A method and apparatus for producing high power, coherent microwave radiaton in which a beam of high speed (above 0.5 light speed) electrons are propagated through a linear path in a waveguide tube including low dielectric constant material adjacent the path so that the electromagnetic radiation is slowed in the material and the beam coupled to the radiation. The beam is focussed and guided to destabilize the guided electro-magnetic modes and to convert the beam energy into high power, high frequency, coherent microwave radiation.