In the context of the FCC IS European study, which investigates the feasibility of a 100 km circular 𝑒 + 𝑒 − collider for the future high-energy physics research, we present the design status of the High Energy Booster (HEB) ring. In order to perform precision measurements of the Z, W and H bosons, as well as of the top quark, unprecedented luminosities are required. To reach this goal and to fill the collider, it is mandatory to continuously top-up inject some beam with emittances comparable to those in the collider, so as to maintain an intensity difference between colliding electron and positron bunches below a few %. The main challenges of the HEB design are achieving the collider equilibrium emittances with a fast cycle, and determining the minimum booster injection energy allowing for stable operation. We present the status of the optics design of the HEB, taking into account the above challenges, and the impact of magnetic field imperfections on the dynamic aperture at injection.

The Future Circular Collider (FCC) study was launched as a worldwide international collaboration hosted by CERN with the ultimate goal of pushing the energy frontier far beyond the LHC. FCC covers two accelerators, namely an energy-frontier hadron collider (FCC-hh) and a highest luminosity, high-energy lepton collider (FCC-ee) serving as electroweak Higgs factory, as a possible first stage. The mass of particles that could be either directly produced at FCC-hh or indirectly detected at FCC-ee is increased by an order of magnitude, relative to today's reach, and the subatomic distances that can be resolved are decreased in the same proportion. Importantly, FCC-hh and FCC-ee share the same ∼100 km tunnel infrastructure. This paper focuses on the FCC-hh, summarising its key features, such as accelerator design, performance reach, and underlying technologies. The discussion is based on the 2019 conceptual design report (CDR) [1], which represents a study milestone, but also describes more recent design activities and indicates future directions.