Superconducting Properties of Composite, Rigid, Tubular Elements with Nb3Sn Diffusion Layers

Nb3Sn-based superconducting materials are produced mainly as ribbons or multifilamentary composites. The Nb3Sn layer is usually not more than 3 to 6 μm thick for two reasons: (1) thicker layers are extremely sensitive to very slight bend deformations and (2) the critical current density of thicker layers drops sharply, and thus the integral current is not proportional to the layer thickness and can even decrease with the thickness growth. In this investigation, rigid, tubular Nb3Sn conductors were used as experimental ac transmission cables, and therefore, bend-deformation sensitivity was not a concern. But increases in critical current are of primary importance for the superconducting cable. Therefore, the behavior of the critical current density in thick Nb3Sn diffusion layers was studied as a function of diffusion process structure and parameters. Other goals of the study were to obtain additional insight into the mechanism and kinetics of the processes involved in Nb3Sn formation as well as into the factors that determine the magnitude of ac losses.