Critical-Current Measurements on an ITER Nb₃Sn Strand: Effect of Axial Tensile Strain

The dependence of transport critical current Icon axial tensile strain was measured for a developmental Nb3Sn multifilamentary strand as a function of magnetic field B between 12 T and 16 T, at the temperature of 4 K. This conductor was from the first stage of strand pre-production for the central solenoid of the International Thermonuclear Experimental Reactor (ITER) project. Straight samples were measured with a stress-free-cooling strain apparatus. The compressive pre-strain and the irreversible strain limit epsivirrwere 0.19% and 0.8%, respectively; and the ultimate strain where the wire physically broke was about 0.95%. The pinning force Fp( = Icx B ) was proportional (B*c2)sbp(1 - b )qto , where b = B / B*c2is the reduced magnetic field, and the scaling constants had values p = 0.58, q = 1.86, and s = 0.7. The strain dependence of the effective upper critical field (the field at which Fpextrapolates to zero) was well described within the measured strain range by B*c2max[1 - alpha |epsiv - epsivmax|u],where B*c2is the maximum value of B*c2as a function of strain, u = 1.7, and alpha was about 1230 for the compressive strains and 1670 for the tensile strains. Ekin's strain scaling law was applied to calculate the strain sensitivity of Icat various intrinsic strains between -0.5% and 0.5%, and magnetic fields from 12 T to 16 T.