New proposal of a fs X-ray facility, which is described in this paper, is based on the use of X-ray SASE FEL combined with a 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 omega(opt). 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 This leads to an amplitude modulation of the beam density at the sidebands omega(0) +/- omega(opt). 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 omega(0) - omega(opt). 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 monochromator is used to distinguish the fs pulses from the sub-ps intense SASE pulses. (C) 2002 Elsevier Science B.V. All riahts reserved. References: 7
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