BBU effect in an ERL-FEL two-purpose test facility

TL;DR

This paper investigates the BBU instability in an ERL-FEL test facility, demonstrating that proper bunch repetition rate control can mitigate orbit fluctuations and confirming the stability of high-charge operation.

Contribution

It introduces a numerical simulation for BBU threshold in ERL-FEL systems and presents a physical model explaining orbit fluctuations, offering strategies for stability control.

Findings

BBU threshold remains high even at large FEL bunch charge

Orbit fluctuations can be controlled by adjusting FEL bunch repetition rate

Numerical simulation effectively predicts BBU behavior in ERL-FEL systems

Abstract

Both the Energy Recovery Linac (ERL) and Free Electron laser (FEL) are considered to be candidates of the fourth generation light source. It is proposed to combine FEL into an ERL facility to integrate the advantages of both ERL and FEL, and to realize a compact two-purpose light source. A test facility to verify this principle is being designed at the Institute of High Energy Physics, Beijing. One main concern is the beam breakup (BBU) instability which limits the available beam current. To this end, we developed a numerical simulation code to calculate the BBU threshold, which is found to have only a small reduction even in a high-FEL-bunch-charge operation mode, compared with that in the case with ERL bunches only. However, even with ERL beam current far below BBU threshold, we observed a fluctuation of the central orbit of the ERL bunches in the presence of FEL beam. We then present a physical model of BBU and understand the mechanism of the orbit-fluctuation in an ERL-FEL two-purpose machine. We found that by choosing an appropriate FEL bunch repetition rate, the central orbit fluctuation amplitude can be well controlled.