Fabrication and characterization of internal Sn and bronze-processedNb3Sn strands for ITER application

Abstract

Long multifilamentary Nb3Sn strands for ITER (International Thermonuclear Experimental Reactor) have beensuccessfully fabricated by internal Sn and bronze processes, respectively. To improve thebonding of Nb and Cu and the diffusion between Sn and Cu for the internal tin process,the traditional RIT (rod-in-tube) and new hot extrusion (HE) processes havebeen employed to manufacture the Cu/Nb rods. The final internal tin processstrand with a unit length of 3000–5000 m comprises a Cu stabilizer, a Cu/Tabarrier and 19 subfilaments. Every subfilament consists of about 280–330Nb filaments in a Cu matrix surrounding an Sn–Ti core. The bronze processNb3Sn strands with 11581 and 9805 filaments of Nb7.5Ta with an unit length of more than 3 km havebeen produced, respectively. The Ta or Nb barriers were used to compare the influence ofdifferent barrier materials on the fabrication process and superconducting properties ofNb3Sn strands. The microstructure details of two kinds of strands beforeand after heat treatment have been investigated by scanning electronmicroscopy (SEM) and energy dispersive x-ray analysis (EDX). The non-CuJc (12 T, 4.2 K) value of 812 A mm − 2 with hysteresis loss of 760 mJ cm − 3 for the bronze-processstrand and 1069 A mm − 2 withhysteresis loss of 956 mJ cm − 3 for the internal tin-process strand have been obtained. The influence of microstructure onthe transport property and hysteresis loss of strands has been discussed.