Fabrication and characterization of internal Sn and bronze-processed Nb3Sn strands for ITER application

Long multifilamentary Nb3Sn strands for ITER (International Thermonuclear Experimental Reactor) have been successfully fabricated by internal Sn and bronze processes, respectively. To improve the bonding 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 have been employed to manufacture the Cu/Nb rods. The final internal tin process strand with a unit length of 3000-5000 m comprises a Cu stabilizer, a Cu/Ta barrier and 19 subfilaments. Every subfilament consists of about 280-330 Nb filaments in a Cu matrix surrounding an Sn-Ti core. The bronze process Nb3Sn strands with 11581 and 9805 filaments of Nb7.5Ta with an unit length of more than 3 km have been produced, respectively. The Ta or Nb barriers were used to compare the influence of different barrier materials on the fabrication process and superconducting properties of Nb3Sn strands. The microstructure details of two kinds of strands before and after heat treatment have been investigated by scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX). The non-Cu Jc (12 T, 4.2 K) value of 812 A mm-2 with hysteresis loss of 760 mJ cm-3 for the bronze-process strand and 1069 A mm-2 with hysteresis loss of 956 mJ cm-3 for the internal tin-process strand have been obtained. The influence of microstructure on the transport property and hysteresis loss of strands has been discussed.