Extending the photon energy coverage of an x-ray self-seeding FEL via the reverse taper enhanced harmonic generation technique

TL;DR

This paper introduces a reverse taper enhanced harmonic generation technique to extend the photon energy range of soft x-ray self-seeding FELs, achieving continuous coverage from 1.5 keV to 4.5 keV with high power.

Contribution

The paper proposes a novel method combining reverse taper and harmonic generation to extend FEL photon energy coverage beyond current limits.

Findings

Photon energy gap between 1.5 keV and 4.5 keV fully covered.

Achieved 100 GW-level peak power in simulations.

Method maintains electron beam quality during energy extension.

Abstract

In this paper, a simple method is proposed to extend the photon energy range of a soft x-ray self-seeding free-electron laser (FEL). A normal monochromator is first applied to purify the FEL spectrum and provide a coherent seeding signal. This coherent signal then interacts with the electron beam in the following reverse tapered undulator section to generate strong coherent microbunchings while maintain the good quality of the electron beam. After that, the pre-bunched electron beam is sent into the third undulator section which resonates at a target high harmonic of the seed to amplify the coherent radiation at shorter wavelength. Three dimensional simulations have been performed and the results demonstrate that the photon energy gap between 1.5 keV and 4.5 keV of the self-seeding scheme can be fully covered and 100 GW-level peak power can be achieved by using the proposed technique.