The Effect of Heat Treatment on the Stability of Nb3Sn RRP-150/169 Strands

Abstract

Superconductins magnets fabricated with multi-filamentary Nb3Sn strands and cables are the best candidates for high field accelerator magnets in the field region of up to 16 T. A key challenge to Nb3Sn wires and cables is their magnetic instability [1]-[5], which can significantly depress their current-carrying capability to a fraction of the theoretical limit. The instability of Nb3Sn composite wires can be attributed to the redistribution of magnetic field inside a single superconducting filament or a strand as a whole (flux jump) induced by a perturbation. It was shown theoretically and confirmed experimentally that small filament size and low RRR of copper matrix are essential to a stable, high-Jc Nb3Sn composite strand. While there have been considerable amount of experimental and theoretical studies on this topic, they largely focused on some early Nb3Sn strands. With the development of Nb3Sn techniques in recent years, it is necessary to continue this work, both experimentally and theoretically, on state-of-the-art conductors. At Fermi National Accelerator Laboratory (FNAL), the design and fabrication of a 15 T dipole demonstrator magnet is in progress [6], [7]. In this work, we present some experimental study of the stability of Nb3Sn composite strands that will be used in this magnet. This work is also done jointly with variation of the heat treatment parameters.