Critical current vs. Strain for LTS wires up to 21 T

We have developed a device for the electro-mechanical characterization of long high current technical LTS wires and tapes up to 21 T. Our device is based on the concept of the Walters spring (WASP), which allows us to measure long length wires (up to 80 cm) and to obtain an absolute measurement of the strain value. The long length of the conductors allowed us to measure the V-I relation under strain over three decades of electric field (down to 0.01 μV/cm) with currents up to 1000 A. This constitutes a progress in view of the characterization of the conductors for NMR applications at high field, which must operate in persistent mode. We report on I_c vs. strain measurements at fields up to 21 T for various types of Nb₃Sn wires prepared by different techniques (industrial wires as well as wires prepared in our laboratory) and for a copper cladded Nb₃Al wire, prepared at NIMS (Tsukuba). We show that the new, sharper criterion gives a new insight on the reversible behavior of the I vs. strain curve as well as of the variation of the exponential n factor under uniaxial tensile strain. The results on Nb₃Sn are analyzed in the context of the scaling laws proposed by J. Ekin.