Critical-Current Measurements on an ITER Nb$_{3}$Sn Strand: Effect of Axial Tensile Strain

摘要

The dependence of transport critical current $(I_{rm c})$ on axial tensile strain $varepsilon$ was measured for a developmental ${rm Nb}_{3}{rm Sn}$ 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 $varepsilon_{max}$ and the irreversible strain limit $varepsilon_{rm irr}$ were 0.19% and 0.8%, respectively; and the ultimate strain where the wire physically broke was about 0.95%. The pinning force $F_{rm p}(=I_{rm c} times B)$ was proportional to $(B_{{rm c}2}^{ast})^{rm s}b^{p}(1-b)^{q}$, where $b=B/B_{{rm c}2}^{ast}$ 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 $B_{{rm c}2}^{ast}$ (the field at which $F_{rm p}$ extrapolates to zero) was well described within the measured strain range by $B_{{rm c}2max}^{ast}[1-avertvarepsilon-varepsilon_{max}vert^{u}]$ , where $B_{{rm c}2}^{ast}max$ is the maximum value of $B_{{rm c}2}^{ast}$ as a function of strain, $u=1.7$, and $a$ 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.