Impedance Modeling and Collective Effects in FCC-ee: Impact of Collimators and Optimization Strategies

by Dora Gibellieri (2nd year PhD student)

Operating at 45.6 GeV with high beam current, low emittances, and long damping times, the FCC-ee low-energy machine is particularly sensitive to collective effects and impedance-induced beam instabilities. Controlling these effects requires a continuously refined impedance budget to guide design choices and establish reliable instability thresholds. Recent studies identify the collimation system as a dominant contributor to the total machine impedance, with geometric effects playing a critical role in beam stability. A flexible, modular, and comprehensive impedance model — including beam pipe, collimators, RF cavities, bellows, tapers, and beam position monitors — enables targeted optimization and systematic stability assessments. This work presents the latest FCC-ee impedance budget, emphasizing the impact of collimators’ geometric impedance and ongoing advances in modeling, threshold evaluation, and instability mitigation.