Methods to evaluate the beam coupling impedance of accelerators: a novel technique for bench measurements and beam-based measurements at the Proton Synchrotron Booster

Current research in high-energy particle accelerator physics aims at enhancing beam energy and intensity therefore, the electromagnetic interactions between particle beams and accelerator components become increasingly significant. These interactions can lead to detrimental effects, such as tune shifts, instabilities, emittance growth, beam loss, and overheating of accelerator structures, if not properly managed. An example of the latter effect is presented in the thesis, the failure of the LHC warm vacuum module of 2023 operational run. To understand and predict these effects, the quantification of this electromagnetic interaction, known as beam coupling impedance, is essential. This involves detailed theoretical analysis, precise computer simulations, and experimental mea-surements of the impedance contribution of the accelerator components. Ideally, the impedance is measured using the beam itself, however, practical constraints necessitate alternative methods. This work explores various bench measurement techniques for beamcoupling impedance, critically analyzing existing methods and introducing innovative approaches, including a novel wireless method for longitudinal impedance measurements and an advanced beadpull technique for transverse impedance measurements. In addition, the thesis presents significant advancements in beam-based measurements at the Proton Synchrotron Booster (PSB), focusing on coherent tune-shift measurements across a new energy range enabled by the LHC injectors upgrade. It further discusses impedance-related stability measurements, specifically a newly observed high-energy horizontal instability. Its experimental verification during Machine Development studies and the successful mitigation strategy improved the PSB beam quality and operation. Overall, this work highlights the importance of accurately quantifying the beam-coupling impedance to optimize accelerator performance and stability.