Microwave vacuum electron devices (MVED) are capable of handling more power in a smaller interaction volume than the solid-state devices. An over-mode high power terahertz backward wave oscillator (BWO) is proposed. The slow wave structure (SWS) is constructed as follow: a series of equidistant annular slots are cut in the inner wall of a cylindrical waveguide. The dispersion relationship of the SWS is theoretically analyzed. The parameters of this structure are optimized by using 2.5D UNIPIC code. The influences of structure parameters and working parameters on the performance of the device are investigated, such as the period number of the SWS, the beam voltage, and the strength of external guiding magnetic field. The numerical results indicate that the frequency of the device is not sensitive to the beam voltage. This is the typical characteristic when the device works at π-mode. Under the condition of the electron beam with the voltage of 200 kV, the current of 1100 A, the inner radius of 2 mm, the outer radius of 2.5 mm, and magnetic field of 5 Tesla, the TM_(01) mode wave begins to oscillate at 3.5 ns, whose average power is about 30 MW, frequency is 0.147 THz. And an efficiency of 14% is also obtained with a fine spectrum characteristic. The numerical results demonstrate that the over-mode electrodynamic structure can be used to decrease internal electric field strength while avoiding multimode generation and maintaining good spectral purity.
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