Positron source yield is a key factor for reaching the luminosity needed in future lepton colliders. Conventional scheme relies on a few GeV e-beam impacting on a high-density solid target to initiate an e.m. shower and collect the positrons after the target. This scheme is limited by the maximum heat load on the target before its structural failure. An innovative approach to overcome such limitations exploits the large photon emission in axial channeling in a crystal radiator, to increase the positron yield and/or decrease the target thickness and therefore the Peak Energy Deposited Density in it*. Together with the conventional scheme, our crystal-based one is under study for the FCC-ee injector design**. We carried out experiments at DESY and CERN PS with high-Z crystals (W and Ir) and tuned e-beam parameters useful for FCC-ee to validate a new simulation model implemented in Geant4***. This model includes the modified photon production in channeling condition and oriented crystals in general. Capable of designing the full FCC-ee source. This new model was employed to simulate the positron source showing reduced energy deposition compared to conventional sources.

In autumn 2023, the FCC Feasibility Study underwent a crucial “mid-term review”. We describe some accelerator performance risks for the proposed future circular electron- positron collider, FCC-ee, identified for, and during, the mid-term review. For the collider rings, these are the collective effects when running on the Z resonance – especially resistive wall, beam-beam, and electron cloud –, the beam lifetime, dynamic aperture, alignment tolerances, and beam-based alignment. For the booster, the primary concern is the vacuum system, with regard to impedance and effects of the residual gas. For the injector, the layout and the linac repetition rate are primary considerations. We discuss the various issues and report the planned mitigations.