  We propose a technique for the production of attosecond X-ray pulses which is
based on the use of X-ray SASE FEL combined with a femtosecond laser system. A
few-cycle optical pulse from a Ti:sapphire laser interacts with the electron
beam in a two-period undulator resonant to 800 nm wavelength and produces
energy modulation within a slice of the electron bunch. Following the energy
modulator the electron beam enters the X-ray undulator and produces SASE
radiation. Due to energy modulation the frequency is correlated to the
longitudinal position within the few-cycle-driven slice of SASE radiation
pulse. The largest frequency offset corresponds to a single-spike pulse in the
time domain which is confined to one half-oscillation period near the central
peak electron energy. The selection of single-spike pulses is achieved by using
a crystal monochromator after the X-ray undulator. Our studies show that the
proposed technique is capable to produce 300 attoseconds long single pulses
with GW-level output power in the 0.1 nm wavelength range, and is applicable to
the European X-Ray Laser Project XFEL and the Linac Coherent Light Source at
SLAC.
