First Considerations on Beam Optics and Lattice Design for the Future Electron-Positron Collider FCC-ee

Abstract

Following the recommendations of the European Strategy Group for High Energy Physics, CERN launched the Future Circular Collider Study (FCC) to investigate the feasibility of a new large circular collider for high energy physics research. This paper presents the constraints on the design of the lattice and optics of the lepton collider version of FCC, that has to be optimised for four different beam energies and parameter sets. Special emphasis is put on the need for a highly flexible magnet lattice in order to achieve the required beam emittances for the four energies and on the layout of the interaction region that will have to combine an advanced mini-beta concept, an effective beam separation scheme and a local chromaticity control to optimise the momentum acceptance and dynamic aperture of the ring. INTRODUCTION The lepton collider part of the FCC study is based on racetrack geometry with a circumference of about 100 km and foresees the design of an electron-positron collider, running at four different centre-of-mass energies to allow precision measurements at the Z resonance, at the energy with maximum Higgs production rate, as well as above the WW and t-tbar thresholds [1]. For each energy the beam parameters depend crucially on the synchrotron light emission and the lattice has to be optimised to provide the emittance target values that are summarised together with the general beam parameters in the parameter list of Table 1. The general layout of the machine is shown in Figure 1. Figure 1: Geometry of the FCC storage ring. MAIN PARAMETRES The general parameters [2] have been optimised for each beam energy and are determined by the overall synchrotron radiation load that can be accepted in the design. In general, for all operation energies the same global synchrotron radiation limit of Pγ =50 MW has been assumed, leading e.g. to a considerable higher number of stored bunches at the lowest operation energy and to a large variation of beam emittances over the complete energy range of the machine. Table 1: FCC-ee Parameters at Four Different Beam Energies