FCC circumference studies and resulting choice of RF frequency.

As the detailed study on the project of the Future Circular Collider (FCC) progresses, the choice of the exact circumference on the basis of the proposed one 91.1 km becomes essential. It is defined by the RF-related constraints, mainly the RF frequency of the FCC-hh. This also determines the tunnel length for the FCC-ee collider. In this note, we first show how a solid circumference option with the present LHC RF frequency has been identified and adopted as a baseline scheme by the FCC project management. Furthermore, the entire frequency range of interest has been systematically scanned, starting from keeping the possibility of SPS and LHC as the potential injectors of FCC-hh and the RF synchronization between two neighboring hadron synchrotrons. Two very attractive alternative circumference options with RF frequency at around 500 MHz and 600 MHz are found. The results are analyzed and compared from the point of view of robustness and beam dynamics.