Impedance modelling and collective effects in the Future Circular e$^+$e$^-$ Collider with 4 IPs

TL;DR

This paper updates the FCC-ee impedance model considering recent design changes, and analyzes the impact of these updates on beam stability and collective effects in a 4 IP configuration.

Contribution

It provides a refined impedance model for FCC-ee including transverse effects and studies the resulting beam instabilities with new parameters.

Findings

Updated impedance model with thicker NEG coating and realistic bellows

Analysis of single bunch instabilities with new parameters

Proposed mitigation for beam-beam head-tail instability

Abstract

The FCC-ee impedance model is being constantly updated closely following the vacuum chamber design and parameters evolution. In particular, at present, a thicker NEG coating of 150 nm (instead of previous 100 nm) has been suggested by the vacuum experts, and a more realistic impedance model of the bellows has been investigated. Moreover, also the transverse impedance has been updated by considering the same sources as for the longitudinal case. Therefore, the FCC-ee impedance database is getting more complete and the impedance model is being refined. In this paper we describe the presently available machine coupling impedance in both longitudinal and transverse planes, and study the impedance-driven single bunch instabilities (with and without beam-beam interaction) for the new FCC-ee parameter set with 4 interaction points (IPs). The results are compared with the previously obtained ones and a further possible mitigation of the beam-beam head-tail instability (X-Z instability) is proposed.