Thesis supervisor: Prof. Migliorati, Prof. Alesini (Dipartimento di Scienze di base ed applicate per l'Ingegneria)
Thesis Title: Studies of the Longitudinal Coupled-Bunch Instability in the LHC Injector Chain
This PhD work will investigate the longitudinal coupled-bunch instabilities in the LHC Injector Chain. These instabilities cause the most relevant limitations for the high-brightness beams in the LHC injectors, particularly in the CERN Proton Synchrotron. The study will be performed in the framework of the LHC Luminosity Upgrade and in the LHC Injectors Upgrade program. The Proton Synchrotron (PS) is the injector where this instability is more critical because the maximum beam intensity is limited by coupled bunch instabilities appearing after transition crossing. The unwanted longitudinal oscillations, if not cured, would induce large beam losses and bunch-by- bunch intensity and longitudinal emittance variations, not compatible with the requirements of the future LHC-type beams. To reach the High Luminosity LHC beam characteristics a new longitudinal wideband feedback using a Finemet(R) cavity kicker will be installed. The aim of the new study will be to determine the maximum intensity that could be provided to the LHC. Simulations will be performed to predict and understand the machine behavior in the parameter space of the LIU (increased beam intensity) and as a further step, the coupled-bunch instability will be investigated taking into account the internal motion of the bunch to verify if intra-bunch oscillations can lead to a further level of instability not yet observed with the present beam intensity. This analysis will be carried out using the HEADTAIL code, a collective effect code that predicts the evolution of a beam interacting with a localized impedance source or an electron cloud. One of the goals of the thesis is to implement and benchmark a longitudinal feedback system in HEADTAIL. This will be the tool to study in detail the coupled-bunch instability also by taking into account higher order oscillation modes. In parallel with the simulation efforts, a measurements campaign of the coupled-bunch motion using the new feedback system of the PS will be performed to validate the simulations results on the existing beam and substantiate the extrapolations and predictions for the future HL-LHC-type beams.