Progress in research on vacuum breakdown of relativistic backward wave oscillator with strong guiding magnetic fields

High-power microwave (HPM) technology has emerged as a critical research frontier owing to its broad application potential in directed energy weapons, high-power radar, and long-distance energy transmission. The relativistic backward wave oscillator (RBWO) based on the Cerenkov mechanism is one of the most promising HPM generators due to its large power capacity, high conversion efficiency, and flexible repetitive operation. The phenomenon of vacuum breakdown remains a primary limiting factor that impedes further advancements in output power and pulse duration of RBWOs. In this paper, the decade-long progress made in the vacuum breakdown of RBWOs with an intensive axially guiding magnetic field has been analyzed and summarized. Initially, we have developed a theoretical framework for current densities that trigger vacuum breakdown and constructed an anodic model to describe the process. In addition, the relevant factors that affect the breakdown threshold of RBWOs, including surface topography, material grain size, and device temperature, are discussed. Finally, several physical methods aimed at mitigating vacuum breakdown are presented to enhance the overall performance of RBWOs. This work provides theoretical guidance for the structural design of RBWOs across various frequency bands, with potential applicability to understanding breakdown mechanisms in other types of high-power microwave devices. These findings are of great significance for exploring effective breakdown suppression strategies and enhancing both the power capacity and operational reliability of such devices.