Electrode shape dependence of output microwave characteristics in reflex triode virtual cathode oscillator

A virtual cathode oscillator (vircator), which can generate a high-power microwave, has been widely studied over the last four decades. This paper describes the dependence of microwaves and electron beams generated in a reflex triode vircator on the electrode shape and material. The reflex triode vircator is driven by a Marx generator with an output voltage of 200kV and a stored energy of 240J. Experiments were conducted with aluminum and velvet cathodes and two types of electrode shapes: (1) circular, with a diameter of 46mm, and (2) elliptical, with an aspect ratio of 2:3. Both shapes have the same emitting area. Experimental results revealed that the electric field strength of the microwave pulse generated with the elliptical area electrode was 1.3 times greater than that with the circular area electrode. In addition, an x-ray pinhole camera with a beryllium window was used to evaluate the cross-sectional shape of the electron beam. It was found from the x-ray images that the shape of the electron beam was nearly the same as that of the electrode and that the cross-sectional area of the elliptical electron beam was larger than that of the circular electrode although both the circular and elliptical electrodes have the same emitting area. These results show that the electrode shape is one of the important parameters for the vircator.