We have investigated an efficiency enhancement of high power microwave generation from cylindrical virtual cathode oscillator by adopting a ring type reflector in this experiment. This high power microwave generator is powered by the "Chundoong" intense relativistic electron beam pulser (Max: 600 kV, 88 kA and 60 ns duration). It is noted in this experiment that the maximum power can be obtained at the cylindrical A-K gap distance of 8 mm with ring-type reflector width 7mm. The microwave frequency has been measured to be 4.8～5.1 GHz by combination of coupled line band pass filters with band gap of 1 GHz and dispersive delay line. It is noted that the output microwave power and its microwave conversion efficiency without reflector are about 244 MW and 2.7 %, respectively, at A-K gap distance 4mm in this experiment. It is especially found in this experiment that the maximum microwave output power and efficiency with optimized ring-type reflector could be enhanced up to 426 MW and 7.1 %, respectively.
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