FCC-ee collimation system design

The Future Circular electron–positron Collider (FCC-ee) is a proposed highest-energy, luminosity-frontier lepton collider designed to enable precision measurements in the electroweak and Higgs sectors. The machine is being designed to operate in four distinct running modes, with beam energies of 45.6 GeV, 80 GeV, 120 GeV, and 182.5 GeV, optimized for the production of different particles (Z, W , H, and tt̄). Owing to beam intensities far exceeding those of any previous lepton collider, required to achieve unprecedented luminosities, the FCC-ee beams will feature stored beam energies up to 17.7 MJ—about two orders of magnitude higher than the current state-of-the-art lepton collider, SuperKEKB, and comparable to the stored beam energy of the heavy-ion beams in the Large Hadron Collider (LHC). The FCC-ee thus represents a new operational regime for lepton colliders, where the combination of high stored beam energy and ultra-low emittances makes the beams highly destructive. This regime is extremely challenging from a beam-collimation standpoint. At the FCC-ee, a beam collimation system is indispensable—not only to reduce beam-induced backgrounds in the experimental detectors, as in previous lepton colliders, but also to protect the machine itself from unavoidable beam losses. A robust and efficient beam collimation system is therefore essential both to minimize detector backgrounds and to ensure machine protection, operational safety, and sustained availability as the FCC-ee progresses toward its integrated luminosity goals. This thesis presents a comprehensive design study of a beam collimation system for the FCC-ee, combining analytical studies and advanced simulations to evaluate performance under a wide range of beam-loss scenarios expected during regular operation as well as abnormal conditions. In this work, dedicated simulation routines have been developed to model beam losses from beam–residual gas scattering and Touschek scattering. The simulation tools and developed routines have been benchmarked against measured data from the SuperKEKB collider, as well as with beam loss data from the LHC, reproducing observed beam-induced background trends or beam loss patterns with excellent accuracy. This validation demonstrates their reliability for predicting collimation performance not only at the FCC-ee but also at other existing or future colliders. The outcome of this thesis is a proposed multi-stage beam collimation system for the FCC-ee, included in the machine’s baseline design. More importantly, this work provides a comprehensive documentation of the principles and methods for developing a robust and efficient collimation system for any future electron–positron collider. The methodologies, tools, and design strategies developed here provide a solid foundation for future optimization of the FCC-ee collimation system and will play a key role in the forthcoming technical design phase of the machine. Finally, complementary studies on a crystal collimation system are presented, demonstrating its promising potential and identifying it as an interesting alternative to a traditional collimation system.