Nb3Sn Wires for the Future Circular Collider at CERN: Microstructural Investigation of Different Wire Layouts

In the challenging project concerning the realization of the CERN Future Circular Collider (FCC), Nb-3 Sn represents the best candidate material for the construction of high-field superconducting dipole magnets, since it is able to satisfy the requirements of J(c) (non-Cu) = 1.5 kA/mm(2) at 16 T and 4.2 K. In that context, a cluster layout of prototype internal tin Nb-3 Sn wires, developed by TVEL and the Bochvar Institute (Russia), was analyzed and compared to a standard layout produced by the same manufacturer. The main reason for dividing the sub-element into clusters is reducing the effective sub-element size (d(eff)). The microstructural characterization of such a wire layout can provide fundamental contributions to steer the manufacturing processes towards higher performing wires. In particular, since the homogeneity in Sn concentration influences the superconducting properties, the effect of cluster and standard layouts on the Sn concentration gradient over the wire cross-section was evaluated. For this purpose, energy dispersive X-ray (EDX) spectroscopy was employed with both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Finally, scanning Hall probe microscopy (SHPM) measurements were performed to understand how these cluster wire sub-elements, with their specific geometry, influence the local currents flowing through the wire cross-section on a microscopic scale. The comprehension of the correlation between the microstructural characteristics and superconducting performance is crucial for obtaining wires meeting the requirements of FCC dipole magnets.