Seminar By Shreyas Balachandran, ASC, National MagLab
Title: Recent Advances in Nb3Sn and Potential Pulsed Magnet Conductor
Abstract: We are seeking to make Nb3Sn conductors with significantly higher critical current densities at magnetic fields > 15 T. One recent approach that has been successfully demonstrated to produce an increase in pinning sites has been to use SnO2 powder in internally oxide Zr in Nb1Zr. However, lack of Ta or Ti doping resulted in reduced irreversibility fields (Hirr) in these initial studies. To achieve better high field performance we performed a systemic study using different in-house fabricated alloys based on the standard high-field Nb7.5Ta alloy. Using these alloys we fabricated monofilamentary conductors with and without excess oxygen (SnO2). After reaction heat treatment we found that the addition of Hf to the NbTa reduced the overall Nb3Sn grain size to well below 100 nm and TEM analysis showed that non-oxide intragranular precipitates had been produced in the Nb3Sn. Physical property measurements on the monofilamentary strands over the complete superconducting range up to 31 T indicated that the alloying additions raised the irreversibility field (Hirr) , and also boost the pinning force maximum (Fpmax). We found better properties in Nb-Ta-Hf conductor without SnO2, Fpmax reaching 2.34 times that of standard Nb7.5Ta alloy. The highest layer Jc (16 T,4.2 K) of 3700 A/mm2, was calculated for the SnO2 free Nb-Ta-Hf alloy. Using a standard A15 cross-section fraction of 60% for modern PIT and RRP wires suggests a non-Cu Jc of 2200 A/mm2 in a standard RRP or PIT conductor geometries, well above the 1500 A/mm2 FCC specification.
We also present some preliminary work on high strength Cu conductors developed in-house using the wire drawing facilities at the Applied Superconductivity Center. We fabricated a stacked composite of Ta-W alloy rods in a Cu matrix. The composite wire containing 133 Ta-W filaments had a strength exceeding 650 MPa, and an IACS of ~80%. The modulus of the conductor is about 140 GPa. The performance of strength and conductivity improves at 77 K, with a RRR (ρ77K/ρ295K) increased to 7.7. The composite fabricated has good ductility, and toughness. These results are comparable to or better than current options and the potential to improve this product is explored.
Acknowledgement: The Nb3Sn conductor development is funded by the U.S. Department of Energy, Office of Science, and Office of High Energy Physics under Award Number DE-SC0012083, and performed under the purview of the US-Magnet Development Program. All the work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement No. DMR-1157490, DMR-1644779 and the State of Florida.