Measurement of undulator section wakefield at the SXFEL test facility

TL;DR

This study measures and analyzes the wakefield effects in the SXFEL undulator section, combining theoretical, simulation, and experimental approaches to assess its impact on free electron laser performance.

Contribution

It provides the first comprehensive measurement and comparison of undulator wakefields at SXFEL, integrating theory, simulation, and experiment to evaluate their influence on FEL output.

Findings

Resistive wall wakefields agree well between theory and simulation.

Experimental measurements confirm simulation results with small differences.

Wakefields significantly reduce FEL performance at 8.8 nm but not at 44 nm.

Abstract

In free electron laser facilities, almost every kind of device will generate wakefield when an electron beam passes through it. Most of the wakefields are undesired and have a negative effect on the electron beam, which means a decrease of FEL performance. As for the SXFEL test facility, the sophisticated layout and the cumulative effect of such a long undulator section lead to an obvious wakefield, which is strong enough that can not be ignored. Based on two deflecting cavities at the entrance and the exit of the undulator section with corresponding profile monitors, we measured the wakefield of the undulator section. In this paper, we give the theoretical and simulation results of resistive wall wakefields which agree well with each other. In addition, the experimental and the simulation results of the overall undulator wakefield are given showing small difference. In order to explore the impact of this wakefield on FEL lasing, we give the simulation results of FEL with and without wakefield for comparison. There is almost no impact on 44 nm FEL in stage-1 of cascaded EEHG-HGHG mode, while the impact on 8.8 nm FEL in stage-2 becomes critical decreasing the pulse energy and peak power by 42% and 27% and broadening the bandwidth.