Axial strain characterization of the Nb 3Sn strand used for China's TF conductor

Abstract

Oxford Superconducting Technology (OST) has developed a type of Nb 3Sn strand which can be used in the ITER TF coils. The strand (billet number is OST 10424FE) is made by an internal tin process. The critical current ( I c) has been measured subjected to uniaxial strain in the Twente University. The strand is destined for the cable-in-conduit conductors (CICC) of the China first short conductor sample for ITER toroidal field coil. For the uniaxial strain characterization, the voltage–current characteristics were measured with an applied axial strain from −0.8% to +0.5%. The strand appears to be fully reversible in the compressive regime during the axial strain testing, while in the tensile regime, the behavior is already irreversibly degraded when reaching the maximum in the critical current versus strain characteristic. The parameters for the improved deviatoric strain description are derived from the I c data, giving the accuracy of the scaling with a standard deviation of 1.5 A, which is by far within the expected deviation for the large scale strand production of such a high I c strand. Analysis of the relation between I c, n index and the axial strain is reported.