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In the last few years, a new type of Nb$_3$Sn superconducting composite, containing a high density of artificial pinning centers (APC) generated via an internal oxidation approach, has demonstrated a significantly superior performance relative to present, state-of-the-art commercial Nb$_3$Sn conductors. This was achieved via the internal oxidation of Nb-4at.%Ta-1at.%Zr alloy. On the other hand, our recent studies have shown that internal oxidation of Nb-Ta-Hf alloys can also lead to dramatic improvements in Nb$_3$Sn performance. In this work we follow up this latter approach, fabricating a 61-stack APC wire based on the internal oxidation of Nb-4at.%Ta-1at.%Hf alloy, and compare its critical current density (Jc) and irreversibility field (Birr) with APC wires made using Nb-4at.%Ta-1at.%Zr. A second goal of this work was to improve the filamentary design of APC wires in order to improve their wire quality and electromagnetic stability. Our new modifications have led to significantly improved RRR and stability in the conductors, while still keeping non-Cu Jc at or above the FCC Jc specification. Further improvement via optimization of the wire recipe and design is ongoing. Finally, additional work needed to make APC conductors ready for applications in magnets is discussed.

In this White Paper for the Snowmass 2021 Process, we propose the establishment of a magnet Leading-Edge technology And Feasibility-directed Program (LEAF Program) to achieve readiness for a future collider decision on the timescale of the next decade. The LEAF Program would rely on, and be synergetic with, generic R&D efforts presently covered - in the US - by the Magnet Development Program (MDP), the Conductor Procurement and R&D (CPRD) Program and other activities in the Office of HEP supported by Early Career Awards (ECA) or Lab Directed R&D (LDRD) funds. Where possible, ties to synergetic efforts in other Offices of DOE or NSF are highlighted and suggested as wider Collaborative efforts on the National scale. International efforts are also mentioned as potential partners in the LEAF Program. We envision the LEAF Program to concentrate on demonstrating the feasibility of magnets for muon colliders as well as next generation high energy hadron colliders, pursuing, where necessary and warranted by the nature of the application, the transition from R&D models to long models/prototypes. The LEAF Program will naturally drive accelerator-quality and experiment-interface design considerations. LEAF will also concentrate, where necessary, on cost reduction and/or industrialization steps., Comment: Contribution to Snowmass 2021, 19 pages, 5 figures. Corresponding Author: G. Apollinari apollina@fnal.gov

In this letter we demonstrate achievement of record non-Cu critical current density (Jc,non-Cu) in ternary, multifilamentary Nb3Sn conductors by the introduction of artificial pinning centers (APC). In the past two years, we have made great progress in the development of APC Nb3Sn wires. Recent resistivity vs magnetic field measurements confirmed the high upper critical field (Bc2) of ternary APC wires, which at 4.2 K was ~28 T, about 1-2 T higher than present state-of-the-art conductors. In addition to high Bc2, it was found that APC wires have noticeably higher Sn content in the Nb3Sn layers as compared to standard wires. The Jc,non-Cu values of the most-recent APC wires have met the Jc,non-Cu-B specification required by the Future Circular Collider (FCC), with the best heat treatment leading to a Jc,nonCu 29% higher than the FCC specification at 21 T. Microscopy analysis shows that the APC wires still have overly high residual Nb fractions due to too low of a Sn/Nb ratio, indicating that there is still great potential for further Jc,non-Cu improvement. The development of APC wires is ongoing; this letter details some of the steps forward in the optimization and lays out a roadmap to push the APC wires towards practical, magnet-grade conductors., Comment: 4 figures