Coupling impedance and single beam collective effects for the future circular collider (lepton option)

In the framework of the Future Circular Collider (FCC) study, the high luminosity electron-positron collider FCC-ee will cover a beam energy range from 45.6 GeV to 182.5 GeV, thus allowing very precise measurements of all known heavy particles. The research activity presented in this Ph.D. thesis analyzes some important limitations for the operation of this machine, i.e. electron cloud (EC) and collective effects, particularly critical on the Z resonance due to the low energy and the high beam current. EC build up simulations have been performed for the main components of the machine, revealing the necessity of a NEG coating in the entire ring to lower the Secondary Electron Yield (SEY) of the surface. The presence of this coating affects the resistive wall (RW) impedance seen by the beam, representing the major source of wakefields in the machine due to its large circumference. The work presented in this thesis proves analytically and numerically that for the FCC-ee beam parameters on the Z resonance the contribution of the RW impedance can be reduced by decreasing the thickness of this layer. However, reducing the thickness of NEG coatings can affect the performance of the material itself and therefore the maximum SEY and related EC mitigation. For this reason, this thesis also includes an extensive set of measurements performed at CERN to characterize experimentally Ti-Zr-V thin films with thicknesses below 250 nm in terms of activation performance and SEY. An impedance model was also developed, through the characterization and optimization of the impedance of some important machine components. This model was crucial for a better understanding of single bunch and multi bunch instabilities, thus allowing to identify adequate mitigation techniques for ensuring beam stability during operation. This work also summarizes the impedance studies in the interaction region (IR) of FCC-ee.