Dynamics of virtual cathode oscillation analyzed by impedance changes in high-power diodes.

Investigations on the dynamics of virtual cathode oscillation are performed with a reflex triode virtual cathode oscillator powered by a 450 kV, 400 ns voltage pulse. Based on the diode impedance changes, the dynamics of virtual cathode oscillation are discussed. Four ensuing processes, corresponding to different stages of microwave emission, exist in the temporal behavior of diode impedance changes. When the turning point of diode impedance forms, the beam current develops into the parapotential flow. The process of microwave emission can be described by diode impedance changes. To confirm this mechanism, self-pinching of the beam current and current-voltage characteristics are introduced. This mechanism is independent of the cathode material and structure. When the experiments were carried out with a carbon fiber cathode, the 300 MW, 180 ns microwave emission was obtained, the beam current density reached kA/cm² level, and the plasma expansion rate was 1.3 cm/μs. These results show that the carbon fiber cathode is a potential candidate for high-power microwave sources. [ABSTRACT FROM AUTHOR]