Spatial Periodic Bending and Critical Current of Bronze and PIT Nb3Sn Strands in a Steel Tube

We measured the influence of spatial periodic bending on the critical current (I_c) of Nb₃Sn strands that were tightly swaged in a steel tube, imposing a compressive strain, in attempt to simulate the conditions representative for an ITER (international thermonuclear experimental reactor) cable in conduit conductor (CICC). The huge electromagnetic transverse force causes both transverse compressive strain in the strand crossover contacts and bending strain on top of the strain induced by the differential thermal contraction of the various conductor components. For a CICC with stainless steel or Incoloy conduit, the effect of the thermal cool-down strain on the reduction in performance due to periodic bending strain is unknown. We performed periodic bending tests in the TARSIS facility (test arrangement for strain influence on strands) on three samples. The degradation of the I_c due to bending appears to be much more severe for a strand swaged in a stainless steel tube than straightforward simulated by available models assuming simply axial strain dependency. It is confirmed by AC loss measurements that the extra degradation can not be attributed to a change in matrix transverse resistivity, possibly affecting the current transfer length, so we believe this is related to the three-dimensional strain state of the Nb₃Sn layer.