Development of interaction point feedback systems for CERN’s Future Circular Lepton Collider

Thesis

CERN’s Future Circular Electron–Positron Collider (FCC-ee) is a proposed high-luminosity circular lepton collider, aimed at delivering unprecedented precision in measurements of the Z, W, Higgs and top quark sectors. Achieving the required luminosity performance demands nanometre-scale control of the relative beam position at the interaction points (IPs). Dynamic perturbations arising from ground motion, magnet vibrations and beam–beam interactions introduce orbit fluctuations that reduce effective luminosity and require active stabilisation. Fast beam-based feedback systems operating at the IP are therefore essential to preserve luminosity and ensure robust operation.

This thesis presents the development of analytic and simulation frameworks for the design and optimisation of IP feedback systems in circular lepton colliders, with particular emphasis on FCC-ee. An analytic model of multi-IP feedback in circular machines is derived, explicitly incorporating the coupling between beam–beam deflections, lattice transport and feedback correction. The formulation enables identification of the physical mechanisms governing stability and gain limits, and provides a computationally efficient tool for parameter optimisation in machines with one, two or four IPs.

Large-scale beam–beam tracking simulations and dedicated machine development studies are used to investigate the practical implementation of IP feedback at SuperKEKB, an operating luminosity frontier lepton collider. At SuperKEKB, the iBump beam–beam deflection feedback system is analysed using operational data to characterise its stability, correction behaviour and operational tuning. Separately, the iBump system is modelled in Xsuite with a realistic lattice description, and its response to driven disturbances is studied as a function of error frequency. Together, these studies provide insight into the behaviour and limitations of IP feedback in a high-luminosity collider environment.

For FCC-ee, performance requirements and tolerances that must be met by the IP feedback are established. A feedback architecture based on integral control is developed and evaluated. Full lattice tracking simulations incorporating beam–beam interaction and feedback correction are performed to assess luminosity recovery under both single-frequency perturbations and realistic models of final-focus quadrupole vibrations. The results demonstrate that appropriately optimised IP feedback can significantly mitigate dynamic orbit perturbations and preserve luminosity under realistic disturbance conditions.

Authors: Salvesen, JPT

• DOI: 10.5287/ora-kkqa6b0xr
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: luminosity stabilisation; FCC-ee; circular lepton colliders; control-system optimisation; Interaction point feedback; beam dynamics; particle tracking simulations; orbit control; beam–beam effects; accelerator physics; high-luminosity colliders; ground-motion mitigation; fast feedback systems
• Subjects: Colliders (Nuclear physics); Electron accelerators; Feedback control systems