Study of mechanical state of Nb3Sn conductors during heat treatment



Mahmoud ABDEL HAFIZ (Université Paris-Saclay)


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The development of future particle colliders such as the HiLumi - Large Hadron Collider (HL-LHC) and the Future Circular Collider (FCC) relies on the ability to generate high magnetic fields allowed by improved superconducting Nb3Sn magnets. The coil fabrication starts with the winding of cables followed by a precipitation of the superconducting brittle Nb3Sn phase during a long 650°C heat treatment. This study focuses on Rod Restacked Process (RRP) conductors which heat treatment is composed of several dwells, activating different diffusion mechanisms and phase transformations. These mechanisms, located into the filament area of each strand, naturally lead to variations of the crystalline structure hence of the mechanical state. As it is well known, Nb3Sn superconducting properties are stresses (or strain) sensitive. Hence estimating the mechanical state of Nb3Sn conductors after the heat treatment might have a considerable implication concerning the tooling and enhancing its properties.

Thus, a multiphysic model of the strand is proposed, taking into account the stresses created by differential thermal expansion and phase transformations. Diffusion kinetics are here determined experimentally using Energy Dispersive X-ray Spectroscopy and Scanning Electron Microscope analysis on strand cross sections. In the other hand, a high temperature in situ Digital Image Correlation (DIC) experimental device has been designed. This experiment allows the determination of a 2D displacement field at the surface of Nb3Sn Rutherford cables during heat treatment.

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Friedrich Lackner
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Sonja Sandell, Macarena Gonzalez Torres
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