Design of the vacuum system of the FCC-ee electron-positron collider

The Future Circular Collider (FCC) Design Study includes a high-luminosity, low-emittance, two-ring storage ring (FCC-ee) where electrons and positrons are stored and made to collide inside two detectors. The vacuum system of FCC-ee must be designed in order to deal with a lower-energy (45.6 GeV), high-current (1390 mA) Z-pole machine and at a final stage with a higher-energy (175-182.5 GeV) low-current (6.4-5.4 mA). Two intermediate energies are also envisioned. The lower-energy machine turns out to be the most challenging one from the point of view of vacuum, since the photon-stimulated desorption (PSD) generated by the copious synchrotron radiation (SR) fans is quite large. Optimization of the pressure profiles has been carried out by means of extensive coupled monte-carlo simulations and optimization, for SR and molecular flow. For the higher energy versions of the machine, for which the SR spectra are characterized by critical energies well above the Compton edge, the localized absorbers facilitate also shielding the tunnel and any radiation-sensitive machine components from scattered gamma-ray photon damage, by installing short high-Z shielding material around the absorbers.