Low Secondary Electron Yield Carbon Coatings for Electron Cloud Mitigation in Modern Particle Accelerators

Doctoral thesis, 2011

In order to upgrade the Large Hadron Collider (LHC) performance to be oriented towards higher energies and higher intensities in the future, a series of improvements of the existing LHC injectors is planned to take place over the next few years. Electron cloud effects are expected to be enhanced and play a central role in limiting the performance of the machines of the CERN complex. Electron cloud phenomena in beam pipes are based on electron multiplication and can be sufficiently suppressed if the Secondary Electron Yield (SEY) of the surface of the beam pipes is lower than unity. The goal of this work is to find and study a thin film coating with reliably low initial SEY, which does not require bake-out or conditioning in situ with photons, is robust against air exposure and can easily be applied in the beam pipes of accelerators. In this work, amorphous carbon (a-C) thin films have been prepared by DC magnetron sputtering for electron cloud mitigation and antimultipactor applications. In the first part of this thesis, the experimental set-ups for the a-C thin film coatings used in this work are described. In the second part, a summary of all the key experimental findings obtained on the a-C thin films in the laboratory, as well as the results of the measurements carried out with LHC type beams in the Super Proton Synchrotron (SPS), i.e. the last injector before the LHC are shown. The influence of different coating parameters, such as power, discharge gas pressure and substrate temperature on SEY has been investigated. The a-C thin films have been characterized by SEY measurements, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Nuclear Reaction Analysis (NRA), X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy. The SEY of the a-C samples after long termexposure to various atmospheres has been studied. Different surface treatments, such as conditioning by electron beam, annealing under vacuum and ion bombardment, on the sample surfaces have been investigated. Apart from the characterizations in laboratory, a-C thin films have also been applied to the liners in the electron cloud monitors and to vacuum chambers of three dipole magnets in the SPS. The electron cloud effect has been studied in the SPS with LHC type beams. In the third and final part, several other coating techniques as well as materials have been investigated for low SEY applications are presented. To gain a deep understanding of the properties of our amorphous carbon coating, the amorphous carbon thin film has been compared with other pure graphite samples. In conclusion, the present study has largely improved the knowledge of the electron cloud understanding with LHC type beams in the SPS. Amorphous carbon coatings are believed to be a potential solution for the electron cloud in the high-energy particle accelerators.