Abstract
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, collective effects and electron cloud can be very 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 attention has been given to the resistive wall impedance associated with a layer of nonevaporable getter (NEG) coating on the vacuum chamber required for electron cloud mitigation. The studies presented in this paper will show that minimizing the thickness of this coating layer is mandatory to increase the single bunch instability thresholds in the proposed lepton collider at 45.6 GeV. For this 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.
Highlights
Among the studies aiming to design post-Large Hadron Collider (LHC) particle accelerators at CERN, the high luminosity electronpositron Future Circular Collider FCC-ee [1] is considered as a potential first step toward the 100 TeV hadron collider FCC-hh, in the same 97.75 km tunnel
This paper presents the main limitations for the operation of the lepton collider FCC-ee
Electron cloud build up simulations have been performed for the main elements of the machine, in both the arcs and the interaction region
Summary
Among the studies aiming to design post-LHC particle accelerators at CERN, the high luminosity electronpositron Future Circular Collider FCC-ee [1] is considered as a potential first step toward the 100 TeV hadron collider FCC-hh, in the same 97.75 km tunnel. An impedance model is needed to study the single bunch instabilities, to predict their effects on beam dynamics and to identify possible mitigation techniques. In the proposed lepton collider on the Z resonance, minimizing the thickness of this coating layer is mandatory to increase the single bunch instability thresholds in both transverse and longitudinal planes. 1.7 1.39 400 100 0.036 1.48 3.5=12.1 0.038=0.132 269.138 269.22 0.025 0.27 1.0 affect the performance of the material itself and the maximum SEY and related electron cloud mitigation For this reason, an extensive measurement campaign was performed at CERN to characterize TiZrV thin films with thicknesses below 250 nm. The BS effect results in a higher energy spread which lengthens the bunches, increasing the single bunch instability thresholds, as will be discussed in the last section
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