Tapering Options and Emittance Fine Tuning for the FCC-ee Collider

The lepton collider version of the FCC study describes a future electron-positron collider with a circumference in the order of 100 km, optimised for operation with collision energies in the range of 90 GeV to 350 GeV (FCC- ee). This paper presents the layout of the machine and the constraints on the design of the arc lattice in the context of the four different beam energies that are foreseen for beam operation. Special emphasis is put on the compensation of the effect of the strong synchrotron radiation losses. The beam orbit as well as the optics have to be re-optimised for a given operation energy in order to achieve the foreseen emittance of ε = 1 nm in the horizontal and 1 pm in the vertical plane. Counter measures of the so-called saw-tooth effect of the design orbit are needed as well as a compensation of the energy loss on the beam optics. The paper summarizes different scenarios of how to achieve this goal as well as the need for additional emittance fine tuning using wiggler magnets.