Abstract
The Future Circular Collider study, which aims at designing post-LHC particle accelerator options, is entering in the final stage, which foresees a conceptual design report containing the basic requirements for a hadron and a lepton collider, as well as options for an electron-proton machine. Due to the high beam intensities of these accelerators, collective effects have to be carefully analyzed. Among them, the finite conductivity of the beam vacuum chamber represents a major source of impedance for the electron-positron collider. By using numerical and analytical tools, a parametric study of longitudinal and transverse instabilities caused by the resistive wall is performed in this paper for the case of the Future Circular Collider lepton machine, by taking into account also the effects of coating, used to fight the electron cloud build up. It will be proved that under certain assumptions the coupling impedance of a two layer system does not depend on the conductivity of the coating and this property represents an important characteristic for the choice of the material itself. The results and findings of this study have an impact on the machine design in several aspects. In particular the quite low threshold of single bunch instabilities with respect to the nominal beam current and the not negligible power losses due to the resistive wall are shown, together with the necessity of a new feedback system to counteract the fast transverse coupled bunch instability. The importance of a round vacuum chamber to avoid the quadrupolar tune shift is also discussed. Finally the crucial importance of the beam pipe material coating and thickness choice for the above results is underlined.
Highlights
The Future Circular Collider (FCC) study [1], which is entering in its final stage, has the goal of exploring post-LHC particle accelerator options in order to produce a conceptual design report containing the basic requirements for a hadron and a lepton collider, as well as for an electronproton machine
As a consequence of this, investigations about the coatings are focusing on the minimum achievable thickness that could be reached with Non Evaporable Getters (NEG) without compromising its properties as means to fight the electron cloud build up and pumping capacity
In order to study the effects of the thickness of these coatings on the single bunch beam dynamics, four different thicknesses will be taken into account: 1 μm, 200 nm, 100 nm and 50 nm
Summary
The Future Circular Collider (FCC) study [1], which is entering in its final stage, has the goal of exploring post-LHC particle accelerator options in order to produce a conceptual design report containing the basic requirements for a hadron and a lepton collider, as well as for an electronproton machine. During the study of the single beam collective effects for the lepton collider FCC-ee [2,3,4,5], it has been observed that. The resistive wall impedance, produced by the finite conductivity of the beam vacuum chamber, represents a very important source of impedance and it is responsible of quite low intensity thresholds, for both the microwave instability in the longitudinal plane and the transverse mode coupling instability (TMCI) in the transverse plane, making this contribution very critical for the machine design.
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