[Paper Review] Feedback systems for FCC-ee

A. Drago|arXiv (Cornell University)|Apr 20, 2017

Particle Accelerators and Free-Electron Lasers3 references4 citations

TL;DR

This paper proposes a high-performance bunch-by-bunch feedback system for FCC-ee, addressing the challenges of 100 km ring length, very low fractional tunes, and over 3000 stored bunches. It adapts proven feedback architectures from previous e⁺e⁻ colliders like PEP-II and SuperKEKB, scaling them with advanced real-time processing to stabilize each bunch individually using harmonic motion correction in longitudinal, horizontal, and vertical planes.

ABSTRACT

In this paper, some preliminary considerations on the feedback systems for FCC-ee are developed. Bunch-by-bunch feedback systems have been designed in the last years for other e+/e- colliders like PEP-II, KEKB, DAFNE, SuperB and SuperKEKB. In all these cases, similar approaches have been implemented, even if some design variations have been suitable or necessary for different reasons. Bunch-by-bunch feedback systems are based on the concept that the barycenter of each bunch moves with harmonic motion around the equilibrium point in three planes (L, H, V). The feedback copes with the forcing excitation by producing damping correction for each individual bunch. This is possible managing every single bunch by a dedicated processing channel in real time. For FCC-ee the very high number of stored bunches requires much more power in terms of processing capability for the feedback systems. Ring length (100 Km) and very low fractional tunes must be also considered requiring for a more effective strategy in the feedback system design.

Motivation & Objective

• Address the challenge of stabilizing over 3000 bunches in the 100 km circumference FCC-ee ring with high precision.
• Overcome limitations of existing feedback systems by scaling them for unprecedented bunch count and long ring length.
• Ensure stability under very low fractional tune conditions, which complicate feedback system performance.
• Adapt proven feedback architectures from PEP-II, KEKB, DAFNE, and SuperKEKB to the unique FCC-ee environment.
• Design a system capable of real-time, individual bunch correction in all three planes (L, H, V) using dedicated processing channels.

Proposed method

• Adopt the bunch-by-bunch feedback concept, where each bunch's barycenter is monitored and corrected in real time.
• Implement a dedicated real-time processing channel for each individual bunch to enable independent correction.
• Model feedback dynamics based on harmonic motion of bunch centroids around equilibrium in longitudinal, horizontal, and vertical planes.
• Use feedback loops that apply damping corrections to counteract forcing excitations such as wakefields or noise.
• Incorporate ring-length and fractional tune effects into system design to ensure stability and performance at scale.
• Leverage experience from prior e⁺e⁻ colliders to inform system architecture, while adapting for FCC-ee's extreme parameters.

Experimental results

Research questions

• RQ1 How can bunch-by-bunch feedback systems be scaled to handle over 3000 bunches in a 100 km ring?
• RQ2 What modifications are necessary to maintain feedback performance under very low fractional tune conditions?
• RQ3 How can real-time processing capability be sufficiently enhanced to support individual bunch correction in FCC-ee?
• RQ4 What design adaptations are required to extend proven feedback systems from PEP-II and SuperKEKB to FCC-ee?
• RQ5 How do ring length and beam dynamics affect feedback system stability and correction efficiency?

Key findings

• The feedback system is designed to handle the extreme number of bunches in FCC-ee, requiring significantly higher processing power than previous colliders.
• The system architecture is based on proven principles from PEP-II, KEKB, DAFNE, and SuperKEKB, adapted for FCC-ee’s unique conditions.
• Ring length of 100 km and very low fractional tunes necessitate a more effective feedback strategy than previously used in shorter rings.
• Bunch-by-bunch feedback remains viable for FCC-ee, provided that real-time processing capacity is substantially increased.
• The approach ensures individual bunch correction in all three planes (L, H, V) through dedicated processing channels.
• The system design accounts for harmonic motion of bunch centroids and applies damping corrections to suppress collective instabilities.