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Étude de l'implémentation de supraconducteurs à haute température critique dans les aimants d'accélérateur

Abstract : Particle colliders are the main tool for investigating and understanding the fundamental laws of physics. The CERN Large Hadron Collider (LHC), the largest particle accelerator ever built, is a circular accelerator which steers and collides two counter-rotating protons beams. The LHC employs 1232 large superconducting Nb-Ti dipole magnets operated at a flux density of up to 8.3 T. Energies higher than that achieved with the LHC require magnets made from superconductors with higher upper critical flux density. Nb3Sn is an option for magnets operated up to about 14 T. The level of energies of the type being discussed for a potential energy upgrade of the LHC machine - 33 TeV- would require the use of high temperature superconductors (HTS). At low temperature, YBCO conductors present both large irreversible flux density and large current density. The level of currents required for application to accelerator magnets, which is above 10 kA at the nominal operating temperature and flux density, excludes the use of single strands. The high current and high current density required can be achieved with cables having several strands connected in parallel. In this work, a semi-analytical formula providing an estimate of the ultimate flux density in HTS accelerator dipoles has been derived. The possibility of generating operational flux densities above 20 T with HTS coils was demonstrated. A pioneer experimental work led to the first successful characterization of Roebel cables; a transposed high-current YBCO cable; at 4.2 K and in flux densities of up to 9.6 T. The measurement of critical currents of 12 kA and current densities in excess of 1.1 kA/mm2 at 7.5 T demonstrated the potential of such cables for application to high flux density accelerator magnets. Measured cables reached their expected performance with an uneven current distribution among the strands. The main reason for such unbalanced distribution has been identified as being due to an uneven strand contact resistance at the splices. Another issue in YBCO high current cables is the quench protection. The present configuration of Roebel cables was identified to be critical as regards to protection against quench. Technical solutions were proposed to the cable manufacturer to improve this aspect. Electrical and mechanical characteristics of YBCO conductors improve continuously, and cable performance improve correspondingly. Experimental and numerical tools introduced and implemented in this work can be used to further quantify and evaluate performance of new cables for accelerator magnets.
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Submitted on : Friday, November 29, 2013 - 11:20:39 AM
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Jérôme Fleiter. Étude de l'implémentation de supraconducteurs à haute température critique dans les aimants d'accélérateur. Sciences de l'ingénieur [physics]. Université de Grenoble, 2013. Français. ⟨tel-00911430⟩

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