Improved techniques of impedance calculation and localization in particle accelerators

In this thesis we mainly focus on particle accelerators applied to high energy physics research where a fundamental parameter, the luminosity, is maximized in order to increase the rate of particle collisions useful to particle physicists. One way to increase this parameter is to increase the intensity of the circulating beams which is limited by the onset of collective effects that may drive the beam unstable and eventually provoke beam losses or reduce the beam quality required by the particle physics experiments. One major cause of collective effects is the beam coupling impedance, a quantity that quantifies the effect of the fields scattered by a beam passing through any accelerator device. The development of an impedance budget is required in those machines that are planning substantial upgrades as shown in this thesis for the CERN PS case. The main source of impedance in the CERN LHC are the collimators. Within an impedance reduction perspective, in order to reach the goals of the planned upgrades, it was proposed to reduce the collimator impedance by means of their segmentation in the longitudinal direction. This motivated the study of electromagnetic techniques able to take into account the finite length of the device, such as the Mode Matching technique. This technique enabled us to study the impedance dependence on the device length and assess that no evident impedance reduction can be achieved by means of a collimator segmentation. The developed model allowed also for an accurate study of the impedance resonant-like behaviour below the beam pipe cut-off frequency in beam pipe flanges. These insertions are very common in particle accelerator and the resonant effect could drive harmful instabilities within circulating bunches. The possibility of detecting the high impedance sources by means of beam-based measurements represents another powerful investigation tool. In this thesis we improved the impedance localization technique based on the impedance-induced phase advance beating with intensity. We improved the theoretical background by means of macro particles simulations showing the effect of distributed and localized impedances, we quantified the impact of the noise over signal ratio in the measurement accuracy and we performed impedance localization measurements in the CERN PS, SPS, LHC and the Brookhaven RHIC.