Analysis of the microbunching instability in a mid-energy electron linac

TL;DR

This paper investigates the microbunching instability in a mid-energy electron linac for FELs, combining analytical and simulation methods, and proposes a new noise mechanism affecting beam quality in soft X-ray FEL facilities.

Contribution

It introduces a novel noise mechanism in the beam current profile during bunch compression and highlights the need for improved theoretical models of the instability.

Findings

Microbunching instability can severely affect beam quality in soft X-ray FEL linacs.

A new noise introduction mechanism during bunch compression is proposed.

Higher-order modes indicate the necessity for advanced theoretical modeling.

Abstract

Microbunching instability usually exists in the linear accelerator (linac) of a free electron laser (FEL) facility. If it is not controlled effectively, the beam quality will be damaged seriously and the machine will not operate properly. In the electron linac of a soft X-Ray FEL device, because the electron energy is not very high, the problem can become even more serious. As a typical example, the microbunching instability in the linac of the proposed Shanghai Soft X-ray Free Electron Laser facility (SXFEL) is investigated in detail by means of both analytical formulae and simulation tools. In the study, a new mechanism of introducing random noise into the beam current profile as the beam passes through a chicane-type bunch compressor is proposed. The higher-order modes that appear in the simulations suggest that further improvement of the current theoretical model of the instability is needed.