Pulse Lengthening of the Microwave Generated by TM₀₂ Mode Ka-Band RBWO Operating With Low Guiding Magnetic Field

The TM02 mode has advantage in power capacity over the fundamental TM01 mode in relativistic backward-wave oscillator (RBWO), whereas in our formal experiment, the microwave pulse seemed not as wide as we expected. In this article, the power capacity of the TM02 mode is investigated theoretically and the mechanisms corresponding to the pulse shortening are analyzed in detail. It is found that the pulse shortening in the RBWO results from the breakdown on the window, the bombardment of the stray electrons traveling in the low guiding magnetic field, and the breakdown induced by the intensive electric field on the surface of the slow wave structure (SWS). The approach of eliminating the pulse shortening is put forward based on the theoretical research and the numerical optimization using our artificial intelligence (AI) code and is validated in the experiment. With larger aperture antenna adopted, post resonator simplified, and emitting electric field greatly decreased, the pulsewidth is lengthened from ~9 to 18 ns with the output power of 360 MW at guiding magnetic field of 0.9 T.