Investigating electron cloud formation in FCC-ee nested magnet designs

The Future Circular Collider is an ambitious international proposal for a next-generation particle accelerator complex, building upon the successes of CERN’s Large Hadron Collider. Specifically, the FCC-ee is a future circular lepton collider. The baseline design for the FCC-ee features four modes of operation, with beam energies ranging from 45.6 GeV to 182.5 GeV. Electron cloud (e-cloud) could be a concern for the FCC-ee due to the high number of bunches foreseen for the Z configuration, which results in small bunch spacing. The bunch spacing is a key parameter for the e-cloud formation process, as very small bunch spacing could lead to the avalanche multiplication and its deleterious effects. Moreover, electron trajectories are strongly influenced by externally applied magnetic fields, which could trap electrons and alter their survival time inside the vacuum chamber. The concept of nested magnets, which involves overlapping dipole fields with quadrupolar and/or sextupolar gradients, is under investigation. This approach aims to increase the dipole filling factor and reduce the synchrotron radiation. In this paper, the nested magnets are studied from the e-cloud point of view.