  Typical parameters of electron bunches available now in undulator X-ray FELs
enable generation of induced radiation in crystal-based X-ray VFEL. An
important peculiarity of an undulator X-ray FEL is electron bunch spatial
modulation with the period equal to the wavelength of the produced radiation.
Therefore, XFEL can be considered as a modulator of electron bunch density. As
a result a facility combining an undulator, which produced a modulated beam,
and a crystal, on which the modulated beam is incident, can be considered to be
similar to klystron, which is a well-known source for microwaves emission. Use
of pre-modulated beams enables to generate and observe superradiation and
induced radiation in a crystal-based X-ray VFEL.
