Wakefield issue and its impact on X-ray photon pulse in the SXFEL test facility

TL;DR

This paper investigates how wakefields in the Shanghai soft X-ray FEL test facility cause beam energy loss, affecting FEL performance, and demonstrates that magnetic tuning can compensate for these effects.

Contribution

It provides detailed simulation analysis of wakefield effects on beam energy in a two-stage FEL, highlighting the impact on FEL output and proposing compensation methods.

Findings

Wakefields cause about 0.8 MeV energy loss in the second stage.

The 1st stage FEL output is unaffected by wakefields.

Magnetic field tuning can restore FEL brightness affected by wakefields.

Abstract

Besides the designed beam acceleration, the energy of electrons changed by the longitudinal wakefields in a real free-electron laser (FEL) facility, which may degrade FEL performances from the theoretical expectation. In this paper, with the help of simulation codes, the wakefields induced beam energy loss in the sophisticated undulator section is calculated for Shanghai soft X-ray FEL, which is a two-stage seeded FEL test facility. While the 1st stage 44 nm FEL output is almost not affected by the wakefields, it is found that a beam energy loss about 0.8 MeV degrades the peak brightness of the 2nd stage 8.8 nm FEL by a factor of 1.6, which however can be compensated by a magnetic field fine tuning of each undulator segment.