Effects of Cr Diffusion on $RRR$ Values of Cr-Plated Nb $_{3}$ Sn Strands Fabricated by Internal-Tin

Nb₃Sn strands which will be used for the ITER TF coils should be chrome plated thickness of 1.8 mum to reduce AC loss and improve the thermal stability of cable. During heat treatment, the RRR value can be affected by Cr diffusion. In this work, the effect of Cr diffusion was systematically studied, using the KSTAR and ITER candidate strands. For the KSTAR strand, the Cu/non-Cu ratio was systematically varied from 1.53 to 0.69 by chemical etching whereas the samples were heat treated by the same schedule. For the ITER candidate strand, on the other hand, the Cu/non-Cu ratio was fixed to the specification value of 1.0, and the variation of the RRR value was studied with respect to the heat treatment schedule, especially the retention time of the 650degC plateau was varied from 100 to 200 hours. It was also compared with the Cr diffusion distance obtained from EPMA (electron probe microanalysis). We found that the RRR value of the ITER candidate strand becomes lower than the ITER requirement of 100, if the heat treatment at 650degC is longer than 200 hours. The RRR can be reached up to 161 when the duration of 650degC plateau is reduced to 100 hours, while satisfying all other ITER requirements including the critical current density.