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.