Characterization of ITER NbSn strands under strain-applied conditions

Nunoya, Yoshihiko; Isono, Takaaki; Koizumi, Norikiyo; Hamada, Kazuya; Matsui, Kunihiro; Nabara, Yoshihiro; Okuno, Kiyoshi

Japan Atomic Energy Agency has developed NbSn strand which can be applied to ITER TF coils. The achieved critical current density is more than 790A/mm in bronze process strand and more than 980A/mm in internal tin process strand under 4.2K temperature and 12T magnetic field as well as hysteresis loss of less than 770mJ/cc under 3T cycle. Because these strand are utilized with an external strain by thermal contraction and hoop force of the coils, it is necessary to evaluate strain dependency of these strands to confirm the ITER conductor design. An apparatus to measure the strain dependency was newly developed. It has a horseshoe-shaped ring to produce uniform axial compressive or tensile strain along strand length and a strand is soldered on its outer surface. ITER TF coils will be operated under 0.77 compressive strain, magnetic field of 11.3T and temperature of 5.7K as design value. Typical obtained critical current for bronze process strand is about 92A under these designed operating conditions, which is about 20 larger than the requirement. The details of the results of strand characterization will be presented.