Scheme for time-resolved experiments based on the generation of femtosecond pulses by a sideband-seeded soft X-Ray SASE FEL
In this paper, we describe the extention of the soft X-ray SASE FEL facility for two-color time-resolved experiments. A new type of pump-probe technique is proposed which is based on the use of soft X-ray SASE FEL combined with an fs quantum laser. An ultrashort laser pulse is used for modulation of the energy and density of the electrons within a slice of the electron bunch at a frequency ωopt. The density modulation exiting the modulator (energy-modulation undulator and dispersion section) is about 10%. Following the modulator the beam enters an X-ray SASE FEL undulator, and is bunched at a frequency ω0. This leads to an amplitude modulation of the beam density at the sidebands ω0± ωopt. The sideband density modulation takes place at the part of the electron pulse defined by the duration of the seed laser pulse that is much shorter than the electron pulse. Following the SASE FEL undulator the beam and SASE radiation enter undulator section (radiator) which is resonant at the frequency ω0- ωopt. Because the beam has a large component of bunching at the sideband, coherent emission is copiously produced within fs slice of the electron bunch. Separation of the sideband frequency from the central frequency by a time-compensated monochromator is used to distinguish the fs pulses from the sub-ps intense SASE pulses. Finally, the fs soft X-ray pulses are combined with optical pulses generated in the seed laser system. In the proposed scheme for pump-probe experiments, fs soft X-ray pulse is naturally synchronized with his fs optical pulse and cancel jitter.