Emittance growth induced by electron cloud in proton storage rings

In proton and positron storage rings with many closely spaced bunches, a large number of electrons can accumulate in the beam pipe due to various mechanisms (photoemission, residual gas ionization, beam-induced multipacting). The so-formed electron cloud interacts with the positively charged bunches, giving rise to instabilities, emittance growth and losses. This phenomenon has been observed in several existing machines such as the CERN Super Proton Synchrotron (SPS), whose operation has been constrained by the electron-cloud problem, and it is a concern for the Large Hadron Collider (LHC), under construction at CERN. The interaction between the beam and the electron cloud has features which cannot be fully taken into account by the conventional and known theories from accelerators and plasma physics. Computer simulations are indispensable for a proper prediction and understanding of the instability dynamics. The main feature which renders the beam-cloud interactions so peculiar is that the the electron cloud evolves during the passage of a bunch. The electrons are attracted by the protons electric field and move toward the beam axis, where they reach very large density and a distribution which is spiked and highly non-uniform. The electron density evolution during the passage of a bunch is studied by means of numerical and analytical tools and its consequence on the beam stability and emittance growth are investigated. The simulations are mainly done with the code HEADTAIL, originally written at CERN in 2002, which computes the localized interaction of a single bunch with an electron cloud over successive turns, using a particle-in-cell approach. The code has been debugged, benchmarked and extended, as part of the thesis work. New features have been added to the code, for a more realistic modelization of the accelerator elements and of the electron distribution in the vacuum chamber. The upgraded code is used for the study of fast single-bunch instabilities induced by electron cloud. These instabilities, which depend on the cloud density, beam intensity and on several other beam and machine parameters, are threshold effects, have a rise time comparable to the synchrotron period and are similar to the strong head-tail instability arising from conventional impedances. For moderate electron-cloud densities, the simulations revealed another effect, previously unrecognized or confused with noise, namely a slow incoherent emittance growth below the fast instability threshold. Extrapolated to the typical store time in the LHC, the magnitude of this growth may be significant. This thesis has identified in the periodic crossing of resonances, due to the combined effect of the electron-cloud induced tune shift and synchrotron motion, the responsible of emittance growth below the instability threshold, with a mechanism similar to halo formation in space-charge dominated beams.