A coherent radiation source which uses an electron beam to drive a resonator which consists of two short sections of metal grating embedded in a parallel-plate waveguide structure has been operated in the millimeter-wavelength regime. The fields in the first of the grating sections imparts velocity modulation to the beam and the second extracts power from a bunched beam. Thus, the device functions like a two-cavity klystron. However, the open quasi-optical coupling structure can be utilized at wavelengths which are shorter than those that are practicable in a conventional closed cavity klystron design. The electron beam energy and current employed in these experiments are modest (10's of kv and 1-2 A) and the primary motivation for the work is to develop convenient moderate power sources for various applications of millimeter- and submillimeter-wavelength radiation.
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