Development of Strain-Applying Apparatus for Evaluation of ITER Nb3Sn Strand

Japan Atomic Energy Agency has successfully developed Nb₃Sn strand which fulfills ITER requirements. Because Nb₃Sn is very susceptible to external strain which reduces critical current, critical temperature, and critical field, it is necessary to evaluate strain dependency of these Nb₃Sn strands to confirm an ITER conductor design. An apparatus to measure the strain dependency was newly developed. It has a horseshoe-shaped ring and a strand is soldered on its outer surface. This shape produces uniform axial compressive or tensile strain along strand length by expanding or closing the opening of the ring. Critical current can be measured by the apparatus under a magnetic field up to 15 T, a temperature range of 4.2 K to 15 K, and strains. The maximum allowable current is about 300 A. The details of the apparatus and results of strand characterization are presented.