Electron cloud buildup and impedance effects on beam dynamics in the Future Circular e+e− Collider and experimental characterization of thin TiZrV vacuum chamber coatings

The Future Circular Collider FCC-ee is a study toward a high luminosity electron-positron collider with a centre-of-mass energy from 91 GeV to 365 GeV. Due to the beam parameters and pipe dimensions, collectiveeffects and electron cloud can bevery critical aspects for the machine and can represent the main limitations to its performance. An estimation of the electron cloud build up in the main machine components and an impedance model are required to analyze the induced instabilities and to find solutions for their mitigation.Special attentionhas been given tothe resistivewall impedanceassociated with alayer of nonevaporablegetter (NEG) coating on thevacuum chamber required for electron cloud mitigation. The studiespresented inthispaper willshowthatminimizingthethicknessofthis coatinglayerismandatory to increase thesinglebunchinstabilitythresholdsinthe proposed leptoncollider at 45.6GeV.Forthis reason, NEG thin films with thicknesses below 250 nm have been investigated by means of numerical simulations to minimize the resistive wall impedance. In parallel, an extensive measurement campaign was performed at CERN to characterize these thin films, with the purpose of finding the minimum effective thickness satisfying vacuum and electron cloud requirements.