Wakefields of the FCC-ee collimation system

The purpose of this paper is to calculate the longitudinal and transverse wakefields of the FCC collimators by using the electromagnetic codes ECHO3D and IW2D. We cross-checked our results using CST particle studio for long bunches, and found them to be in good agreement. The obtained results show that the collimators give one of the highest contributions to the overall FCC-ee wake potentials. Using the code PyHEADTAIL, we have found that the presence of the geometric wakefield of the collimators leads to the occurrence of transverse mode coupling instability (TMCI) at a significantly lower bunch population as compared to that of all other contributions and solutions to reduce this geometric term must be found

FCC-ee Collective Effects and their mitigation

The high luminosity foreseen in the future electron-positron circular collider (FCC-ee) necessitates very intense multi-bunch colliding beams with very small transverse beam sizes at the collision points. This requires emittances comparable to those of the modern synchrotron light sources. At the same time, the stored beam currents should be close to the best values achieved in the last generation of particle factories. This combination of opposite factors represents a big challenge in order to preserve a high beam quality avoiding, at the same time, a machine performance degradation. As a consequence, a careful study of the collective effects and some solutions for the mitigation of foreseen instabilities is required. In this contribution, we discuss the current status of these studies

Combined Effect of Beam-Beam Interaction and Beam Coupling Impedance in Future Circular Colliders

The future large scale electron-positron colliders, such as FCC-ee in Europe and CEPC in China, will rely on the crab waist collision scheme with a large Piwinski angle. Differently from the past generation colliders both luminosity and beam-beam tune shifts depend on the bunch length in such a collision scheme. In addition, for the future circular colliders with extreme beam parameters in collision several new effects become important such as beamstrahlung, coherent X-Z instability and 3D flip-flop. For all these effects the longitudinal beam dynamics plays an essential role and should be taken into account for the collider luminosity optimization. In this paper we discuss an impact of the longitudinal beam coupling impedance on the collider performance

Transverse and Longitudinal Single Bunch Instabilities in FCC-ee

Improving the accuracy of the impedance model of an accelerator is important for keeping beam instabilities and power loss under control. Here, by means of the PyHEAD- TAIL tracking code, we first review the longitudinal mi- crowave instability threshold for FCC-ee by taking into ac- count the longitudinal impedance model evaluated so far. Moreover, we present the results of beam dynamics simula- tions, including both the longitudinal and transverse wake- fields due to the resistive wall, in order to evaluate the influ- ence of the bunch length on the transverse mode coupling instability. The results of the transverse beam dynamics are also compared with the Vlasov solver DELPHI

Studies and Mitigation of Collective Effects in FCC-ee

In order to achieve a high luminosity in the future electron-positron circular collider (FCC-ee), very intense multi-bunch colliding beams should have nanometer scale transverse beam sizes at the collision points. For this purpose the emittances of the colliding beams are chosen to be very small, comparable to those of the modern synchrotron light sources, while the stored beam currents should be close to the best values achieved in the last generation of particle factories. In order to preserve beam quality and to avoid collider performance degradation, a careful study of the collective effects and techniques for their mitigation is required. The current status of these studies is discussed in the paper

MD3308 – Growth rate versus chromaticity measurement at injection

This report summarises the results of MD3308 on the measurement of the instability rise time as a function of chromaticity in the LHC. The measurement was held in the MD block 3 of 2018, on the afternoon of the 14th of September. With nominal bunches at the injection energy, the horizontal and vertical chromaticities were trimmed in the negative range from ∼-40 to -2 units. The transverse damper (ADT) was switched off to let an instability develop. The turn-by-turn position signal was then recorded with the ADT ObsBox. From these signals, the rise-time was deduced and compared to a prediction derived from the LHC impedance model

MD 4145: Study of Landau damping with an Antidamper

Landau damping is an essential mechanism for ensuring the transverse beam stability in LHC and its upgrades. It suppresses transverse head-tail modes within the bunch that cannot be otherwise damped by a narrow-band feedback or chromaticity. The goal of this study was to test an experimental procedure that would allow quantifying the strength of Landau damping using the ADT feedback as a controllable source of beam impedance. The procedure can potentially provide an accurate way of measuring the stability diagram for LHC and HL-LHC

Beam Instrumentation Challenges for FCC-ee

For the accelerator-based future of high energy physics at the energy frontier, CERN started to investigate a 92 km circumference Future Circular Collider (FCC), as e⁺/e⁻ collider the FCC-ee will operate at beam energies up to 182.5 GeV. Beside the machine operational aspects, beam instrumentation will play a key role in verifying and optimizing the machine to achieve the ambitious beam parameters and quality. This paper gives a brief overview of the various challenges to develop the required beam instruments, with focus on beam position, beam size and bunch length measurements, and well as an outline of the planned R&D; activities

HL-LHC impedance and related effects

This note aims at gathering the work done by CERN and collaborators on the HL-LHC beam coupling impedance in the recent years and identifying the next steps. The note describes the status of the impedance, beam stability and beam induced heating at the moment of writing with the present design and layout (HL-LHCv1.3)

Building the impedance model of a real machine

International audienceA reliable impedance model of a particle accelerator can be built by combining the beam coupling impedances of all the components. This is a necessary step to be able to evaluate the machine performance limitations, identify the main contributors in case an impedance reduction is required, and study the interaction with other mechanisms such as optics nonlinearities, transverse damper, noise, space charge, electron cloud, beam-beam (in a collider). The main phases to create a realistic impedance model, and verify it experimentally, will be reviewed, highlighting the main challenges. Some examples will be presented revealing the levels of precision of machine impedance models that have been achieved