of the built in composition gradients inevitable in any filamentary design of composite. The gradients are much smaller than reported for bronze-route conductors but higher than the overall layer gradient found in the latest generation of high critical current density internal Sn design strands. Coupled to recent modelling simulations, our data show the great value of high-Sn designs in developing high layer J c values in Nb 3 Sn conductors.

AbshacMultilayers of Nb47Ti superconductor (S) andtitanium pins (N) have been made with a critical temperature T, and an upper critical field Hc2 approaching bulk values. There is no proximity effect suppression of T, and Hc2 for layer thickness ds = dN = 10 nm, in contrast to the strong suppression for comparable multilayers made with Cu, Cu-alloy, or Nb pins. This may be because the proximity length of the Ti layers, -10 nm, is less than or equal to their thickness. The critical current density J, exhibits multiple peaks in J,(H), which suggests that matching effects contribute to the overall pinning. J, for 20 nm bilayers is comparable to that of Nb47Ti tapes, which have a higher number density of pins but a lower pin volume fraction. The weak proximity coupling suggests that much higher J, can be obtained if smaller bilayer periods can be made with good adhesion. The results are compared to other Nb-Tfli multilayer experiments, and stability limitations are also discussed.

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