Study of the beam-cavity interaction in the CERN PS 10 MHz cavities and investigation of hardware solutions to reduce beam loading

In the Proton Synchrotron (PS), where the LHC protons longitudinal structure (bunch spacing) is determined as the result of a sophisticated series of Radio Frequency (RF) gymnastics, collective effects were identified as a major limitation to the achievable beam current delivered to the LHC. Dedicated machine development studies pointed out the RF cavities to be one of the major source of instability in the PS. In particular, the 10 MHz RF system, responsible for beam acceleration, was identified as the most probable impedance source in the machine. The cavity impedance limits the circulating intensity in the accelerator since the beam-induced voltage could trigger longitudinal instabilities causing beam losses. For this reason the cavity impedance seen by the beam must be kept as low as possible. In the framework of the LHC Injector Upgrade (LIU) project, the present PS 10 MHz RF system requires an upgrade, in order to reach higher beam intensities and to reduce beam loading. This thesis focuses on the improvements of the wide band feedback system that encloses the 10 MHz cavities and their driving amplifier. It describes the upgrade it underwent to reduce the cavity impedance seen by the beam, avoiding a complete redesign of the amplifier-cavity system and keeping the present configuration of the vacuum tubes amplifier driving the cavity. This work describes the studies that were carried out to quantify the contribution of the 10 MHz RF system to the PS longitudinal impedance. It, indeed, summarizes the measurements and simulations that led to a full characterization and evaluation of the effective impedance of the eleven 10 MHz cavity-amplifier systems installed in the PS.