Search

In the framework of the Future Circular Collider design study for a 100 TeV circular collider, 16 T superconducting bending magnets based on Nb3Sn technology are being developed. A pre-stress on the conductor during magnet assembly at room temperature (RT) is needed to counteract the Lorentz forces during operation. The superconducting properties of the brittle Nb3Sn superconductor are strain sensitive and excessive pre-stress leads to an irreversible degradation of the superconductor. In order to determine the level of acceptable pre-stress during the magnet assembly process, reacted and impregnated Nb3Sn cables were exposed to increasing transverse compressive stress up to a maximum stress level of 200 MPa at RT. After each stress cycle, the critical current of the cable specimens were characterized at 4.3 K in the FRESCA cable test station. No significant critical current degradation was observed up to 150 MPa, followed by degradation less than 4% after a nominal stress of 175 MPa. A dramatic permanent critical current degradation occurred after applying a nominal stress of 200 MPa. A comprehensive post analysis consisting of non-destructive micro-tomography followed by microscopic characterization of metallographic cable cross sections was carried out after the critical current test to reveal cracks in the Nb3Sn sub-elements of the loaded specimen. [ABSTRACT FROM AUTHOR]

High critical current density (Jc) Nb3Sn A15 multifilamentary wires require a large volume fraction of small grain (SG), superconducting A15 phase, as well as Cu stabilizer with high Residual Resistance Ratio (RRR) to provide electromagnetic stabilization and protection. In powder-in-tube (PIT) wires the unreacted Nb7.5 wt%Ta outer layer of the tubular filaments acts as a diffusion barrier and protects the interfilamentary Cu stabilizer from Sn contamination. A high RRR requirement generally imposes a restricted A15 reaction heat treatment to prevent localized full reaction of the filament that could allow Sn to reach the Cu. In this study we investigate recent high quality PIT wires that achieve a Jc (12 T, 4.2 K) up to ∼2500 A mm−2 and find that the minimum diffusion barrier thickness decreases as the filament aspect ratio increases from ∼1 in the inner rings of filaments to 1.3 in the outer filament rings. We found that just 2–3 diffusion barrier breaches can degrade RRR from 300 to 150 or less. Using progressive etching of the Cu we also found that the RRR degradation is localized near the external filaments where deformation is highest. Consequently minimizing filament distortion during strand fabrication is important for reducing RRR degradation. The additional challenge of developing the highest possible Jc must be addressed by forming the maximum fraction of high Jc SG A15 and minimizing low Jc large-grain (LG) A15 morphologies. In one wire we found that 15% of the filaments had a significantly enhanced SG/LG A15 ratio and no residual A15 in the core, a feature that opens a path to substantial Jc improvement. [ABSTRACT FROM AUTHOR]