Subgigawatt Level X-Band BWO Using Adiabatically Tapered SWS at Low Magnetic Field

Improving the efficiency of backward wave oscillators (BWOs) operating in a low magnetic field regime remains a significant research challenge. In this field regime, increased transverse motion of electrons leads to variable interaction impedance. Consequently, the beam wave synchronization with the desired mode is disrupted, resulting in reduced efficiency. To partially overcome this challenge, an adiabatically varying nonuniform slow wave structure (SWS) with a circular ridged wall profile is proposed. The SWS helps in maintaining the synchronization of the beam bunch with the desired normal mode throughout the interaction length. The circular SWS increases the field breakdown limit of the electrodynamic structure. This, in turn, results in an average output power of 810 MW at 9.8 GHz with a power increment of 50% compared to the uniform SWS, guided by a 0.6 T magnetic field. The low magnetic field operation helps in the implementation of a permanent magnet, which results in a repetitive microwave system. The novelty of this work lies in the use of an adiabatically tapered, nonuniform SWS geometry to ensure continuous synchronism at low magnetic fields, which is rarely addressed in conventional BWO designs. This method offers practical significance in reducing system size, weight, and cost by eliminating bulky electromagnets. Further efficiency enhancement is limited by overbunching, which induces Coulomb instability and leads to back-streaming of electrons, as observed in particle-in-cell simulation conducted using CST Microwave Studio.