Numerical and experimental studies to model and reduce the impedance in the CERN Super Proton Synchrotron (SPS)

Intensity-driven beam instabilities are studied in simulations with a refined impedance model and in experimental measurements for this thesis. The High Luminosity Large Hadron Collider (HL-LHC) project at CERN aims for an increase in the integrated luminosity in the Large Hadron Collider (LHC). To achieve this goal the LHC injector chain has to be upgraded. These upgrades are summarized in the LHC Injectors Upgrade (LIU) project. The last LHC injector, the Super Proton Synchrotron (SPS), intends to accelerate beams with twice the present intensity. To successfully accelerate beams with such intensities, impedance-driven instabilities have to be studied. In accelerator physics the impedance describes the interaction between the particle beam and the accelerator. Impedances can drive beams unstable and can result in particle losses. To model these effects in the SPS its impedance model is updated and completed. An impedance model merges all design related impedance contributions of the different accelerator components. To obtain a reliable model, the included components have to be modelled realistically. The CST PARTICLE STUDIO® code is used to further improve the model of the extraction and injection kickers impedance in the SPS. The simulations are validated by bench measurements. Also the Electrostatic Septum (ZS) in the SPS is initially characterized in terms of impedance. A scaling technique is used to determine the contribution of the whole ZS chain. Simulations show that the proposed ZS upgrade will significantly reduce its impedance contribution. These simulations are then included in the impedance model of the SPS. The updated models include two new versions, that model the SPS impedance in the year 2018 and after the next Long Shutdown (LS2) in 2021. The models are validated by beam-based measurements in the accelerator and are then used to study observations in the SPS. The horizontal instability occurring in the SPS during high intensity operations for example, is also observed in beam dynamics simulations. This instability can be damped with higher chromaticity values. Beam dynamics simulations are used to support the initial measurements of the intensity threshold in a newly proposed configuration of the SPS, the so-called Q22 optics. The post- LIU operation of the SPS may benefit from this optics. The intensity threshold is measured in detail and is found at "N_{thr} = 2.5\cdot 10^{11}" ppb for a longitudinal beam emittance of "ϵ_{z} = 0.32" eVs. These values are also obtained in beam dynamics simulations. Finally, measurements investigating the stabilizing effects of the octupole magnets on instabilities in the horizontal plane are presented. With all this research this thesis contributes to the impedance modelling of accelerators and to their operation with high intensity beams.