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

The dependence of transport critical current I_c on 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 epsiv_irr were 0.19% and 0.8%, respectively; and the ultimate strain where the wire physically broke was about 0.95%. The pinning force F_p ( = I_c x B ) was proportional (B*_c2)^sb^p (1 - b )^q to , where b = B / B*_c2 is 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 F_p extrapolates to zero) was well described within the measured strain range by B*_c2max [1 - alpha |epsiv - epsiv_max |^u _], where B*_c2 is the maximum value of B*_c2 as 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 I_c at various intrinsic strains between -0.5% and 0.5%, and magnetic fields from 12 T to 16 T.