The construction of the Future Circular Collider (FCC) has very stringent requirements for Nb3Sn conductor critical current density, Jc, with a target non-Cu Jc(16T, 4.2K) of at least 1500 A/mm2. Nowadays the best commercial Nb3Sn strands can only reach 1300 A/mm2, consequently it will be necessary to significantly increase the high field Jc. To meet this challenge requires new approaches that can introduce additional pinning centers while maintaining a high irreversibility field, HIrr. In this work, we focus on Nb3Sn wires prepared using Nb-Ta-Zr and Nb-Ta-Hf alloys. Both Zr and Hf had been partially investigated in the ’80 and ’90 but only recently it has been shown that greatly enhanced pinning can be obtained in Nb3Sn with Zr or Hf, while maintaining high HIrr by incorporating Ta doping. In the Zr case, an internal oxygen source (SnO2) is required, and ZrO2 nanoparticles have been shown to form in the Nb3Sn. However, we find that Fp and Jc at high field can be further improved with Hf additions without supplemental oxygen. The introduction of a significant point defect pinning contribution increases the maximum of Fp by more than a factor 2 and shifts its peak position from 4.6 to 5.8 T with respect to Ta-only doped wires. This leads to a layer Jc(16T, 4.2K) of about 3710 A/mm2, corresponding to a potential non-Cu Jc(16T, 4.2K) of 2230 A/mm2. We will also discuss the sensitivity of these properties to heat treatment, the different HIrr and Fp behavior with respect to standard Ta/Ti-doped conductors and the recent high field characterizations of newer alloyed wires.
Acknowledgments. This work was supported by the U.S. DOE No.DE-SC0012083. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by NSF Cooperative Agreement No.DMR-1644779 and the State of Florida.