  The experimental device for generation of undulator radiation in terahertz
wavelength region by use of undulator on ferromagnets is created. The device is
based on a beam of a microtrone with the energy 7.5 MeV. The radiation
wavelength is 200 micron. Registered spontaneous radiation has a power of 10
microwatt at a current of a beam 2 mA in a pulse. With the optical resonator,
in a mode, the amplification of 6 percent is received, that in some times is
more than expected value. This effect is explained as a result of partial
coherence of radiation.
