Formation and Electromagnetic radiation of ion-focused virtual cathode oscillation

Due to the ability of enhancing the space-charge-limited current, the ion-focus regime of electron beam-plasma system can lead to virtual cathode (VC) oscillations with a much higher frequency than that in vacuum. Based on particle-in-cell simulation including Mont-Carlo collision model, the self-consistent formation and radiation process of the ion-focused VC is studied. It is found that the virtual cathode has a multistage structure caused by the plasma wave modulation, and an ion cavity where almost all the plasma electrons are evacuated is found at the second stage, which is significant for electron trapping. Under the demonstrated parameters, the radiation frequency of VC reaches the sub-terahertz band.