Study of microbunching instability in the linac of a soft X-ray FEL Facility

TL;DR

This paper investigates the microbunching instability in a soft X-ray FEL linac using analytical and numerical methods, highlighting growth rates, simulation limitations, and proposing a noise introduction mechanism.

Contribution

It provides a detailed analysis of microbunching instability development and compares growth rates under various conditions, also addressing simulation limitations and higher-order mode excitations.

Findings

Growth rates vary with conditions

Simulation models have limitations

Higher-order mode excitations observed

Abstract

The development of the microbunching instability is studied for the linac of the proposed Shanghai Soft X-ray Free-Electron-Laser facility (SXFEL) by analytic formulae as well as numerical simulations with the aid of two different codes. The process is investigated in detail and the growth rates (gains) of the instability under various conditions are compared. The results indicate that the limitations from numerical computations in the present simulation model must be taken into account. Moreover, the appearance of higher-order mode excitations in the simulations suggests that further improvement of the current theory is necessary. A mechanism of introducing shot noise into beam profile as the beam passes through a chicane is proposed. The key issues that drive the instability are analyzed.