Enhancing Efficiency of Relativistic Backward Wave Oscillator by Mitigating the Impact of High-Energy Secondary Electrons

In a relativistic backward wave oscillator (RBWO), the backflow of high-energy secondary electrons (HSEs) negatively impacts the beam-wave conversion efficiency (BWCE). To mitigate the influence of backflowing secondary electrons (SEs), an electron collection cavity (ECC) is introduced. A design method for the ECC, which considers the emission characteristics of SEs, is proposed to enhance its capability to capture SEs. Particle-in-cell (PIC) simulations confirm the efficacy of the ECC, leading to a significant increase in output power from 6.41 to 7.13 GW, and an improvement in BWCE from 35.3% to 39.4% in a klystron-like RBWO with an operating frequency of 4.3 GHz. A detailed physical analysis reveals that backflowing SEs do not significantly affect the phase matching between axial electric field and axial electron-beam current. However, the SEs disturb the electric field and harmonic current near the extractor, impacting the beam-wave interaction. In addition the SEs can extract energy from microwaves during their backflow, thereby reducing the BWCE.