Critical current and strand stiffness of three types ofNb3Sn strand subjected to spatial periodic bending

Abstract

Knowledge of the influence of bending on the critical current(Ic) ofNb3Sn strands is essential for the understanding of the reduction in performance due to transverseelectromagnetic load. In particular, for the large cable-in-conduit conductors (CICCs)meant for the international thermonuclear experimental reactor (ITER), we expect thatbending is the dominant mechanism for this degradation. We have measured theIc of a bronze, a powder-in-tube and an internal tin processedNb3Sn strand when subjected to spatial periodic bending using bending wavelengths from 5 to 10 mm.Two of these strands were applied in model coils for the ITER. We found that the testedstrands behave according to the so-called low interfilament resistivity limit, confirming fullcurrent transfer between the filaments. This is supported by AC coupling loss measurementsgiving an indication of the interfilament current transfer length. The reduction ofIc due to bending strain can then be simply derived from the bending amplitude and theIc versus axialapplied strain (ε) relation. This Ic(ε) sensitivity can vary for different strand types but since the electromagnetic force is thedriving parameter for strand bending in a CICC, the stiffness of the strands definitivelyplays a key role, which is confirmed by the results presented.