New Understanding of the Heat Treatment of Nb-Sn Superconducting Wires

Enhancing the beam energy of particle accelerators like the Large Hadron Collider (LHC), at CERN, can increase our probability of finding new fundamental particles of matter beyond those predicted by the standard model. Such discoveries could improve our understanding of the birth of universe, the universe itself, and/or many other mysteries of matter—that have been unresolved for decades—such as dark matter and dark energy. This is obviously a very exciting field of research, and therefore a worldwide collaboration (of universities, laboratories, and the industry) is attempting to increase the beam energy in the LHC. One of the most challenging requirements for an energy increase is the production of a magnetic field homogeneous enough and strong enough to bend the high energy particle beam to keep it inside the accelerating ring. In the current LHC design, these beam bending magnets are made of Nb-Ti superconductors, reaching peak fields of ~8 T. However, in order to move to higher fields, future magnets will have to use different and more advanced superconducting materials. Among the most viable superconductor wire technologies for future particle accelerator magnets is Nb₃Sn, a technology that has been used in high field magnets for many decades. However, Nb₃Sn magnet fabrication has an important challenge: the fact the wire fabrication and the coil assembly itself must be done using ductile metallic components (Nb, Sn, and Cu) before the superconducting compound (Nb₃Sn) is activated inside the wires through a heat treatment. The studies presented in this thesis work have found that the heat treatment schedule used on the most advanced Nb₃Sn wire technology (the Restacked Rod Process wires, RRP®) can still undergo significant improvements. These improvements have already led to an increase of the figure of merit of these wires (critical current density) by 28%.