Virtual cathode oscillator under various cathode radii with intense relativistic electron beam

It has been reported that the virtual cathode is formed when the electron beam current in the drift region exceeds the space charge limiting current. In this study, the axially extracted virtual cathode oscillator (vircator), which is new conceptual device and the ultra-high power (/spl sim/GW) microwave generator, with "chundoong" intense relativistic electron beam pulser (max. 600 kV, 70 kA, 60 ns) has been constructed. The virtual cathode formation has been numerically simulated by 2-1/2 dimensional PIC code "MAGIC", from which the dominant frequency and the extracted power have been evaluated before the experiment. In experiments, the incident current density into drift tube was measured to be about 0.6 kA/cm/sup 2/ under the diode voltage and the A-K gap were kept to be 300 kV and 4 mm respectively. Maximum power of extracted microwave is about 520 MW and its conversion efficiency is about 9% under cathode radius of 2.5 cm. It has been also found that the propagating microwave mode is the combination of TM/sub 01/ and the other higher mode based on the electric field (Ez) profile versus cathode radius. The dominant microwave frequency is shown to be 5/spl square/7 GHz from this experiment.