Mode selection in an S-band relativistic backward wave oscillator based on a coaxial waveguide

The competition of oscillations in a relativistic backward wave oscillator (RBWO) based on a coaxial waveguide has been studied by theoretical and experimental methods. It is established that the starting current for asymmetric magnetic TE m1 type waves is lower than that for the TEM wave. Predominant excitation of the former waves is confirmed by numerical simulations using the PIC-code KARAT. In an experimental RBWO prototype, the excitation of asymmetric oscillations was suppressed by longitudinal cuts in the central conductor of the coaxial waveguide. As a result, stable generation of the TEM wave has been achieved at a frequency of 1.23 GHz, a peak power of 260 MW, and a 28% efficiency of electron-beam-power conversion into radiation power for a microwave-pulse duration of 33 ns.