Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Vita. / Includes bibliographical references. / by Joseph Vito Minervini. / Ph.D.

Thesis (Ph.D.)--University of Wollongong, 2005. / Typescript. Includes bibliographical references: leaf 188-200.

Superconducting Magnetic Energy Storage (SMES) offers many advantages over conventional forms of energy storage. The higher unit costs of SMES make it economically feasible only for large-scale applications (5000 MWh or more). Early studies showed that low aspect ratio solenoids have the lowest overall costs and most of the subsequent research and conceptual design was centered around low to moderate aspect ratio solenoidal designs. Toroids, poloids and force-reduced magnets are some alternate magnet structures that can be used for SMES but have not received much attention. These structures have some advantages over solenoidal designs such as lower peak fields with greater energy storage capability (poloid), lower support structure requirement (force-reduced magnet) and zero external field (toroid). For some applications of SMES such as pulsed power for fusion reactors or particle accelerators, where the large external field of the solenoid may be unacceptable but the fast response and high efficiency of SMES are required, alternate magnet structures or geometries must be used even if the costs are somewhat higher. Therefore it is useful to study the relative costs of each magnet structure in order to choose a suitable magnet structure for a given application. Four magnet structures viz. the solenoid, toroid, poloid and a force-reduced magnet are evaluated for their energy storage capability, support structure requirements and stray field characteristics. The variation of these parameters with the geometry of the magnet as well as the size of the SMES system are also studied. The objective of this study is to provide a quantitative comparison of important magnet parameters as well as to develop a simple procedure for the preliminary magnetic design of SMES magnets of any size, based on the detailed analysis of a reference design. Due to the high costs of SMES particularly for smaller sizes, it is important to optimize magnet design as well as to look for new magnet configurations to make SMES more cost effective. Superconductor and support structure material are major components of the overall cost of SMES magnets. Various methods of optimizing these parameters are explored. Force-reduced magnets have attracted some controversy in SMES research due to various claims made for and against them. The virial theorem and its implications for force-reduced magnets are analyzed with reference to a specific force-reduced configuration and it is shown that the claims that force-reduced magnets do not offer any savings in structure are unjustified. Methods for further reducing the structure requirements in force-reduced magnets and toroidal magnets are discussed. Due to the unique and highly desirable characteristic of zero stray field of toroidal magnets, it is important to seek ways of increasing its energy storage capability. A variational problem is formulated to determine the optimal cross-sectional shape which maximizes the stored energy with a given quantity of superconductor. The optimal shape stores 16 p.c. more energy than the circular cross section toroid and is shown to be identical to the structurally superior constant tension D shape. The desired characteristics of an ideal SMES magnet are shown to be a uniform magnetic field within a closed magnet configuration. A twisted toroidal magnet combining the features of an ideal solenoid and the general toroidal configuration is studied as a candidate for the ideal magnet. Geometric arguments are used to prove that no such closed surface can be found in three dimensions with the minimum smoothness conditions required from physical considerations. / Ph. D.

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Elektronen- und Positronenstrahlen mit niedrigsten Emittanzen und hohen Strömen werden in zukünftigen Linearbeschleunigern, wie zum Beispiel dem Compact Linear Collider (CLIC), benötigt, um die geforderte Leuchtkraft für physikalische Experimente bereit zu stellen. Diese Strahlen können in Dämpfungsringen, ausgestattet mit starken, supraleitenden Dämpfungswigglermagneten, erzeugt werden. In dieser Arbeit sind Designkonzepte verschiedener supraleitender Dämpfungswigglermagnete entwickelt worden. Testspulen sowie Modelle sind gebaut und getestet, elektrische Verbindungstechniken entwickelt worden. Eine Wärmelastrechnung für den Betrieb in Dämpfungsringen und ein Designkonzept für den kryogenen Betrieb bei 4.2 K ist erstellt worden. Es konnte theoretisch und experimentell gezeigt werden, dass supraleitende Dämpfungswigglermagnete mit Nb-Ti und Nb3Sn Niedertemperatursupraleitern die magnetischen, mechanischen, elektrischen und thermischen Anforderungen erfüllen und in Dämpfungsringen betrieben werden können.

Les tokamaks visent à produire de l'énergie par fusion thermonucléaire en chauffant un plasma d'hydrogène jusqu'à 150 millions K et en le confinant à l’aide d’un champ magnétique intense créé par des aimants transportant d’importants courants. La supraconductivité est un atout précieux ici car permettant de réduire la taille des aimants et leur consommation énergétique en contrepartie d’un refroidissement cryogénique. Cependant, dans les tokamaks, des variations de champ magnétique apparaissent (ex : décharge du solénoïde central) et génèrent des pertes par induction dans les aimants. Si leur température augmente trop, ils peuvent perdre leur état supraconducteur lors d’une transition brutale appelée "quench": afin de les protéger, ils sont déchargés de leur courant entraînant ainsi la perte du plasma. Nous avons concentré notre travail sur la modélisation de ces pertes car leur connaissance est cruciale pour le bon dimensionnement du refroidissement des aimants et la prédiction des limites opérationnelles du tokamak. Afin d'améliorer la compréhension physique de ce phénomène complexe et de proposer des solutions simples mais réalistes, facilement intégrables dans des plateformes multiphysiques déjà fortement sollicitées par la modélisation d'autres effets, nous avons choisi d'adopter une approche analytique. Les câbles présents dans les tokamaks ayant une architecture assez complexe (centaines de brins torsadés ensemble), nous avons mené des études analytiques et expérimentales aux différentes échelles du câble; nous comparons ensuite les résultats de notre approche à ceux d'autres modèles existants (ex : numériques) et, lorsque cela est possible, à l'expérience. / Tokamaks aim at producing energy by thermonuclear fusion heating a hydrogen plasma up to 150 million K and confining it with an intense magnetic field created by magnets carrying important currents. Superconductivity is a very valuable asset in this field since it allows to reduce the size of the magnets and their energy consumption in exchange for cooling them down to cryogenic temperatures. However, in tokamaks, magnetic field variations occur (e.g. due to the central solenoid discharge) and generate induction losses in the magnets. If their temperature increases too much, they lose their superconducting properties in a brutal transition called "quench": to protect their integrity, they are then discharged and the magnetic confinement of the plasma is lost. We have therefore focused on the modeling of these losses - more precisely on the “coupling losses” - since their knowledge is crucial to safely adapt the cryogenic cooling of the magnets and predict the operating limits of the tokamak. In order to both enhance the physical understanding of this complex phenomenon and provide simple but realistic solutions that can easily be integrated in multiphysics platforms already heavily solicited by the modeling of other effects, we have chosen to adopt an analytical approach on this problem. The cables commonly considered for tokamaks presenting a rather complex architecture (several hundreds of strands twisted together in specific patterns), we have carried out analytical and experimental studies at the different scales of the cable; we then compare the results of our approach to other existing ones (e.g. numerical models) and, when possible, to the experiment.

Since a superconductor has no resistance below a certain temperature and can therefore save a large amount of energy dissipated, it is a 'green' material by saving energy loss and hence reducing carbon emissions. Recently the massive manufacture of high-temperature superconducting (HTS) materials has enabled superconductivity to become a preferred candidate to help generation and transportation of cleaner energy. One of the most promising applications of superconductors is Superconducting Magnetic Energy Storage (SMES) systems, which are becoming the enabling engine for improving the capacity, efficiency, and reliability of the electric system. SMES systems store energy in the magnetic field created by the flow of direct current in a superconducting coil. SMES systems have many advantages compared to other energy storage systems: high cyclic efficiency, fast response time, deep discharge and recharge ability, and a good balance between power density and energy density. Based on these advantages, SMES systems will play an indispensable role in improving power qualities, integrating renewable energy sources and energizing transportation systems. This thesis describes an intensive study of superconducting pancake coils wound using second-generation(2G) HTS materials and their application in SMES systems. The specific contribution of this thesis includes an innovative design of the SMES system, an easily calculated, but theoretically advanced numerical model to analyse the system, extensive experiments to validate the design and model, and a complete demonstration experiment of the prototype SMES system. This thesis begins with literature review which includes the introduction of the background theory of superconductivity and development of SMES systems. Following the literature review is the theoretical work. A prototype SMES system design, which provides the maximum stored energy for a particular length of conductors, has been investigated. Furthermore, a new numerical model, which can predict all necessary operation parameters, including the critical current and AC losses of the system, is presented. This model has been extended to analyse superconducting coils in different situations as well. To validate the theoretical design and model, several superconducting coils, which are essential parts of the prototype SMES system, together with an experimental measurement set-up have been built. The coils have been energized to test their energy storage capability. The operation parameters including the critical current and AC losses have been measured. The results are consistent with the theoretical predictions. Finally the control system is developed and studied. A power electronics control circuit of the prototype SMES system has been designed and simulated. This control circuit can energize or discharge the SMES system dynamically and robustly. During a voltage sag compensation experiment, this SMES prototype monitored the power system and successfully compensated the voltage sag when required. By investigating the process of building a complete system from the initial design to the final experiment, the concept of a prototype SMES system using newly available 2G HTS tapes was validated. This prototype SMES system is the first step towards the implementation of future indsutrial SMES systems with bigger capacities, and the knowledge obtained through this research provides a comprehensive overview of the design of complete SMES systems.

Actuellement, l’immense majorité des aimants supraconducteurs, notamment d’IRM, sont refroidis par un bain d’hélium liquide à pression atmosphérique. Néanmoins, ce type de refroidissement est onéreux et impose des contraintes sécuritaires importantes pour les grands volumes. Pour ces raisons, le refroidissement des aimants supraconducteurs est souhaitable sans l’hélium liquide. L’utilisation de cryogénérateur permet de refroidir par conduction solide jusqu’à 4 K et ainsi supprimer l’hélium liquide. Néanmoins, les faibles puissances disponibles combiner aux difficultés de mise en œuvre de ce type de refroidissement rendent difficile l’utilisation dans ces conditions du NbTi. En revanche à 10 K, la puissance des cryogénérateurs augmente d’un facteur 10, mais l’utilisation d’un supraconducteur à haute température critique est alors nécessaire. Notre choix s’est porté sur les conducteurs MgB2 R & W qui ont l’avantage d’être relativement économique à mettre en œuvre, mais qui ont, en revanche, le défaut d’être sensible à la déformation. Il est donc nécessaire d’être soigneux lors de leurs bobinages pour ne pas dégrader leurs performances supraconductrices. Dans le cadre de cette thèse, nous avons développé un insert froid refroidis par conduction solide permettant de mesurer le courant critique des conducteurs MgB2 R & W ainsi que des maquettes. Pour ce faire, un nouveau type de contact thermique à base de nitrure d’aluminium a été développé. En complément, nous avons conçu deux maquettes d’aimant MgB2 R & W : un solénoïde et une double galette. Cette dernière a été fabriquée (grâce à une nouvelle méthode de bobinage brevetée) et testée avec succès. / Currently, the majority of superconducting magnets, including MRI, are cooled by a bath of liquid helium at atmospheric pressure. Nevertheless, this type of cooling is expensive and imposes significant security constraints for large volumes. For these reasons, the cooling of superconducting magnets is desirable without liquid helium. Cryocooler provides dry cooling to 4 K without any liquid helium. However, the power available is low and dry cooling is difficult. In these conditions, it is complicate to use NbTi with dry cooling. But if we increase the operating temperature to 10 K, the power of cryocooler increases by a factor of ten. Nevertheless in this case, it is necessary to use of a high critical temperature superconductor. We choose to use MgB2 R & W conductors because it is relatively low cost but it has the handicap to be sensible at mechanical stress. It is therefore necessary to be careful during their winding to not degrade their superconducting performance. As part of this thesis, we have developed a dry test facility to measure the critical current of MgB2 R & W conductors as well as mock-ups. To do this, a new type of thermal contact based on aluminum nitride has been developed. In addition to this development, we designed two MgB2 R & W magnet mock-ups: a solenoid and a double pancake. The double pancake was manufactured (with a new patented winding method) and it has been successfully tested.

The transport critical current density (J(_c)) of a 37 filament Bi-2223/Ag tape has been measured as a function of field and temperature from 4.2 K up to 90 K. Data have been obtained over a large current range from 10 mA up to 100 A and in fields up to 23 T with the tape in 3 orientations with respect to field. These comprehensive data have been used to test the predictions of the flux creep and weak link models used to explain J(_c) in Bi-2223 tapes. The J(_c)(B,T) dependence of optimised Bi-2223 tapes has been calculated using a curved film model. The model assumes perfect grain connectivity and that the local superconducting properties are equivalent to those in the best reported thin films. A comparison between the calculations and measured J(_c)(B,T) dependencies suggest that in high fields at 20 K, J(_c) in presently available industrially processed tapes is only a factor of 8 below the performance of ideal fully optimised tapes. Transport measurements have been made on Bi-2223 single filaments extracted from an alloy sheathed multifilamentary tape in liquid nitrogen at 77 K in fields up to 300 mT with the field aligned parallel and perpendicular to the a-b planes. Further Jc(B,T) data have been taken in a variable temperature insert at temperatures between 60 to 90 K in fields up to 15 T. In a study of the electric field-current density {E-J) characteristics of the c-axis orientated data at 77 K, negative curvature is observed in traces below 280 mT. However, the 280 mT trace exhibits both positive and negative curvature in different current regimes in contrast to the predictions of standard theory. A laboratory scale Bi-2223 superconducting magnet producing a maximum field of 1.29 T at 4.2 K has been designed and fabricated. The magnet comprises 6 resin impregnated double wound pancakes with a 40 mm bore fabricated via the react and wind route. Critical current density measurements have been made as a function of magnetic field, angle and strain at 4.2 K and 77 K on short samples of the constituent tape. The E-J characteristics of all component coils have been measured and a comparison with short sample data shows that minimal additional damage occurred beyond that produced by the bending strain on the tape and the long length variation in J(_c). Sufficient detail is provided for the non-specialist to assess the potential use of brittle superconducting tapes for magnet technology and construct a laboratory scale magnet.

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.