Future Highest-Energy Circular Colliders
Prof. Dr. Frank Zimmermann (CERN, Genf, Schweiz)

In preparation for the post LHC era, the Future Circular Collider (FCC) study, launched in early 2014, is developing a 100-TeV c.m. proton-proton collider (FCC-hh), based on 16-T Nb3Sn magnets and a new 100-km tunnel infrastructure. The FCC study also includes the design of a high-luminosity e+e-collider in the same tunnel (FCC-ee, formerly TLEP), operating at c.m. energies from 91 GeV up to 350-500 GeV, as a potential intermediate step. In addition, the FCC study considers a highest-energy-lepton option (FHH-he). A parallel design study in China prepares for a similar, but smaller e+e-/pp collider, called CepC/SppC. FCC-hh, FCC-ee, and FCC-he offer a rich and complementary physics programme at the high-energy frontier.

The key design challenges for FCC-ee include efficient high-gradient cw SRF systems, top-up injection scheme, optics design for arcs and final focus, effects of beamstrahlung, beam polarization, energy calibration, as well es power consumption. FCC-hh faces other challenges, such as high-field magnets, machine protection, and the effective handling of large synchrotron radiation power in a superconduction machine. Depending on the LHC results and on decisions taken elsewhere, several possible scenarios and alternative evolutions on the CERN complex can be conceived.