| Summary: | BackgroundX-ray free electron lasers (FEL), especially fully coherent femtosecond free-electron laser pulses, are widely used in numerous fields.PurposeThis study aims to propose a new principle for generating fully coherent femtosecond X-ray pulse based on the Shanghai soft X-ray Free Electron Laser User Facility (SXFEL-UF).MethodsThe principle was based on fresh-slice technique. First of all, the electron beam was kicked transversely to get a time-related transverse tilt. Then, the sub-10-femtosecond bunch was achieved because of the spatiotemporal synchronization effect of the seed laser modulation. The FEL pulse duration was even shorter because of harmonic lasing. In the cascaded high-gain harmonic generation (HGHG) mode, the laser generated by the beam tail modulated the beam head in the second stage to reach higher harmonics, while in the echo-enabled harmonic generation (EEHG) mode, the same part of the electron beam was modulated twice. The influence of emittance and energy chirp of the electron beam on the scheme was analyzed, and the instability caused by transverse position jitter and energy jitter of the chirped beam was evaluated.Results & ConclusionsIn the cascaded HGHG mode of SXFEL-UF, an X-ray pulse with peak power as high as 3 GW and pulse duration (FWHM) as short as 3.3 fs is obtained in the 3D simulation at 45th harmonic, and the relationship between the pulse duration and the transverse deflection of the beam is verified. The result of the 3D simulation for the EEHG mode is similar. The scheme is also explored to generate linearly polarized femtosecond pulse at 6 nm and circularly polarized femtosecond pulse at 3 nm simultaneously by means of the elliptically polarized undulator (EPU) afterburner.
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