Effects of electron beam focusing on virtual cathode formation in virtual cathode oscillator

As a high power microwave (HPM) source, virtual cathode oscillator has been studied due to its simplicity. A lot of experiments on virtual cathode oscillator has been conducted and reported to study the characteristics and to increase its efficiency1, 2. In order to increase its efficiency, ballistic focusing of an electron diode has been studied3. And it is reported that the efficiency of virtual cathode oscillator is somewhat increased through the ballistic focusing.