Record-breaking high power coherent radiation at a subterahertz frequency region from a gyrotronutilizing second harmonic resonance modes was attained with a simple cavity. In order to aim at highpower and high frequency simultaneously, the oscillation mode was selected carefully enough to realizestable radiation free from mode competition. The cavity radius was determined from the viewpoints of theoscillation frequency, the coupling coefficient between the electron beam, and the rf-electric field. Thecavity length was also optimized for the highest perpendicular efficiency. In addition, a new electron gunwhich is capable of generating a thin laminar beam for a large current was introduced. Consequently,single mode second harmonic radiation with powers of 52 and 37 kW at frequencies of about 349 and390 GHz, respectively, was achieved.
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