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International audience; In order to improve the current performance of the Large Hadron Collider (HL-LHC project) or to construct the particle accelerators for the Future Circular Collider (FCC), the development of high field Nb3Sn magnets is of high importance. A Heat Treatment (HT) is required for the formation of the Nb3Sn superconducting phase that leads to various physic-chemical phenomena inside the superconducting wires with significant dimensional changes of the coils. The current study is focused on Rod Restacked Process (RRP) conductors. The heat treatment is composed of several dwells, activating different diffusion mechanisms and phase transformations. During the HT formation of different intermetallics between Sn, Cu and Nb take place. The growth of intermetallics is mainly controlled by diffusion mechanisms at large times. These mechanisms, located into each strand's sub-element area, naturally lead to variations of the crystalline structure and compactness. To predict the mechanical state and therefore the stress conditions of Nb3Sn conductors at a higher scale-cables and coils-it is primordial to first study the phase transformations occurring at the level of sub-elements inside the strands. Thus, a simplified one-dimensional model is proposed to evaluate the temporal evolution of different phases in the system during HT. Special attention is paid to the consideration of residual stresses inside the system and their influence on the diffusion process. The set of observations was performed on cross-sections of Nb3Sn strands at the different times of HT. The amount of each component is here determined experimentally using Scanning Electron Microscopy. Experimental observations are compared with the model predictions.

International audience; In the continuity of the Large Hadron Collider (LHC) large-scale projects are under development, such as the HiLumi-LHC and the Future Circular Collider (FCC), in order to produce particle collisions at higher luminosities and energies. The achievement of these projects strongly relies on the development of high field Nb3Sn magnets. The Nb3Sn intermetallic superconducting phase is produced through diffusion and phase transformations during a heat treatment at about 650 ℃. Due to the brittleness of the conductor, Coils are usually wound and then heat treated in a dedicated tooling. During the heat treatment, phase transformations and internal stresses relaxation lead to significant dimensional changes. If these changes are not taken into account by the fabrication tooling, resulting stress may lead to mechanical damage and degradation of the superconducting properties. In order to limit these degradations it is of crucial importance to characterize and quantify the dynamics of the deformations occurring during the heat treatment. An innovative experimental device, using high temperature in situ Digital Image Correlation (DIC), has been designed. This experiment allows the measurement of a 2D displacement field at the surface of Nb3Sn Rutherford cables as a function of temperature. A significant longitudinal contraction (of Rutherford cable) is first observed, that is consistent with results reported in the literature. The experiment provided on the other hand a measurement of a transversal swelling as a function of temperature. Associated dilatation reaches values higher than 1% at the end of the process. Observations at the microscopic scale allow a fine analysis of the strand motions within the cable and help for a global interpretation of dimensions changes.

International audience; In order to develop future particle colliders such as the HiLumi - Large Hadron Collider (HL-LHC) and the Future Circular Collider (FCC), high field superconducting Nb3Sn magnets are necessary. Following the wind- ing, the conductor requires a heat treatment at 650 °C during which significant dimensional changes occur. If dimensional changes are not allowed by the tooling, mechanical stresses build up in the coils and the perfor- mances of the magnet degrade. Therefore, coil fabrication tooling must authorize these dimensional changes in order to prevent conductor degradation and improve magnet performances. However, quantitative dynam- ics of the thermomechanical behavior of Nb3Sn conductors during heat treatment remains unknown. This study describes how a DIC (Digital Image Correlation) approach has been used for the first time to observe the displacements fields of Nb3Sn samples during heat treatment at 650°C. To do so, a furnace equipped of port- holes allows observing the sample at high temperature. Two digital cameras record the displacements at the surface of the sample (for example a Rutherford cable), from different viewing angles. Finally a DIC algorithm reconstructs the evolution of the displacements fields. The advantages of the method, compared to classical methods, are to have access to in-situ dimensional changes at high temperature, in the three directions of space, with the dynamics during the heat treatment cycle. An analysis of measurements on representative samples is given and data are compared to the literature.

La communication présente les principauxrésultats théoriques et expérimentaux obtenus depuis unequinzaine d'années dans le domaine de la compensationmagnétique de pesanteur. Cette technique, destinéeprincipalement à des études de comportement de fluides enenvironnement spatial, utilise des sources de champ magnétiquegénéralement élevé. Les avantages et les limites de la lévitationsont présentés ; son application à des problèmes decomportement de fluides sous forme diphasique est décrite.

The thesis deals with magnetic gravity compensation. First of all magnetic levitation experiments are explained with the help of a magneto-gravitational potential SL. Next, a general analysis method of the magnetic force is developed which employs magnetic field harmonics. The method underlines both the significance and role of the first three magnetic field harmonics on the resulting forces inherent in magnetic gravity compensation. In cylindrical geometry with translational invariance various combination of magnetic, gravitational and centrifugal forces open new possibilities for the magnetic levitation. A suitable combination of both magnetic and centrifugal forces allows exactly compensating gravity on diamagnetic materials. In axisymmetric geometry with rotational invariance designs of feasible oxygen magnetic levitation stations are introduced (NbTi@4,2K). Levitation of oxygen volumes more than one litre with inhomogeneities less than 1% can be accomplished within these magnetic levitation facilities. The constitution of the stations makes possible both spatial and temporal variations of the resulting acceleration configurations. At last the dynamic magnetic compensation of gravity with a real coil system is studied so as to simulate both acceleration and deceleration of spaceships, Cette thèse traite de la compensation magnétique de pesanteur. Tout d'abord, des expériences de lévitation magnétique de fluides sont interprétées à l'aide d'un potentiel magnéto-gravitationnel SL. Puis, l'utilisation d'une méthode générale d'analyse de la force magnétique grâce aux harmoniques du champ magnétique est développée. Elle souligne l'importance et le rôle de chacun des trois premiers harmoniques du champ magnétique sur les configurations de forces résultantes inhérentes à la compensation magnétique de pesanteur. En géométrie cylindrique (invariante par translation) diverses combinaisons de forces d'origines magnétique, gravitationnelle et centrifuge offrent des perspectives nouvelles pour la lévitation magnétique. Une combinaison judicieuse des forces magnétiques et centrifuges permet de compenser exactement la pesanteur sur des matériaux diamagnétiques. En géométrie axisymétrique (invariante par rotation), le dimensionnement de stations de lévitation d'oxygène, techniquement réalisables (NbTi@4,2K), est présenté. Ces stations permettent de léviter des volumes d'oxygène supérieurs à 1 litre avec des inhomogénéités inférieures à 1%. La constitution de ces stations rend possible les variations spatiales et temporelles des configurations d'accélérations résultantes. Enfin, la compensation magnétique dynamique de gravité, à l'aide d'une station de lévitation réelle, est étudiée afin de simuler des phases d'accélération ou de décélération d'engins spatiaux