Power and efficiency enhancement of reflex-triode virtual-cathode oscillator operating at short pulse

A reflex-triode virtual-cathode oscillator operating at short-pulse duration is numerically investigated. On the time scale of a few nanoseconds, it turns out to be possible to decrease the gap between the cylindrical surface of the cathode and the anode tube wall before vacuum breakdown occurs. Simulations are performed at fixed cathode radius of 3.4 cm for two anode models, namely, thin-foil and anode-mesh models. Both models lead to a significant enhancement (/spl ap/2-5 times) of the total output microwave power upon reducing the anode-cathode radial gap from 2.2 to 1.0 cm for the input voltage range of 300-700 kV. For the same reduction in the radial gap, a considerable enhancement in the electron-beam-to-microwave-power conversion efficiency is obtained-approximately 1.5 times at lower input voltage (300 and 400 kV), using the thin-foil model and about 1.5-2 times at higher voltages (500 and 600 kV), when the anode-mesh model is considered. The results from the present numerical investigation can be applied in the design of high-growth rate high-power microwave (HPM) sources such as short-pulse small-size axial vircators, operating at single-pulse or pulsed-repetition mode. These vircators have a potential to be more efficient and to generate considerably higher output microwave power in comparison with long-pulse vircators.