The problem of X-ray Free-Electron Laser operating on self-amplified
spontaneous emission in irregular microundulator is considered. The case when
the spectrum width of spontaneous radiation is conditioned by the spatial
distribution of sources creating the undulating field is considered. In this
case gain function of the stimulated radiation is dozens of times higher than
that of the conventional undulators. We propose a model of irregular
microundulator, which can be used to construct a drastically cheap and compact
X-ray free-electron laser operating on medium energy electron bunch.Comment: 6 pages, 5 figures, revtex4, accepted by Armenian Journal of Physic