Critical Current Characterization Under Pure Bending Strains of Prebent Cu-Nb/Nb3Sn Strands for Practical React-and-Wind Process

Abstract

Nb-rod-method Cu-Nb reinforced Nb3Sn (Cu-Nb/Nb3Sn) wires have been successfully developed for commercialization of the react-and-wind (R&W) processed Nb3Sn coils. Superconducting performance of the Cu-Nb/Nb3Sn wires is improved by applying the prebending treatment at room temperature, because of releasing the residual stress of Nb3Sn filaments (prebending strain effects). The purpose of this study is to confirm the efficient use of the prebending strain effects for optimizing critical current $(I_{c})$ characteristics under pure bending strain on the R&W coil design. The Cu-Nb/Nb3Sn strands of 0.8-mm diameter were heat treated at 670 °C for 96 h on stainless steel grooved cylindrical holders of different radii of 10.1–32.1 mm. After half of the reacted strands were applied repeating prebending strain of ±0.5%, each of the prebent strands and the as-reacted (without prebending) strands was transferred to a Ti-6Al-4V alloy grooved cylindrical holder of 15.6-mm radius. Then, those $I_{c}$ measurements were carried out under applied pure bending strains ranging from −0.9% to +0.84% at 11–17 T, 4.2 K. As a result, the prebent strand showed no deterioration of $I_{c}$ in the applied pure bending strain ranging from −0.5% to +0.5% and had larger $I_{c}$ than that of the as-reacted strand at no pure bending strain. In this paper, the $I_{c}$ enhancements of the prebent Cu-Nb/Nb3Sn strands under pure bending strains are analyzed by using the $I_{c}$ characteristics under axial strains.