The Bundle-Barrier PIT Wire Developed for the HiLumi LHC Project

For the HiLumi Large Hadron Collider Project, CERN is developing dipole and quadrupole magnets based on state-of-the-art high-J
_c
Nb
₃
Sn wires that are expected to operate at 1.9 K and at fields larger than 11 T. Two different types of Nb
₃
Sn wires are considered for the project: the powder in tube (PIT) and the restacked rod process conductors manufactured, respectively, by Bruker-EAS and Oxford superconducting technology. During the last 18 months, CERN and Bruker-EAS have being collaborating to develop a new variant of the PIT conductor in order to further improve its electromechanical properties. This collaboration led to the introduction of an additional Nb barrier around the whole bundle of filaments that allowed drastically reducing the effect of mechanical deformation and of the heat treatment cycle on the residual resistivity ratio (RRR) of the stabilizing wire copper. Furthermore, the new wire has already a slightly larger engineering critical current density with respect to the previous generation of PIT wire and it has the potential to further improve. In this paper, the bundle-barrier PIT wire is presented together with the critical current, magnetization, and RRR measurements carried out at CERN to: characterize its electro-mechanical properties; quantify the effect of the filament size on the critical current performance and; study the effect of the heat treatment cycle.