[en] Undesired background due to beam halo hitting the beam pipe of some machine components could limit the performance and experiments of ATF2 and FLCs. In order to control and reduce the beam halo, beam halo collimation systems are necessary. The design of such a systems is a complex balance between the efficiency needed requiring very small apertures and the wakefields induced which can compromise the beam luminosity and stability. In this context, in the first part of this thesis, we have performed a feasibility design study, construction, installation, commissioning and posterior experiments of a single vertical collimation system for ATF2 with the main objective of reducing the background photons in the Post-IP region. In the second part of the thesis, we have used the ATF2 vertical collimation system to perform a detailed study of the wakefields induced by such a system in the context of wakefield studies for ATF2 and FLCs. The wakefield impact is being studied deeply in ATF2 in order to elucidate the impact on the beam sizes at high intensities. In this sense, it was crucial to optimize the design of the collimation system. Furthermore, in order to define an optimum operation mode of the vertical collimation system in terms of efficiency and wakefields we have performed a systematic benchmarking study of the vertical collimation system wakefield kick in terms of analytical calculations, numerical simulations and measurements. Concerning the beam halo collimation topic, in this thesis, we focus on the linear collimation systems. We studied the efficiency of a betatron and an energy collimation system for ATF2. The studies were made for different beam halo models, locations and optics using the tracking code MADX-PTC in order to investigate the efficiency and required collimation depth for different ATF2 operation modes. From these studies we could conclude that the best candidate system to reduce the background photons generated in the Post-IP is a vertical collimation system with a vertical half aperture of 5 mm. Because of that, the rest of this thesis focused only on a vertical collimation system. In order to make a more realistic study of the vertical collimation system efficiency taking into account the interaction of the beam halo particles with the accelerator components, the tracking code BDSIM has been used. These simulations, shows that 4 mm half aperture is required to avoid any beam halo loss before the IPBSM window for photons in the BDUMP. In these simulations, we observed that even collimating to 3 mm we have some background photons generated in the BDUMP, one possible explanation to this, is the fact that we are using a single collimation system and we will have always some particles scattered by the surface of the jaws, loosing some energy but without being stopped. Furthermore, with BDSIM we modeled the reduction of background photons in the Post-IP as a function of the vertical collimation system aperture for later comparison with measurements. The tracking code BDSIM has been improved and the ATF2 model update including a more realistic magnets geometry and beamline apertures by the RHUL team. In this context, our studies also contribute to the benchmarking campaign been carried out in ATF2 in the last years in order to validate the recent BDSIM improvements. The benchmarking of this kind of tracking codes it is of great importance towards future machines, especially, high energy and intense beam machines and accelerators using photons based diagnostics as the Shintake monitor in ATF2, where undesired losses could limit their performance. The vertical collimation system was constructed and tested in the LAL laboratory and installed in ATF2 in 2016. After some functionality and calibration tests performed in March 2016 an experimental program was carried out by the author of this thesis to measure the efficiency of the vertical collimation system. The efficiency of the vertical collimation system was demonstrated being about 35% for 4 mm vertical collimation system half aperture. The efficiency measurements are in a reasonable agreement with the BDSIM simulations tacking into account that in our simulation with BDSIM we do not consider any orbit error source. We could conclude that simulations taking into account the interaction of the particles with the jaws are essential to study the performance of the collimation system and to defined a more realistic collimation depth. The efficiency of the vertical collimation system has been compared with the effect of the TBP, device that was used efficiently in the past as a kind of collimation system but limited to a maximum collimation depth of 18 σy for a beam passing through the center of this device. In order to achieve the same impact the vertical collimation system has to be closed between 5-6 mm. In the February 2017, in order to perform new wakefield experiments a C-band cavity was placed at the location of the TBP and the vertical collimation system is operated as the main collimation system. The other main topic of this thesis is the detailed study of the wakefields induced by the vertical collimation system. The ATF2 vertical collimation system jaws were design in terms of wakefield minimizations. In the design of such a system, the wakefield impact has been compared with the expected value of the reference cavity and the TBP corresponding to two ATF2 structures with well know wakefield impact. From all these studies we conclude that the best design parameters for the rectangular jaws of the vertical collimation system were: a flat part of 100 mm of Cu and with a tapered angle of 3° In addition, we considered that a retractable collimation system with an aperture ranging between 3 and 12 mm is needed in order to cover different ATF2 operation modes. For these specifications, the vertical collimation system wakefield impact is a factor 2 higher than the TBP and a factor 4 lower than the reference cavity one. These specifications were were given to the engineer at LAL as starting point to perform the 3D vertical collimation system mechanical design. The vertical collimation system induced wakefields on the beam dynamics were also studied using the tracking code PLACET. Some discrepancies were found between the results obtained with PLACET (v1.0.0) and the linear propagation of the numerical wakefield kick calculated with CST PS by one order of magnitude. A detailed study was performed and the source of the discrepancies was found to be on the definition of the limits of the analytical calculation relevant for the ATF2 case. The program has been modified according to the results of our studies in a new version PLACET (v1.0.1) . The analytical and numerical studies were completed by an experimental program carried out during 2016 with the main objective of measuring the vertical collimation system wakefield impact on the orbit. These measurements were done to investigate the optimum operation mode of the vertical collimation system installed in ATF2 in terms of efficiency and acceptable wakefield impact. In addition, we performed a systematic benchmarking study of theoretical models, numerical simulations and measurements. These studies were motivated due to the fact that there are discrepancies in the wakefield kick described in different analytical models for the same regime, in the models implemented in the tracking codes and between simulations and measurements (ESA (SLAC) 2001. In addition, there are different analytical models (inductive, intermediate, diffractive) depending on the geometry of the jaws and beam parameters and when the parameters of the problem sit close to the limits the estimations are not accurate. The study presented on this thesis applies for structures laying in the inductive geometric wakefield regime and long range resistive one}. From the geometric point of view our collimation system is similar to some of the ILC spoilers and absorbers with the current design parameters (as illustrated in Annex 2). Therefore, the results of the studies presented on this thesis contribute to the understanding of the applicability of the tools used to simulate the wakefield impact, crucial for future design of FLCs collimation system. The vertical collimation system wakefield impact was measured on the orbit. In these measurements, the contribution of the good performance of the C-BPMs system with resolution about to 250 nm and the ATF2 beam orbit stability were essential. The results of this benchmarking study show that measurements with the associated errors are in agreement with numerical simulations of the realistic vertical collimation system model. However, the benchmarking accuracy is at the 10 % level. The difference observed within measurements and analytical calculation of the jaws is about 20 %. Notice here, that the analytical models only describe the jaws of the collimation system. The numerical simulations and measurements presented on this work have been performed for a complete structure including the transition to the beam pipe and the beam pipe. The impact of simulating the complete device and only the jaws was studied for different half apertures of the collimation system and the difference observed is about a 15-30%. These numerical calculations with CST PS have been performed for the ATF2 bunch length, the proper scaling to the ILC scenario has to be done in order to perform realistic simulation of the wakefield impact in such a case.

[es] Pérdidas de partículas del haz en zonas no deseadas puede limitar el funcionamiento y los experimentos en los aceleradores. Con el fin de controlar y reducir el halo de partículas y las pérdidas en zonas no deseada a lo largo del acelerador, se necesitan los sistemas de colimación de halo. El diseño de dichos sistemas es un complejo equilibrio entre la eficiencia requerida y los wakefields inducidos que pueden comprometer la luminosidad del haz y la estabilidad. En este contexto, en la primera parte de esta tesis hemos realizado un estudio de diseño de viabilidad, construcción, instalación, puesta en marcha y experimentos de un sistema de colimación vertical para ATF2 con el objetivo principal de reducir los fotones de fondo en el Post-IP. En la segunda parte de la tesis hemos utilizado el sistema de colimación vertical de ATF2 para realizar un estudio detallado de los wakefields inducidos por tal sistema en el contexto de estudios de wakefields para ATF2 y FLCs. Hemos realizado un estudio sistemático de comparación del wakefield kick entre cálculos analíticos, simulaciones numéricas y medidas. En cuanto al tema de colimación de halo, mediante simulaciones de tracking llegamos a la conclusión de que el sistema más eficiente para reducir las pérdidas en el BDUMP es un sistema de colimación betatrónico vertical. Por lo que el resto de la tesis se centro en desarrollar dicho sistema. Se utilizó el código de tracking MADX-PTC y BDSIM para estudiar la eficiencia del sistema de colimación vertical. Si bien a partir de simulaciones con MADX-PTC concluimos que la apertura necesaria era de 5 mm para evitar pérdidas en la entrada del BDUMP, los resultado de las simulaciones más realistas con BDSIM mostraron que necesitamos cerrar el colimador hasta 4 mm . Con BDSIM también calculamos la reducción de los fotones de fondo en el Post-IP en función de la apertura del sistema de colimación vertical para luego compararla con las medidas experimentales. El sistema de colimación vertical fue construido y probado en laboratorio del LAL e instalado en ATF2 en Marzo del 2016. Después de algunas pruebas de funcionalidad y calibración realizadas en marzo de 2016 se llevó a cabo un programa experimental por el autor de esta tesis para medir la eficiencia del sistema de colimación. La eficiencia del sistema de colimación vertical fue demostrada. Además, las medidas son consistentes con las simulaciones de BDSIM. También se comparó la eficiencia del sistema de colimación vertical con la de la TBP. La TBP se utilizaba como una especie de sistema de colimación pero limitado a una profundidad máxima de colimación de 18 σy para un haz que pasa por el centro de este dispositivo. Para lograr el mismo impacto, el sistema de colimación vertical tiene que estar cerrado a una media apertura de entre 5-6 mm correspondiente a 15-18 σy. Una profundidad de colimación simétrica menor que 18 σy sólo es posible con el sistema de colimación vertical. En febrero de 2017, con el fin de realizar nuevos experimentos de wakefields una cavidad radiofrecuencia (RF) se colocó en lugar de la TBP y el sistema de colimación vertical es ahora utilizado como el principal sistema de colimación. El otro tema principal de la tesis consiste en un estudio detallado y sistemático de los wakefields inducidos por el sistema de colimación vertical. La geometría y el material de las piezas centrales del colimador se han optimizado para reducir los wakefields inducidos por el sistema. Se realizó un estudio detallado de como reducir el impacto de los wakefields y los parámetros resultantes son los siguientes: una parte plana central de 100 mm de cobre, con un ángulo de 3 grados y un apertura variable entre 3 y 12 mm. Para estas especificaciones, el impacto de los wakefields del sistema de colimación vertical es un factor 2 mayor que el de la TBP (con un bajo impacto de wakefields) y un factor 4 inferior al de la cavidad de referencia (con un alto impacto de wakefields). Estas especificaciones fueron dadas al ingeniero del LAL como el punto de partida para la realización del diseño mecánico del sistema. El impacto de los wakefields en la dinámica del haz se ha estudiado utilizando el código de seguimiento PLACET. Se encontraron algunas discrepancias entre los resultados obtenidos con PLACET (v1.0.0) y la propagación lineal del wakefield kick calculado numéricamente con CST PS en un orden de magnitud. El motivo de las discrepancias se estudió con detalle y se encontró que la fuente de las discrepancias estaba en la definición de los límites del cálculo analítico relevantes para el caso ATF2. El problema fue resuelto y el programa ha sido modificado en consecuencia en una nueva versión PLACET (v1.0.1) versión utilizada para continuar con los estudios de ATF2. Estos estudios analíticos y numéricos fueron completados con un programa experimental llevado a cabo durante 2016 con el objetivo principal de medir el impacto de los wakefields inducidos por el sistema de colimación vertical en la órbita. Estas medidas se realizaron con el objetivo principal de investigar el modo de operación óptimo del sistema de colimación vertical instalado en ATF2 en términos de eficiencia y wakefield y realizar un estudio sistemático de comparación entre cálculos analíticos, simulaciones numéricas y medidas. Estos estudios son cruciales para el futuro diseño del sistema de colimación del FLC porque existen discrepancias en el wakefield kick descrito en los distintos modelos analalíticos, en los modelos implementados en los códigos de tracking y entre simulaciones y medidas experimentales (ESA (SLAC) 2001-2007). Además, existen diferentes modelos analíticos (inductivos, intermedios, difractivos) dependiendo de la geometría del colimador y del haz y cuando los parámetros del problema están cerca de los límites los cálculos no son precisos. En mis estudios realicé un estudio de comparación sistemático incluyendo simulaciones numéricas con CST PS y medidas. Además, los modelos analíticos solo describen las jaws y en nuestro estudio, las simulaciones numéricas y las medidas se han realizado para la estructura completa. Concluimos que las medidas experimentales están de acuerdo con las simulaciones numéricas con el error experimental asociado. La diferencia entre las medidas y el cálculo analítico que tiene en cuenta solo la parte central del sistema, es de aproximadamente un 20 %. Además, observamos una diferencia entre simular solo la parte central y todo el sistema de colimación vertical de aproximadamente un 15-30 %.