Irreversible degradation of Nb₃Sn Rutherford cables due to transverse compressive stress at room temperature

Abstract

In the framework of the Future Circular Collider design study for a 100 TeV circular collider, 16 T superconducting bending magnets based on Nb 3 Sn technology are being developed. A prestress 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 Nb 3 Sn 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 Nb 3 Sn 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 Nb 3 Sn sub-elements of the loaded specimen.

Effect of Applied Compressive Stress and Impregnation Material on Internal Strain and Stress State in Nb3Sn Rutherford Cable Stacks

“…Transverse compressive stress can degrade the superconducting properties of the strain sensitive Nb3Sn [4]. A permanent conductor damage occurs when at room temperature the mechanical stresses exceed about 150 MPa, [5]. Therefore, this stress level must not be exceeded during coil manipulations and magnet assembly.…”

Machine learning applied to X-ray tomography as a new tool to analyze the voids in RRP Nb3Sn wires

“…Finally, the tool is not limited to study single wires, more complex system can also benefit from its use. The analysis of voids and cracks distribution of Nb 3 Sn Rutherford cables after load test 58 can provide precious information of the most sensible area of the cable and how to improve the cable design. While the study of the deformations of the wires in cable in conduit conductors (CICCs) for fusion application 59 can offer new solutions for the wires twisting pattern in order to optimize such conductors.…”

Nb3Sn Wire Shape and Cross-Sectional Area Inhomogeneity in Rutherford Cables

“…The double cable stack has been used for mechanical transverse compression tests at ambient temperature [14,15]. The tomogram of the double cable stack has been acquired in a cable region showing half the indent with a transverse pressure of 230 MPa-tomogram and half of the tomogram is outside the indent ( Fig.…”