The authors present the first bandwidth measurements of a novel gyrotron amplifier. Coupling between the second harmonic cyclotron mode of a gyrating electron beam and the radiation field was achieved in the region of near infinite phase velocity over a broad bandwidth by exploiting a cylindrical waveguide with a helical corrugation on its internal surface. With a beam energy of 185 keV, the amplifier achieved a maximum output power of 1.1 MW, saturated gain of 37 dB, linear gain of 47 dB, saturated bandwidth of 8.4 to 10.4 GHz (21% relative bandwidth) and an efficiency of 29%. These results were in good agreement with theoretical predictions.
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