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

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.

Harmonic-enhanced high-gain harmonic generation for a high repetition rate free-electron laser

“…Besides, a few methods can be used to further improve the FEL performance. For example, the first undulator segment of U 2 can be reversely tapered, so that it mainly acts as a dispersive section [44] , which might shorten the FEL saturation length significantly. It is also possible to further increase the FEL pulse energy by tapering the radiation undulator [45,46] .…”

Status and future of the soft X-ray free-electron laser beamline at the SHINE

“…Usually, harmonic lasing is used for SASEdependent harmonic lasing schemes based on two different undulator periods [49][50][51]. In order to expand the coherent X-ray coverage range, EEHG harmonic cascading can be operated at 0.25-0.6 keV, and self-seeding harmonic lasing can cover 1.5-4.5 keV [52]. Consequently, the full coverage of the coherent X-ray FEL can be expected due to the flexible infrastructure.…”

Few-femtosecond X-ray pulse generation and pulse duration control in a seeded free-electron laser

“…The basic working scheme of XFELs is self-amplified spontaneous emission (SASE) [22 -24], which has a relatively low temporal coherence and high pulse-energy fluctuations since the initial signal is derived from a shot noise of the electron beam. Throughout the last decade, the self-seeding method [25][26][27][28][29] has been proven highly effective to significantly increase the temporal coherence of the SASE XFEL pulses. Using a monochromator and a bypass chicane implemented between two undulator sections, the self-seeding method can obtain monochromatic light from the SASE FEL pulse developed in the up-front undulator section and amplify it to saturation in the subsequent undulator section.…”