Influence of anode-cathode gap distance on output characteristics of high-power microwave from coaxial virtual cathode oscillator

We have investigated the influence of anode-cathode (A-K) gap distance on the output characteristics of high-power microwave from coaxial virtual cathode oscillator (vircator). This high-power microwave generator is powered by the "Chundoong" (Max 600 kV, 88 kA, 60 ns), which is an intense relativistic electron beam-pulse generator. In this experiment, it was observed that the microwave power has been strongly dependent upon the A-K gap distance, where the A-K gap has been varied from 4 to 8 mm. The maximum microwave power is observed to be 244 MW, whose efficiency is about 2.75% at the A-K gap distance 4 mm. Also the microwave frequencies are measured to be 5.1/spl les/f/spl les/5.9 GHz from the combination of a coupled line band pass filter with a 1-GHz bandwidth and dispersive delay line method. It is noted that measured output microwave frequency is in good agreement with that of simulation. It is also found that the dominant mode of output microwave is a TM/sub 01/ in this simulation result.