Centre d'études et d'expertise sur les risques, l'environnement, la mobilité et l'aménagement

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A beam optics scheme has been designed for the Future Circular Collider-e + e − (FCC-ee). The main characteristics of the design are: beam energy 45 to 175 GeV, 100 km circumference with two interaction points (IPs) per ring, horizontal crossing angle of 30 mrad at the IP and the crab-waist scheme [1] with local chromaticity correction. The crab-waist scheme is implemented within the local chromaticity correction system without additional sextupoles, by reducing the strength of one of the two sextupoles for vertical chromatic correction at each side of the IP. So-called "tapering" of the magnets is applied, which scales all fields of the magnets according to the local beam energy to compensate for the effect of synchrotron radiation (SR) loss along the ring. An asymmetric layout near the interaction region reduces the critical energy of SR photons on the incoming side of the IP to values below 100 keV, while matching the geometry to the beam line of the FCC proton collider (FCC-hh) [2] as closely as possible. Sufficient transverse/longitudinal dynamic aperture (DA) has been obtained, including major dynamical effects, to assure an adequate beam lifetime in the presence of beamstrahlung and top-up injection. In particular, a momentum acceptance larger than ±2% has been obtained, which is better than the momentum acceptance of typical collider rings by about a factor of 2. The effects of the detector solenoids including their compensation elements are taken into account as well as synchrotron radiation in all magnets.The optics presented in this paper is a step toward a full conceptual design for the collider. A number of issues have been identified for further study.

The proposed Frontier Circular Collider (FCC) project integrates in sequence e + e − and hadron colliders in the same 100 km infrastructure. The FCC provides a most effective and comprehensive exploration of open questions in modern particle physics, by a combination of much increased precision, sensitivity, and centre-of-mass energy. The first stage is a high-luminosity electron-positron storage ring collider (FCC-ee) with centre-ofmass energy ranging from 88 to 365 GeV, to study with high precision the Z, W, Higgs and top particles, with samples of 5 × 10 12 Z bosons, 10 8 W pairs, 10 6 Higgs bosons and 10 6 top quark pairs. A cornerstone of the FCC-ee physics program lays in the precise (ppm) measurements of the W and Z masses and widths, as well as forward-backward asymmetries. To this effect, the centre-of-mass energy and its distribution should be determined with the highest feasible precision. This document describes the capacity offered by FCC-ee, starting with the possibility to obtain transverse polarization of the beams around both the Z pole and the W pair threshold. A running scheme based on a regular (several times per hour) measurement of the beam energy by means of resonant depolarization of pilot bunches, during physics data taking, is proposed. Feasible designs for polarization wigglers, polarimeters and RF depolarizer are outlined, resulting in the ability to monitor the beam energies E ± b of the e ± beams with a relative precision of around 10 −6 . The centre-of-mass energy,, is derived subject to further corrections, related to the beam acceleration and to the energy losses due to synchrotron radiation and beamstrahlung; these effects are identified and evaluated. Dimuon events e + e − → µ + µ − recorded in the detectors, provide with great precision the average beam crossing angle α, the centre-of-mass energy spread, and the difference between e + and e − beam energies. Monitoring methods to minimize both the absolute error and the relative uncertainties of the different energy settings with each other are discussed. The final impact on the physics measurements is given. Elements of a programme of further simulations, design, monitoring and R&D are outlined.

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The CERN Proton Synchrotron (PS) routinely crosses transition energy at around 6 GeV in order to accelerate protons that are injected in the Super Proton Synchrotron (SPS) or transferred to users of fixed target experiments. Depending on the beam parameters and intensity, a fast vertical coherent instability occurs during transition crossing. The instability, characterized by beam losses and a frequency spectrum in the range of 500-900 MHz, represents an important intensity limitation for the neutron time-of-flight (nTOF) beam, and in general could represent a bottleneck for future high intensity beams. In order to better understand the nature and the source of the instability and to find possible mitigations, a dedicated measurement campaign took place. Parallel to the measurements, beam dynamics simulations have been performed to study the observed instability. In particular, single bunch effects have been simulated using the PS transverse beam coupling impedance model developed over recent years. In this paper we present the measurements results along with the obtained instability thresholds. Different beam configurations and stabilizing effects, such as the gamma jump scheme and the octupole-induced tune spread, are also considered. The measurements results are compared with simulations.