Ultra-low magnetization and hysteresis loss in APC Nb3Sn superconductors
For the accelerator magnets of the next hadron collider, reducing superconductor persistent-current magnetization is not only important for achieving the desired field quality, but also crucial for its sustainability because the magnetization loss is the major heat load to the magnet cold mass. For conventional Nb3Sn conductors this requires reduction of effective subelement size (Deff). For the restacked-rod-process (RRP) conductors a physical subelement size (Dsub) as small as 35 um (corresponding to a Deff close to 45 um) can be reached, but at a significant price in Jc. Another way to reduce the magnetization is by introducing artificial pinning centers (APC) using the internal oxidation approach. APC conductors outperform conventional Nb3Sn wires in two aspects: 1) higher Jc at high fields, and 2) much lower Jc and magnetization at low fields (e.g., below 5 T). In this work we explored the fabricability of APC wires with small Dsub. A 180-stack APC wire was produced and drawn to 0.7- and 0.5-mm diameters with good quality, with Dsubs of 34 and 24 um (Deffs of 36 and 25 um), respectively. For the 34-um-Dsub wire, its non-Cu Jc is higher than that of an RRP wire used for the High-Luminosity Large Hadron Collider (HL-LHC) project above 13 T (e.g., 36% higher at 4.2 K, 18 T), while its non-Cu magnetization at 1 T, {\Delta}M(1 T), is only 29% of the RRP wire. Its non-Cu hysteresis loss for a cycle between 1 and 14 T, Qh(1-14 T), is 37% of the RRP wire. For the 24-um-Dsub wire, its non-Cu Jc surpasses the HL-LHC RRP wire above 17.5 T, while its {\Delta}M(1 T) and Qh(1-14 T) are only 17% and 23% of the RRP wire, respectively. Its non-Cu Qh(+/-3 T) even meets the specification of the International Thermonuclear Experimental Reactor (ITER) project.