Wakefields and the corresponding frequency-domain phenomenon beam coupling impedance have been well studied for a number of years as a source of beam instabilities within particle accelerators. With the development of the Large Hadron Collider (LHC) and the large beam currents stored in the LHC during fills for physics production, wakefield driven instabilities and strong beam induced heating have become a limiting factors in luminosity production due to both instantaneous luminousity and the available time for collisions.In this thesis is presented an in depth study of the beam coupling impedance of two important (from an impedance and operational point of view) devices in the LHC; the collimation system and the injection kicker magnets (MKIs). These systems have both been sources of concern for the beam impedance of the LHC, the collimators due to their large transverse impedance and the MKIs due to the strong heating observed during the increased of beam current during operation in 2011 and 2012. The source of the heating for the MKIs is studied in depth, found to be power lost by the beam to wakefields in the MKIs. Simulations and measurements are used to characterise the impedance and localise the areas responsible for the high impedance, here the beam screen and ferrite yoke of the magnet; improvements are proposed to better screen the ferrite yoke and verified. A new RF damping system using ferrite for the collimation system is studied and compared to the existing RF damping system, focusing on the heating of the damping system. Highlights include a new method for measuring the quadrupolar and constant transverse impedances of an asymmetric structure using a coaxial wire technique is proposed and verified using computational simulations, and a study of the heat loss in a ferrite damped cavity, focusing on the location of the power loss for cavities being damped to varying degrees.

Electric power systems have been in existence for over a century. Electric power transmission line systems play an important role in carrying electrical power to customers everywhere. The number of transmission lines in power systems is increasing as global demand for power has increased. Parallel transmission lines are widely used in the modern transmission system for higher reliability. The parallel lines method has economic and environmental advantages over single circuit. A fault that occurs on a power transmission line will cause long outage time if the fault location is not located as quickly as possible. The faster the fault location is found, the sooner the system can be restored and outage time can be reduced. The main focus of this research is to develop a new accurate fault location algorithm for parallel transmission lines to identify the fault location for long double-circuit transmission lines, taking into consideration mutual coupling impedance, mutual coupling admittance, and shunt capacitance of the line. In this research, the equivalent PI circuit based on a distributed parameter line model for positive, negative, and zero sequence networks have been constructed for system analysis during the fault. The new method uses only the voltage and current from one end of parallel lines to calculate the fault distance. This research approaches the problem by derivation all equations from positive sequence, negative sequence, and zero sequence network by using KVL and KCL. Then, the fault location is obtained by solving these equations. EMTP has been utilized to generate fault cases under various fault conditions with different fault locations, fault types and fault resistances. Then the algorithm is evaluated using the simulated data. The results have shown that the developed algorithm can achieve highly accurate estimates and is promising for practical applications.

Un des défis pour les futurs projets en physique basé sur les accélérateurs de particules est le besoin de faisceaux à hautes intensités. Les effets collectifs sont cependant une limitation majeure qui peuvent détériorer la qualité du faisceau ou limiter l'intensité maximale à cause des pertes. Le CERN SPS, qui est le dernier injecteur pour le LHC, n'est actuellement pas en mesure de délivrer les faisceaux requis pour les futurs projets à cause des instabilités longitudinales.Les nombreux équipements dans la machine (les cavités RF accélératrices, les aimants d'injection et d'extraction, les brides de vide, etc.) entrainent des variations dans la géométrie et les matériaux de la chambre dans laquelle le faisceau transite. Les interactions électromagnétiques internes au faisceau (charge d'espace) et du faisceau avec son environnement sont représentées par une impédance de couplage qui affectent le mouvement des particules et mènent à des instabilités pour des intensités élevées de faisceau. Par conséquent, les sources d'impédance critiques doivent être identifiées et des solutions évaluées. Pour avoir un modèle d'impédance fiable d'un accélérateur, les contributions de tous les équipements dans l'anneau doivent être évaluées à partir de simulations et de mesures électromagnétiques. Dans cette thèse, le faisceau lui-même est utilisé comme une sonde de l'impédance de la machine en mesurant le déplacement de la fréquence synchrotronique avec l'intensité et la longueur du paquet, ainsi que la modulation de longs paquets injectés avec la tension RF éteinte. Ces mesures sont comparées avec des simulations par macroparticules en utilisant le modèle d'impédance du SPS existant, et les déviations sont étudiées pour identifier les sources d'impédance manquantes pour raffiner le modèle.L'étape suivante consiste à reproduire en simulations les instabilités mesurées pour un paquet unique durant l'accélération. Grâce à l'amélioration du modèle d'impédance, une meilleure compréhension des mécanismes de l'instabilité est rendue possible pour les faisceaux de protons et d'ions. Finalement, le modèle pour les simulations étant digne de confiance, il est utilisé pour estimer les caractéristiques du faisceau après les améliorations prévues du SPS pour le projet High Luminosity-LHC au CERN. / One of the main challenges of future physics projects based on particle accelerators is the need for high intensity beams. However, collective effects are a major limitation which can deteriorate the beam quality or limit the maximum intensity due to losses. The CERN SPS, which is the last injector for the LHC, is currently unable to deliver the beams required for future projects due to longitudinal instabilities.The numerous devices in the machine (accelerating RF cavities, injection and extraction magnets, vacuum flanges, etc.) lead to variations in the geometry and material of the chamber through which the beam is travelling. The electromagnetic interaction within the beam (space charge) and of the beam with its environment are described by a coupling impedance which affects the motion of the particles and leads to instabilities for high beam intensities. Consequently, the critical impedance sources should be identified and solutions assessed. To have a reliable impedance model of an accelerator, the contributions of all the devices in the ring should be evaluated from electromagnetic simulations and measurements.In this thesis, the beam itself is used to probe the machine impedance by measuring the synchrotron frequency shift with intensity and bunch length, as well as the line density modulation of long bunches injected with the RF voltage switched off. These measurements are compared with macroparticle simulations using the existing SPS impedance model, and the deviations are studied to identify missing impedance sources and to refine the model.The next important step is to reproduce in simulations the measured single bunch instabilities during acceleration, in single and double RF system operation. Thanks to the improved impedance model, a better understanding of instability mechanisms is achieved for both proton and ion beams.Finally, as the simulation model was shown to be trustworthy, it is used to estimate the beam characteristics after the foreseen SPS upgrades the High Luminosity-LHC project at CERN.