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

Nb-rod-method Cu-Nb reinforced Nb
₃
Sn (Cu-Nb/ Nb
₃
Sn) wires have been successfully developed for commercialization of the react-and-wind (R&W) processed Nb
₃
Sn coils. Superconducting performance of the Cu-Nb/Nb
₃
Sn wires is improved by applying the prebending treatment at room temperature, because of releasing the residual stress of Nb
₃
Sn 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/Nb
₃
Sn 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-4ν 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/Nb
₃
Sn strands under pure bending strains are analyzed by using the I
_c
, characteristics under axial strains.