A new approach to high-field dipole design is being developed at Texas A&M University. The goal of the development is to facilitate the use of high-field conductors (Nb3 and Bi-2212) and to manage Lorentz stress and magnetization so that field strength can be extended to 25 Tesla. The new design incorporates several innovations, including stress management, flux plate suppression of multipoles, and bladder preload. A series of model dipoles is being built and tested to validate and optimize each of these innovations. The second such model dipole, TAMU2, has been completed and was recently tested at Lawrence Berkeley National Laboritory. It achieved 93% of cable short sample limit on the first quench and every subsequent quench and did not suffer from any detectable training. This level of performance corrisponds to currents over 8800 A and a measureable field strength of 4.6 T. Ramp rate studies indicate robust behavior under fast ramping; we interpret this to be a beneficial result of the block coil geometry and the chrome-plated conductor.

Les câbles supraconducteurs sont largement utilisés dans le domaine des aimants à haut champ et sont en plein développement pour le transport de l’énergie. Un câble est un assemblage complexe de fils composites, euxmêmes constitués de filaments supraconducteurs torsadés dans une matrice métallique et entourés d’une couronne. La dépendance des brins supraconducteurs à la déformation est connue pour être responsable de la dégradation des performances électriques des câbles. La compréhension et la prédiction du comportement mécanique des brins est donc nécessaire afin de prédire les propriétés électriques des câbles dans le but d’optimiser leur mise en forme pour augmenter leurs performances (champ magnétique et capacité de transport).Une caractérisation mécanique multi-échelle de brins Nb3Sn et MgB2 a été réalisée au travers d’essais sur brins complets et sur brins dont la couronne a été dissoute. Un dispositif d’essais a été développé dans le cadre d’essais uniaxiaux sur fil fragile de faible diamètre. Des essais de nano-indentation ont permis d’accéder aux propriétés locales des matériaux constituants les brins. Une stratégie de modélisation et d’identification du comportement mécanique des brins a été développée. La modélisation repose sur une représentation simplifiée de la structure construite à partir des fractions volumiques et des essais de nano-indentation. L’identification des paramètres des lois de comportement est réalisée en utilisant la base de données expérimentales construite préalablement. Les modèles ainsi identifiés vont nourrir les futures simulations mécanique et électrique couplées de câbles.Une discussion sur l’endommagement des brins est menée au travers de l’étude de la localisation de la déformation observée dans certains brins, d’observations après des essais de traction interrompus et d’essais de traction in situ à un tomographe à rayons X. / Superconducting cables are widely used in high field magnets and have recently been extended to electricity transport. Cables are composed of a complex assembly of superconducting strands, themselves composed of superconducting filaments twisted in a metallic matrix and surrounded by an outerlayer. The electrical-strain dependence of individual strands is known to be responsible for the degradation of the electrical performance of cables. Thus, it is necessary to understand and predict the mechanical behavior at the strand scale to predict the electrical properties and optimize the manufacturing process of cables to achieve higher fields and better transport capabilities.A multi-scale mechanical characterization of Nb3Sn and MgB2 strands was carried out through tests on complete strands and strands without an outer-layer. A specific device was developed for uniaxial tests on small brittle wire. Nano-indentation tests were also carried out to access to the local properties of each material in the strand.A strategy for the modeling and identification of the mechanical behavior of strands was developed. The model is based on a simplified representation of the structure built from the component volume fractions and the nanoindentation tests. The identification of the parameters is carried out using the experimental database previously developed. The model is intended to be subsequently used to simulate the coupled mechanical and electrical behavior of cables.Discussion about damage phenomenon in strands is carried out through the study of strain localization observed in strands, observations and analyses after interrupted tests, and tensile tests performed in an X-ray tomograph.

Em trabalhos anteriores de nosso grupo de pesquisa foram desenvolvidos fios supercondutores nanoestruturados de Nb3Sn com introdução de centros de aprisionamento artificiais (CAAs) de Cu(Sn), numa tentativa de melhorar as propriedades supercondutoras. Porém, os perfis de tratamentos térmicos utilizados não foram otimizados. No presente trabalho as amostras desses fios compósitos foram utilizadas para otimização dos perfis de tratamentos térmicos visando determinar a influência destes perfis no processo de difusão atômica, reação e formação de fases nestes supercondutores nanoestruturados, e, com isso, otimizar a capacidade de transporte de corrente desses materiais. As micrografias e as análises composicionais obtidas produziram um conjunto de informações sobre os mecanismos de difusão e formação da fase supercondutora de Nb3Sn. As amostras foram analisadas quanto às suas propriedades e características supercondutoras. Com as medidas de transporte e magnetização pôde-se obter o comportamento global do supercondutor e o comportamento de cada filamento individual de Nb3Sn, respectivamente. Os compósitos otimizados apresentaram valores de temperatura crítica Tc próximos de 17 K, valores de Bc2 da ordem de 18 T, e densidades de corrente crítica tão altas quanto 1750 A/mm² a 4,2K e 12 T. Os máximos das curvas de força de aprisionamento aconteceram em campos magnéticos tão altos quanto 10 T, aproximadamente 0,5 Bc2, em distinção ao valor de 4,5 T (cerca de 0,2 Bc2) para Nb3Sn convencional, deslocamento que demonstra mudanças no comportamento de aprisionamento de fluxo nestes materiais. Um comportamento inédito e importante, que mistura os mecanismos de aprisionamento de fluxo devido aos contornos de grãos e as fases normais, está presente e pôde ser quantificado em detalhe. A análise dos comportamentos de todas as amostras possibilitou um entendimento satisfatório da influência dos CAAs na fase supercondutora e da influência do efeito de proximidade no aprisionamento de fluxo e nas propriedades de transporte destes materiais. / In previous works of our research group it was shown the development of Nb3Sn nanostructured superconducting wires with introduction of artificial pinning centers (APCs) of Cu(Sn), in an attempt to improve the superconducting properties. However, the used heat treatment profiles were not optimized. In the present work samples of these composite wires were used to optimize the heat treatment profile aiming to determine the influence of these profiles on the processes of atomic diffusion, reaction and formation of phases in these nanostructured superconductors and, consequently, optimize the transport properties of these materials. The obtained micrographs and compositional analysis gave a set of information about the diffusion mechanisms and formation of the Nb3Sn superconducting phase. The samples were analyzed in relation to their superconducting properties and characteristics. The transport and magnetization measurements showed the global behavior of the superconductor and the behavior of each individual Nb3Sn filament, respectively. The optimized composites presented values of critical temperature Tc around 17 K, values of Bc2 around 18 T, and critical current densities as large as 1750 A/mm2 at 4,2 K and 12 T. The peaks of the pinning force curves, occurred at magnetic fields as large as 10 T, approximately 0.5 Bc2, in distinction to the value of 4.5 T (about 0.2 Bc2) for conventional Nb3Sn, displacement that demonstrates changes on the flux pinning behavior in these materials. An inedit and important behavior, which mixes the flux pinning mechanisms due to the grain boundaries and to the normal phases, is present and could be quantified in detail. The behavior analysis of all samples enable a satisfactory understanding of the APCs influence on the superconducting phase and of the influence of the proximity effect on the flux pinning and on the transport properties of these materials.

Em trabalhos anteriores de nosso grupo de pesquisa foram desenvolvidos fios supercondutores nanoestruturados de Nb3Sn com introdução de centros de aprisionamento artificiais (CAAs) de Cu(Sn), numa tentativa de melhorar as propriedades supercondutoras. Porém, os perfis de tratamentos térmicos utilizados não foram otimizados. No presente trabalho as amostras desses fios compósitos foram utilizadas para otimização dos perfis de tratamentos térmicos visando determinar a influência destes perfis no processo de difusão atômica, reação e formação de fases nestes supercondutores nanoestruturados, e, com isso, otimizar a capacidade de transporte de corrente desses materiais. As micrografias e as análises composicionais obtidas produziram um conjunto de informações sobre os mecanismos de difusão e formação da fase supercondutora de Nb3Sn. As amostras foram analisadas quanto às suas propriedades e características supercondutoras. Com as medidas de transporte e magnetização pôde-se obter o comportamento global do supercondutor e o comportamento de cada filamento individual de Nb3Sn, respectivamente. Os compósitos otimizados apresentaram valores de temperatura crítica Tc próximos de 17 K, valores de Bc2 da ordem de 18 T, e densidades de corrente crítica tão altas quanto 1750 A/mm² a 4,2K e 12 T. Os máximos das curvas de força de aprisionamento aconteceram em campos magnéticos tão altos quanto 10 T, aproximadamente 0,5 Bc2, em distinção ao valor de 4,5 T (cerca de 0,2 Bc2) para Nb3Sn convencional, deslocamento que demonstra mudanças no comportamento de aprisionamento de fluxo nestes materiais. Um comportamento inédito e importante, que mistura os mecanismos de aprisionamento de fluxo devido aos contornos de grãos e as fases normais, está presente e pôde ser quantificado em detalhe. A análise dos comportamentos de todas as amostras possibilitou um entendimento satisfatório da influência dos CAAs na fase supercondutora e da influência do efeito de proximidade no aprisionamento de fluxo e nas propriedades de transporte destes materiais. / In previous works of our research group it was shown the development of Nb3Sn nanostructured superconducting wires with introduction of artificial pinning centers (APCs) of Cu(Sn), in an attempt to improve the superconducting properties. However, the used heat treatment profiles were not optimized. In the present work samples of these composite wires were used to optimize the heat treatment profile aiming to determine the influence of these profiles on the processes of atomic diffusion, reaction and formation of phases in these nanostructured superconductors and, consequently, optimize the transport properties of these materials. The obtained micrographs and compositional analysis gave a set of information about the diffusion mechanisms and formation of the Nb3Sn superconducting phase. The samples were analyzed in relation to their superconducting properties and characteristics. The transport and magnetization measurements showed the global behavior of the superconductor and the behavior of each individual Nb3Sn filament, respectively. The optimized composites presented values of critical temperature Tc around 17 K, values of Bc2 around 18 T, and critical current densities as large as 1750 A/mm2 at 4,2 K and 12 T. The peaks of the pinning force curves, occurred at magnetic fields as large as 10 T, approximately 0.5 Bc2, in distinction to the value of 4.5 T (about 0.2 Bc2) for conventional Nb3Sn, displacement that demonstrates changes on the flux pinning behavior in these materials. An inedit and important behavior, which mixes the flux pinning mechanisms due to the grain boundaries and to the normal phases, is present and could be quantified in detail. The behavior analysis of all samples enable a satisfactory understanding of the APCs influence on the superconducting phase and of the influence of the proximity effect on the flux pinning and on the transport properties of these materials.

Dans le contexte des améliorations du LHC, des efforts importants sont fournis pour concevoir des aimants d'accélérateurs utilisant l'alliage supraconducteur Nb3Sn, qui permet d'atteindre des champs magnétiques plus élevés (>12T). Le but de cette thèse est de proposer de nouvelles méthodes de calcul et de fabrication de dipôles à haut champ en Nb3Sn. Une isolation céramique, mise au point précédemment au CEA Saclay, a été testée pour la première fois sur des câbles, dans les conditions d’utilisation d’un aimant d’accélérateur. Des mesures de courant critique sous champ magnétique et contrainte mécanique ont notamment été réalisées. Ces campagnes d’essais ont révélé que l’isolation céramique actuelle est trop fragile mécaniquement et que les propriétés de courant critique sont dégradées. Une étude a ensuite été menée, afin d’améliorer la tenue mécanique de l’isolation et de mieux répartir les contraintes à l’intérieur du câble. Des méthodes de conception magnétique ont par ailleurs été proposées afin d’optimiser la forme des bobinages, tout en respectant des contraintes d’homogénéité de champ, de marges de fonctionnement, de minimisation des efforts… Pour cela plusieurs codes d’optimisation ont été élaborés. Ils se basent sur des méthodes nouvelles utilisant des formules analytiques. Un code 2D a d’abord été élaboré pour des conceptions en blocs rectangulaires. Ensuite, deux codes 3D ont été conçus pour l’optimisation des têtes de dipôles. Le premier consiste à modéliser le bobinage à l’aide de blocs élémentaires, et le deuxième se base sur une modélisation des câbles supraconducteurs par des rubans. Ces codes d’optimisation ont permis de proposer des configurations magnétiques pour des aimants à haut champ. / In the framework of LHC upgrades, significant efforts are provided to design accelerator magnets using the superconducting alloy Nb3Sn, which allows to reach higher magnetic fields (>12T). The aim of this thesis is to propose new computation and manufacturing methods for high field Nb3Sn dipoles. A ceramic insulation, previously designed at CEA Saclay, has been tested for the first time on cables, in an accelerator magnet environment. Critical current measures, under magnetic field and mechanical stress, have been carried out in particular. With this test campaign, the current ceramic insulation has been shown to be too weak mechanically and the critical current properties are degraded. Then a study has been conducted, with the objective to improve the mechanical strength of the insulation and better distribute the stress inside the cable. Methods of magnetic design have also been proposed, in order to optimize the coils shape, while fulfilling constraints of field homogeneity, operational margins, forces minimization… Consequently, several optimization codes have been set up. They are based on new methods using analytical formulas. A 2D code has first been written for block designs. Then two 3D codes have been realized for the optimization of dipole ends. The former consists in modeling the coil with elementary blocs and the latter is based on a modeling of the superconducting cables with ribbons. These optimization codes allowed to propose magnetic designs for high field accelerator magnets.

Le travail effectué dans cette thèse a pour thème central la conception d’un aimant quadripolaire supraconducteur en Nb-Ti destiné à remplacer à l’horizon 2014 les aimants d’insertions actuellement utilisés dans le grand collisionneur de Hadrons (LHC) du CERN de Genève. Ce nouveau quadripôle, caractérisé par un diamètre d’ouverture encore jamais atteint (120 mm), ouvre la voie vers les quadripôles à grandes ouvertures. Tout d’abord, pour rapidement estimer l’énergie magnétique stockée dans un quadripôle de type cos2?, une formule analytique basée sur la décomposition en série de Fourier du courant et permettant d’estimer l’énergie avec une précision de 10 % est développée. Le design magnétique de la section transverse de la bobine du quadripôle est ensuite réalisé en utilisant une nouvelle méthode d’optimisation basée sur les équations analytiques du champ magnétique. Puis, pour la première fois, une estimation de la reproductibilité dans le positionnement des blocs de conducteurs dans des aimants Nb3Sn est faite. Elle a été réalisée à l’aide d’une méthode existante et grâce à la production récente de deux séries d’aimants Nb3Sn. Une comparaison avec les valeurs obtenues pour des aimants en Nb-Ti est présentée. Ensuite, une méthode analytique basée sur les statistiques et permettant d’expliquer certains phénomènes observés sur la dispersion des mesures magnétiques dans une série de quadripôles est développée. Enfin, on montre que l’incertitude sur la moyenne des harmoniques de champ est due pour la majorité des harmoniques à un phénomène statistique lié au nombre limité d’aimants dans la série et non à des erreurs systématiques / The main objective of the work presented in this thesis is the design of a quadrupole magnet based on Nb-Ti. It aims at replacing the current insertion quadrupoles used in the Large Hadron Collider (LHC) at CERN by 2014. This new quadrupole features an unprecedented large aperture (120 mm) and opens the way toward large aperture quadrupoles. First, to rapidly estimate the magnetic energy stored in a cos2?-type quadrupole, an analytical formula based on the Fourier transform of the current is developed. It allows estimating the energy with a precision of 10 %. Secondly, the magnetic design of the quadrupole coil cross-section is realized using a novel optimization method based on analytical equations of the magnetic field. Subsequently, for the first time, an estimate of the reproducibility in the coil-blocks positioning in Nb3Sn magnets is given. The estimate has been obtained by using an existing method and from tow recently built Nb3Sn magnet series. A comparison with values obtained for Nb-Ti magnets is also presented. Following this, an analytical method based on statistics is developed. It makes possible to explain some phenomenon observed on the dispersion of the magnetic measurement in a quadrupole series. Finally, we show that the uncertainty in the mean of the magnetic field errors is for most of the harmonics related to statistical errors due to the limited number of magnets in the series, and not because of systematic defects in the coil

Nb3Sn Rutherford cables are used in CERN’s superconducting 11 T dipole and MQXF quadrupole magnets, which are proposed for the instantaneous luminosity (rate of particle collisions) upgrade of the Large Hadron Collider (LHC) by a factor of five to a High Luminosity-Large Hadron Collider (HL-LHC). Nb3Sn-based conductors are the key technology for the envisioned Future Circular Collider (FCC) with an operating magnetic dipole field of 16 T. The baseline superconductor of the LHC dipole magnets is Nb–Ti, whereas an operation above 10 T is not possible due to the current carrying performance limitations of this superconductor at higher magnetic fields. Therefore, a superconducting material such as Nb3Sn has to be used with proven performance capabilities of 10 T and above. The conductor choice towards Nb3Sn-based cables affects the magnet manufacturing process, as it requires a heat treatment up to 650°C, an epoxy resin impregnation and introduces mechanical diffculties as the superconducting filaments are brittle and strain sensitive. A mechanical over loading of the filaments lead to irreversible conductor damage. The designs of 11 and 16 T magnets are supposed to push the conductor towards its mechanical and electrical performance limitations. The magnetic field induced forces on the current carrying conductor are balanced by mechanical pre-loading of the magnet. Thereby the highest controlled mechanical pre-load for the 11 T dipole magnet is set at ambient temperature. The mechanical stress limits of Nb3Sn-based cables have been investigated at cryogenic temperatures. The material strength and stiffness of the cable insulation system, formed by glass-fibre-reinforced resin, is increased at low temperatures. The ultimate stress values, determined at cryogenic temperature, are therefore not conservative. The ultimate stress limitation of the insulated conductor is assumed to be lower at ambient temperature. The cable limitations at ambient temperature need to be known for the ongoing magnet manufacturing process and also for future design approaches. Furthermore, the compressive stress–strain behaviour of a coil conductor block at ambient temperature is the key material characteristic, in order to recalculate the stress level in the coil during the assembly process. Existing approaches using an indirect strain measurement method provide uncertainties in the low-strain regime, which is the essential strain range for a material compound consisting of major fractions composed of heat-annealed copper and epoxy resin. Compressive stress–strain data of an impregnated conductor block are required, based on a direct strain measurement system, as available data have been collected on samples based on a different strand type and insulation system. The elaborated direct strain measurements can be correlated to strain gauge data, measured directly on a coil. The stress distribution in a Nb3Sn Rutherford cable need to be understood and validated to understand strain-induced degradation effects in the insulated conductor. This knowledge can also help to optimise the stress distribution envisioned magnet designs. The stress–strain state in the copper and Nb3Sn phase of a loaded conductor block has to be determined experimentally. This dissertation describes a test protocol and first elaborated results on the investigated stress limitations of a Nb3Sn Rutherford cable under homogeneous load applied in transversal direction. The compressive stress–strain behaviour of impregnated Nb3Sn Rutherford cable stacks was investigated experimentally. This includes a detailed report on the sample manufacturing process, measurements performed and validation of results through a comparison with the elaborated data of cable stacks extracted from a coil. The presented results from neutron diffraction measurements of loaded cable stacks allow the determination of the stress–strain level of the copper and Nb3Sn phase in the impregnated conductor. The relevant measured results have been recalculated with numerical calculations based on the Finite Element Method (FEM).:1. Introduction 1 1.1. The LHC and the HL-LHC project 1.2. The FCC study 1.3. Superconducting materials for accelerator magnets 1.4. Multi-filamentary wires and Rutherford cables 1.5. Coil manufacturing process 1.6. Magnet coil assembly 1.7. Objectives of this thesis 2. Theory: fundamental principles 17 2.1. Analytical calculation: sector coil dipole 2.2. Mechanical behaviour of composite materials 2.3. Failure criteria and strength hypotheses for materials 2.4. Compressive tests 2.5. Fundamental principles of Neutron scattering 2.5.1. Test apparatus and measurement method 2.5.2. Lattice plane and Miller indices 2.5.3. Bragg diffraction and interference 2.5.4. Diffraction-based strain calculation 2.5.5. Diffraction-based stress calculation 2.6. Fundamental principles of FEM 3. Homogeneous transversal compression of Nb3Sn Rutherford cables 3.1. Superconducting cable test stations 3.2. The FRESCA test facility and specific sample holder 3.3. The sample description 3.4. Experimental procedure 3.5. Review of existing contact pressure measurement system 3.6. Compressive test station 3.7. Validation of the pressure-sensitive films 3.8. Press punch 3.9. Improvement of the contact stress distribution 3.9.1. First test: cable pressed between the bare tools 3.9.2. Second test: tool shimmed with a soft Sn96Ag4 3.9.3. Third test: tool shimmed with a soft Sn60Pb40 3.9.4. Fourth test: tool shimmed with a soft indium 3.9.5. Fifth test: tool shimmed with a polyimide film 3.10. Test results 3.11. Conclusion 4. Material characterisation by a compression test 4.1. Test set-ups for compressive tests and validation 4.2. Sample preparation 4.3. Compressive stress–strain measurement 4.4. Ten-stack sample stiffness estimation-based composite theories 4.5. Dye penetration test on loaded and unloaded samples 4.6. Conclusion 5. Neutron diffraction measurements 80 5.1. Test set-up for neutron diffraction measurement 5.2. The samples 5.3. Experiment: lattice stress–strain measurements 5.4. Conclusion 6. Simulation and modelling of Nb3Sn cables 6.1. The models 6.2. The 2D simulation results 6.3. The 3D simulation results 6.4. Conclusion 7. Comprehensive summary 7.1. Summary 7.2. Critical review 7.3. Next steps Appendix 113 A. Calculation of the magnetic field components in a sector coil without iron B. Approaches for the determination of diffraction elastic constants C. Manufacturing drawings D. FEM calculation results of the 2D model E. FEM calculation results of the 3D model F. Source Codes Bibliography

Dans le contexte des améliorations du LHC, des efforts importants sont fournis pour concevoir des aimants d'accélérateurs utilisant l'alliage supraconducteur Nb3Sn, qui permet d'atteindre des champs magnétiques plus élevés (>12T). Le but de cette thèse est de proposer de nouvelles méthodes de calcul et de fabrication de dipôles à haut champ en Nb3Sn. Une isolation céramique, mise au point précédemment au CEA Saclay, a été testée pour la première fois sur des câbles, dans les conditions d'utilisation d'un aimant d'accélérateur. Des mesures de courant critique sous champ magnétique et contrainte mécanique ont notamment été réalisées. Ces campagnes d'essais ont révélé que l'isolation céramique actuelle est trop fragile mécaniquement et que les propriétés de courant critique sont dégradées. Une étude a ensuite été menée, afin d'améliorer la tenue mécanique de l'isolation et de mieux répartir les contraintes à l'intérieur du câble. Des méthodes de conception magnétique ont par ailleurs été proposées afin d'optimiser la forme des bobinages, tout en respectant des contraintes d'homogénéité de champ, de marges de fonctionnement, de minimisation des efforts... Pour cela plusieurs codes d'optimisation ont été élaborés. Ils se basent sur des méthodes nouvelles utilisant des formules analytiques. Un code 2D a d'abord été élaboré pour des conceptions en blocs rectangulaires. Ensuite, deux codes 3D ont été conçus pour l'optimisation des têtes de dipôles. Le premier consiste à modéliser le bobinage à l'aide de blocs élémentaires, et le deuxième se base sur une modélisation des câbles supraconducteurs par des rubans. Ces codes d'optimisation ont permis de proposer des configurations magnétiques pour des aimants à haut champ.

For a long time voltage spikes has been seen in measurement data from accelerator magnets during current ramps. These has been believed to be caused by movements, but has never before been studied in depth. The purpose of this thesis is therefore to prove, or disprove, that these events are caused by movements and to analyse what kind of displacements that actually occur. Measurement data from coil voltage, magnetic pick-up coils and current during transients has been acquired and analysed for the Nb3Sn-dipole magnets FRESCA2 and 11T models—named MBHSP107 and MBHSP109. The measurement data is compared to movement simulations that was done with the ROXIE-program, which is used to calculate mutual inductance change for a number of different movement types. The study strongly suggests that the transients are caused by movements, and also indicates that the maximal length of a single slip-stick motion can be up to around 10 micrometers, mostly in the direction of the magnet’s internal forces. The study has proven that transients in measurement data occur due to coil movements, and that these can be quantified—a discovery that can possibly affect future construction and design of accelerator magnets.

No description available.