Influence of diffusion annealing on residual resistivity of Nb3Sn-based chromium-plated strands obtained by a bronze process

Abstract

The residual resistivity ratio, R273/R20, is an important parameter for multifilament superconductors (strands) based on Nb3Sn that are used to manufature cables of magnetic systems. High values of RRR impart stability to the cable with regard to thermal excitations. Nb3Sn strands for magnetic system of the International Thermonuclear Experimental Reactor are manufactured from high purity oxygen-free copper with RRR > 250 units; however, after extended diffusion annealing intended to form superconducting phase Nb3Sn, the residual resistivity ratio values of the strands decrease. This work investigates the influence of diffusion annealing for 55–200 h during the final stage at 650°C on the residual resistivity ratio of chromium-plated Nb3Sn strands. The contents of chromium and oxygen have been analyzed using X-ray spectral microanalysis of the strand surface and peripheral copper layers. Mass spectrometry with inductively coupled plasma (ICP mass spectrometry) has been used to determine the total chromium content in the copper shell. The influence of chromium and oxygen diffusion from coating during annealing at 650°C on the residual resistivity ratio has been demonstrated. Based on the data of ICP mass spectrometry, the depth of the penetration of chromium in a copper shell has been assessed.