Analysis, Design, and 3-D Simulation of a High-Efficiency Overmoded Nonuniform Relativistic Backward-Wave Oscillator

A high-efficiency X-band overmoded relativistic backward-wave oscillator (RBWO) using a nonuniform slow-wave structure (SWS) and cavity resonant reflector (RR) is designed in order to increase the beam- wave interaction efficiency. The electrodynamic SWS is a sinusoidally corrugated and moderately overmoded (D/λ > 1) structure that decreases the strength of electric field per unit area at its surface and RR. This reduces the probability of occurrence of RF breakdown, hence the pulse shortening. The nonuniform SWS enhances the beam-wave interaction efficiency by providing variable coupling impedance between -1st harmonic of backward fundamental mode and slow space-charge wave associated with the electron beam. The 3-D particle-in-cell simulation of the designed RBWO generated an RF output power of ~700 MW at 9.24 GHz, while driving by a trapezoidal voltage pulse of 600 kV. The axial guiding magnetic field is 2.5 T that restricts the velocity of electron beam in the axial direction only. The output signal has a high mode purity because the higher order modes are cutoff by the structure. The frequency tuning is obtained by varying the distance between RR and SWS.