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The generation of the coherent THz radiation in long undulators at the resonant wavelength is affected by the relatively broad spectrum of radiation that results in poor temporal coherence. The presence of the waveguide can essentially modify the continuous undulator radiation spectrum by converting it to a discrete one when radiation diffraction size exceeds the vacuum-chamber dimensions. In this paper the effect of a small radius circular waveguide on helical undulator radiation in the THz region is examined. It is shown that under certain conditions the presence of the waveguide can lead to radiation energy redistribution between the waveguide modes, enhancing the single resonance mode.

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