Characterization of ITER Nb3Sn strands under strain-applied conditions

Japan Atomic Energy Agency has developed four types of Nb(3)Sn strand which can be used in the ITER TF coils. One is a strand made by an internal tin process strand and the others are bronze process strands. The achieved critical current density is more than 790 A/mm(2) in the bronze process strands and more than 980 A/mm(2) in the internal tin process strand under 4.2 K temperature and 12 T magnetic field and there is hysteresis loss of less than 770 mJ/cc under +/- 3 T cycle. Since these strands are utilized with an external strain, it is necessary to evaluate strain dependency 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 the strand length, a strand being soldered on the outer surface of the ring. The detailed strand characteristics were investigated subjecting the developed strands to a magnetic field from 10 T to 13 T, a strain from about -0.8% to 0.5%, and a temperature from 4.2 K to the critical temperature. When the critical current is normalized to that under the conditions where strain is intrinsically zero, the bronze process strands exhibit better performance than the internal tin process strand. However, the three bronze process strands do not exhibit the same I(c)-strain characteristics. Two types of scaling relations are applied to the data, and good expressions of strand performance were obtained by the least square method within 3 A as RMS.