Development of high performance Nb-Ti(Fe) multifilamentary superconductor for the LHC insertion quadrupoles

A development program was initiated in order to develop strand with improved current density at 10.5 T and 1.9 K over existing SSCL designs. The two successful strand designs reported on here both utilized high Fe content Nb-47 wt%Ti alloys to improve the critical current density at high field by 7%. At 10.5 T and 1.9 K, critical current densities exceeding 1450 A/mm$^2$were obtained. In this paper we report detailed quantification of the macro- and micro-structures of these strands and correlate these with critical current density measurements at 1.9 K and 4.2 K. The high Fe content significantly reduced the $\alpha$-Ti precipitate size. The linear relationship between critical current density and precipitate volume found is in agreement with earlier studies. High resolution FESEM electron backscatter contrast suggests a thin layer of high atomic number at grain boundaries. (12 refs).