High-power microwave generation in an axially-extracted virtual cathode oscillator has been studied experimentally. The microwave emission of 120 MW at frequencies in the range of 10-12 GHz is observed in the presence of annular electron beam operated at a 300 kV and 20 kA. The electron beam current in the diode region is well characterized by the electron space-charge-limited current in bipolar flow, which is well below the critical current defined for diode pinching. The observed microwave emission is accompanied by the strongly pinched electron beam. Essential feature of such a pinching effect for the microwave generation is confirmed by the absence of emission when the pinch of electron beam is suppressed by the application of the axial magnetic field.
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