The FCCee collider and booster: study of the SRF cryogenic systems and machine architecture

Abstract The first step of the Future Circular Collider (FCC), under study in the framework of an international collaboration led by CERN, features a highest-luminosity high-energy lepton collider (FCC-ee), optimised to study with high precision the Z, W, Higgs and top particles over a period of about 15 years starting from 2047. Beams of electrons and positrons will have to be accelerated over increasing levels of energies (from 45.6 GeV to 182.5 GeV) while reducing current intensities (from 1.28 A to 5 mA) to keep the level of synchrotron radiation (SR) losses at 100 MW (50 MW per beam). The superconducting radiofrequency (SRF) system of the collider will have to cope with this wide range of operating parameters. The SRF system combines elliptical cavities at 400 MHz, niobium-sputtered on copper, operated at 4.5 K, and cavities at 800 MHz, bulk niobium, operated at 2.0 K. The increase in particle energy needs is met by increasing the number of cavities installed, housed in cryomodules (CM), during scheduled machine upgrades. A second accelerator, the booster, in the same tunnel and requiring an SRF system of its own, will also be needed for continuous injection of new particles into the collider to compensate the beam current as it loses particles colliding in the detectors. The collider and booster SRF are separate systems, installed in two different machine points and operated independently. At its highest energy, the FCCee collider and booster will count 66 CM at 400 MHz, and 272 CM at 800 MHz, each housing four cavities, and covering a total SRF length of about 3.3 km, for a total RF electrical power consumption of almost 150 MW and requiring an unprecedented cryogenic cooling capacity in particle accelerators. This paper describes the SRF system integration, illustrating the machine architecture and the choice made on cryogenic schemes, including considerations to cope with a staged installation. Concepts of the CM, their interfaces to the Cryogenic Distribution System (CDS) and to the RF power distribution will also be covered.