Quasi-Strong-Strong Beam-Beam Modeling of Bootstrapping Injection in FCC-ee

TL;DR

This paper introduces a quasi-strong-strong beam-beam simulation method for FCC-ee, enabling efficient modeling of injection stability under asymmetric beam interactions, with implications for operational feasibility.

Contribution

The paper develops a self-consistent QSS simulation scheme that balances accuracy and computational efficiency for modeling FCC-ee injection processes.

Findings

Stable injection paths identified in W and H modes.

Z mode shows saturation below nominal bunch population.

QSS method effectively models beam-beam interactions during injection.

Abstract

The FCC-ee is designed to operate with exceptionally strong beam--beam interactions, making continuous injection a critical and non-trivial aspect of its operation. During the injection process, an unavoidable charge imbalance between the two colliding beams leads to asymmetric beam--beam forces, potentially compromising transverse stability. In this paper, we introduce a quasi-strong-strong (QSS) beam--beam scheme, implemented in the SAD simulation framework. The method preserves a self-consistent beam--beam lens by coupling paired weak--strong simulations, while avoiding the computational cost of full strong--strong tracking. The injection process is modeled as a gradual increase of the stored bunch population, allowing the isolated study of beam--beam--driven optics deformation under charge imbalance. Using the QSS approach, we investigate the feasibility of bootstrapping injection in the Z, W, and H operating modes of FCC-ee. Stable injection paths up to the nominal bunch population are identified in the W and H modes. In contrast, in the explored parameter region, the Z mode exhibits saturation of the stored population below the nominal value.