The phenomenon of radio-frequency breakdown is a critical bottleneck that limits the achievement of higher output power and longer pulse width by relativistic backward wave oscillators. The unexpected plasmas generated in pulse duration is associated with radio-frequency breakdown, which include cathode plasmas, collector plasmas and plasmas emerging in electrodynamic structures. In generally, the cathode plasmas and collector plasmas can be effectively suppressed by lengthening the distance between cathode and anode and applying coaxial collector, respectively. By contrast, the plasmas generated in electrodynamic structures is a more intricate problem. The expanding plasmas appearing in electrodynamic structures not only have a direct impact on the microwave generation, including the pulse shortening and operating frequency drift, but also result in irreversible structural damage, which seriously limits the lifetime of the device. In this paper, two main physical mechanisms that trigger the plasma generation in electrodynamic structures, namely anode mechanism and cathode mechanism, are introduced. Firstly, the physical connotation and action process of these two mechanisms are described. Afterwards, the critical emission current corresponding to the plasma production calculated by the two mechanisms is compared. If the external reinforced magnetic field of 4.3T is considered, the current density of the corresponding cathodic mechanism during the pulse duration of 40 ns is J₁=3.60×10⁷ A/cm², while the current density of the anodic mechanism is J₂=3.50×10⁵ A/cm². This calculation implies the anodic mechanism is more likely to trigger plasma generation and breakdown of electrodynamic structures than the cathodic mechanism under an externally strong magnetic field. On the contrary, the transverse size of the electrons is determined by the strength of the external magnetic field. Hence, the critical current density corresponding to the anodic plasmas depends on the external magnetic field, while the critical current density corresponding to the cathodic plasmas is hard to be affected by the l magnetic field strength. Further calculation indicates that the critical current density corresponding to the anodic and cathodic plasmas is at the same level as the magnetic field strength is below 1 T. Due to the sharp increase in current density during the transition from field electron emission to explosive electron emission, the cathodic mechanism may play a more important role when compared with the anodic mechanism.

radio-frequency breakdown,relativistic backward wave oscillator,breakdown plasmas,cathodic mechanism,anodic mechanism