Influences of the Modulation Cavity and Extraction Cavity on Microwave Generation and Starting Oscillation in a Klystron-Like Relativistic Backward Wave Oscillator

This paper presents a comprehensive understanding of the influences of the modulation cavity and extraction cavity on microwave generation and starting oscillation in a klystron-like relativistic backward wave oscillator. Near the region of the modulation cavity, the axial electric field is small, which is favorable to the reduction of the energy spread of the modulated beam electrons. In addition, the presence of the modulation cavity increases the fundamental harmonic current in the extraction cavity, where the beam-wave interaction is predominant due to the larger axial electric field. The introduction of the extraction cavity leads to a large increase in the amplitude and a sudden change in the phase of the axial electric field, which is beneficial for decelerating the bunched electron beam that has fallen into the acceleration phase. Furthermore, the peak electric field interval in the second sectional slow wave structure (SWS) and extraction cavity is decrescent, making them similar to the role of nonuniform SWS. The extraction cavity enhances the fundamental harmonic current significantly. The modulation cavity acts like a localized wave absorber during the process of starting oscillation and enhances the start current, and the extraction cavity decreases the start current due to the increased coupling impedance.