Romano, Annalisa (2018)
Electron cloud formation in CERN particle accelerators and its impact on the beam dynamics.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

In high energy accelerators operating with positively charged particles, photoemission and secondary emission can give rise to an exponential electron multiplication within the beam chamber, which leads to the formation of a so-called Electron Cloud (EC). The formation of an EC in a particle accelerator can be responsible for local detrimental phenomena (e.g. heat load on the chamber's wall, pressure rise, noise in beam diagnostics) and for the deterioration of the beam quality due to the electromagnetic forces exerted by the EC on the beam particles. The present thesis work addresses EC effects in the CERN Large Hadron Collider (LHC) and in its injector the Super Proton Synchrotron (SPS) by means of numerical simulations and experimental studies. At the LHC, the formation of ECs in the cryogenic arcs has been identified as one of the main limitations for the performance of the machine, due to the additional heat load deposited on the perforated beam screen. In the framework of the design of the beam screens of the new High Luminosity LHC magnets, the impact of the pumping slot shields (added to preserve the superconducting state of these magnets) on the multipacting process has been addressed by introducing new features in the PyECLOUD simulation code. Besides these local effects, the EC can also significantly influence the beam dynamics in terms of both coherent and incoherent effects. The understanding of these phenomena heavily relies on numerical simulations carried out with the PyECLOUD-PyHEADTAL interface. This new setup offers significant improvements to the modeling of EC induced beam dynamics. Indeed, it allowed simulating for the first time the impact on the beam stability of the EC in the quadrupoles and explaining the driving mechanism of several EC observations at the LHC during the 2015-2017 proton run. Furthermore, potential mitigation techniques have been investigated. EC effects are also found to be a major concern for the SPS, in particular for the production of the high intensity beam foreseen by the LHC Injectors Upgrade project. In this framework, the efficiency of the coating realized with a newly developed technique to suppress the electron multipacting has been investigated with numerical simulations. In this case, the beam chambers have been modeled with a non-uniform Secondary Electron Yield profile in order to reproduce the coating as achievable with the hollow cathode procedure.

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