Efficiency Enhancement of Reflex Triode Virtual Cathode Oscillator Using the Carbon Fiber Cathode

This paper presents the investigation on the reflex triode (RT) virtual cathode oscillator (vircator) using a carbon fiber cathode. Experimental results show that the carbon fiber cathode can obviously improve the electron beam quality and dramatically enhance the beam-to-microwave efficiency of the RT vircator. It was found that the beam-to-microwave efficiency increased from about 4%-6% in the case of the stainless steel cathode to over 10% in the case of the carbon fiber cathode. In order to understand the phenomenon of efficiency enhancement, experimental studies on the plasma formation on the surface of carbon fiber cathode were also done using the facility of the RT vircator. Physically, the electron emission of the carbon fiber cathode is attributed to the plasma formation due to the surface flashover along the whole surface of carbon fiber. This mechanism is different from the conventional explosive emission of metal cathode. Experimental results clearly show that the electrons emitted not only from the top of the carbon fiber but also from the side surface. Particularly, the plasma is more uniform for carbon fiber cathode due to the side surface flashover of the carbon fibers, and plasma expansion velocity is slower than that for conventional metal cathode, which significantly improves the electron beam quality of the carbon fiber cathode