Coherent photon beam based diagnostics for a seeded extreme ultraviolet free-electron laser

TL;DR

This paper proposes a photon beam based diagnostic method for seeded free-electron lasers, using coherent harmonic radiation to optimize beam alignment, verify undulator settings, and improve FEL performance, demonstrated through preliminary experiments.

Contribution

It introduces a novel photon beam based diagnostic technique for seeded FELs utilizing coherent harmonic radiation, independent of electron beam diagnostics.

Findings

Coherent harmonic radiation distribution helps optimize electron beam trajectory.

The method verifies undulator magnetic gap settings.

It assists in phase matching between undulator segments.

Abstract

Independently from electron beam based procedures, photon beam based diagnostics is an alternative way for alignment and commissioning of the numerous undulator cells in a high-gain short-wavelength free-electron laser (FEL). In this paper, using the seed laser modulated electron beam and the undulator fine tuning technique, a coherent photon beam based diagnostic was proposed for seeded FEL, and some preliminary experimental results at Shanghai deep ultraviolet FEL test facility were presented. It demonstrates that the spatial distribution of the coherent harmonic radiation from one individual or two consecutive undulator segments can be used to optimize the electron beam trajectory, to verify the undulator magnetic gap, and to adjust the phase match between two undulator segments.