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

The most known accelerator at CERN is the Large Hadron Collider (LHC), in which proton beams are accelerated to the highest energy in the world and then put in collision to probe into the heart of matter. However, the LHC top energy is gradually built up in a chain of accelerators of equal importance. Each machine boosts the energy of the particles beam, before injecting it into the next machine in the sequence. Since the quality of delivered beam from the injector chain determines greatly the overall performance of the LHC, studies are carried out to overcome eventual beam parameters limitations in the injectors, in the perspective of increasing the LHC beam brightness. In particular, 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. One major cause of collective effects is the beam coupling impedance, the quantity describing the effect of the fields induced by a beam passing through any accelerator device back on itself. 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. 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.