Lawrence Berkeley National Laboratory

The high-luminosity upgrade of the large hadron collider (HL-LHC) at CERN will replace the main ring inner triplet quadrupoles, identified by the acronym MQXF, adjacent to the main ring intersection regions. For the past decade, the U.S. LHC Accelerator R&D Program, LARP, has been evaluating conductors for the MQXFA prototypes, which are the outer magnets of the triplet. Recently, the requirements for MQXF magnets and cables have been published in [P. Ferracin et al., IEEE Trans. Appl. Supercond., vol. 26, no. 4, Jun. 2016, Art. no. 4000207], along with the final specification for Ti-alloyed Nb3Sn conductor determined jointly by CERN and LARP. This paper describes the rationale beneath the 0.85-mm-diameter strand's chief parameters, which are 108 or more subelements, a copper fraction not less than 52.4%, strand critical current at 4.22 K not less than 631 A at 12 T and 331 A at 15 T, and residual resistance ratio of not less than 150. This paper also compares the performance for ∼100 km production lots of the five most recent LARP conductors to the first 163 km of strand made according to theHL-LHCspecification.Two factors emerge as significant for optimizing performance and minimizing risk: a modest increase of the subelement diameter from 50 to 55 μm, and a Nb:Sn molar ratio of 3.6 instead of 3.4. The statistics acquired so far give confidence that the present conductor can balance competing demands in production for the HL-LHC project.