Longitudinal limitations for high-intensity beams in the upgraded Proton Synchrotron Booster

During CERN’s Long Shutdown 2 (LS2), the Proton Synchrotron Booster (PSB) underwent a major upgrade program as part of the LHC Injectors Upgrade (LIU) project. Key modifications include the integration of Linac4 as the new injector, increased injection and extraction energies, and the installation of new wideband Finemet-loaded RF cavities. Along with the new cavities, the digital low-level RF (LLRF) was upgraded with multi-harmonic control loops aimed at compensating beam loading. While these upgrades enhanced operational flexibility and strengthened the PSB’s potential for high-intensity beam production, they also substantially altered the machine impedance and, consequently, the longitudinal stability landscape. This called for a renewed experimental assessment of the PSB longitudinal performance limits. This thesis addresses the need to identify the dominant longitudinal limitations and their influence on the ultimate high-intensity performance of the upgraded PSB. A systematic investigation of longitudinal instability thresholds is carried out for different RF configurations through dedicated beam-based measurements. Additionally, the work combines experimental studies with advanced modelling of the main impedance contributions and RF feedback systems, with particular emphasis on the role of the Finemet cavities and beam-loading compensation mechanisms have been performed. The measured results are used to validate and refine the models and to provide quantitative benchmarks for longitudinal simulations, ultimately improving the predictive capability of beam dynamics tools and the understanding of operational stability margins. Finally, the thesis presents the outcomes of high-intensity Machine Development (MD) campaigns, performed to demonstrate the machine’s potential and to establish a solid experimental basis supporting future PSB upgrades. Overall, this work provides a consistent and experimentally anchored description of the post-LS2 longitudinal limitations of the PSB, contributing to the definition of operational strategies and modelling approaches for high-intensity performance.