The upper critical field of filamentary Nb3Sn conductors

TL;DR

This study investigates the upper critical magnetic field of various Nb3Sn superconducting wires and bulk samples, revealing inhomogeneity effects and consistent high-field properties across different sample types.

Contribution

It provides a comprehensive analysis of the field-temperature phase boundary in Nb3Sn wires, emphasizing the effects of inhomogeneities and demonstrating the applicability of the Maki-DeGennes model.

Findings

Upper critical fields are around 29.5 T at zero temperature.

Inhomogeneities broaden the resistive transitions with increasing inhomogeneity.

The phase boundary fits well with the Maki-DeGennes model across samples.

Abstract

We have examined the upper critical field of a large and representative set of present multi-filamentary Nb3Sn wires and one bulk sample over a temperature range from 1.4 K up to the zero field critical temperature. Since all present wires use a solid-state diffusion reaction to form the A15 layers, inhomogeneities with respect to Sn content are inevitable, in contrast to some previously studied homogeneous samples. Our study emphasizes the effects that these inevitable inhomogeneities have on the field-temperature phase boundary. The property inhomogeneities are extracted from field-dependent resistive transitions which we find broaden with increasing inhomogeneity. The upper 90-99 % of the transitions clearly separates alloyed and binary wires but a pure, Cu-free binary bulk sample also exhibits a zero temperature critical field that is comparable to the ternary wires. The highest mu0Hc2 detected in the ternary wires are remarkably constant: The highest zero temperature upper critical fields and zero field critical temperatures fall within 29.5 +/- 0.3 T and 17.8 +/- 0.3 K respectively, independent of the wire layout. The complete field-temperature phase boundary can be described very well with the relatively simple Maki-DeGennes model using a two parameter fit, independent of composition, strain state, sample layout or applied critical state criterion.