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Effect of transverse compressive stress on Nb3Sn Rutherford cables for accelerator magnets.
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Europe/Zurich
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Description
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Abstract:
The accelerator magnets for the High Luminosity upgrade of the Large Hadron Collider (HL-LHC) use Nb3Sn conductors to achieve in-field performance exceeding Nb-Ti-based technologies. To sustain the Lorentz forces during operation, a pre-compression is applied to the conductor during the fabrication of the magnet. This can lead to an irreversible degradation due to the mechanical sensitivity of the Nb3Sn material.
In this study, the impact of the pre-compression is investigated using a reacted double-stack of Rutherford cables. The stack is submitted to transverse stress at room temperature. The critical current is then measured in liquid helium and a background field of up to 9.6 T in the FRESCA test station at CERN. The pressure applied at room temperature covers the range from 130 MPa to 190 MPa with a 10 MPa step increase. Monotonic and cumulated pressures were applied to analyse the impact of the cyclic loading. Microscopic analyses of cross-sections were performed following procedures specifically developed to minimize surface damage during samples’ preparation.
These observations were compared to the electrical measurements to correlate the irreversible effect of the transverse pressure to the A15 damage in the cross-section. On the non-inverted double-stack configuration, cracks were observed from the 160 MPa stress level. It was observed clear differences in the crack density between monotonic and cumulated samples. Electrical measurements show degradations at 170 MPa and 160 MPa for relative critical current and n-value. Further understanding of the correlation between the macroscopic electrical measurements and the damage process is mandatory to propose relevant limits for Nb3Sn magnet design.
Carlo Petrone
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