Spatial periodic contact stress and critical current of a Nb3Sn strand measured in TARSIS

We have developed a new probe for testing the influence of local contact load from crossing superconducting Nb3Sn strands. The probe is part of the TARSIS setup for strand stress–strain characterization. The results from the TARSIS setup with different probes developed for the characterization of axial tensile stress–strain, spatial periodic bending, contact from crossing strands and homogeneous transverse load enable discrimination in performance reduction per specific load type and per strand type. In particular, the electromagnetic charging of Nb3Sn cable in conduit conductors (CICC) causes transverse contact and bending strains in the wires and hence in the Nb3Sn filaments. More than ever, for high electromagnetic loads, such as in the conductors designed for the International Thermonuclear Experimental Reactor (ITER), the transverse load causes significant local strain concentrations in combination with strain differences due to the thermal contraction of the composite materials. These high local strains in the strands degrade the superconducting properties significantly. We report on the design of the probe and the first results demonstrating the influence of periodic transverse contact load from crossing strands, using a wavelength of 5 mm on an Nb3Sn powder-in-tube processed strand. The cyclic behaviour in terms of critical current and n-value involves a component representing a permanent reduction as well as a factor expressing reversible (elastic) behaviour as a function of the applied load. The results of the probe are used as input for the mechanical and electromagnetic modelling of a full-size ITER Nb3Sn conductor in order to optimize the final cable design