An Analysis of Spatially Localized Clusters of Space Charge in Non-Laminar Electron Beams of Low-Voltage Vircators

One of the actual problems of modern vacuum microwave electronics is the transition to the shorter wavelengths of output radiation. One of the well-known methods of operation frequency increasing is the operation at the higher harmonics. Such an approach is used, for example, in gyrotrons. It is expedient to apply such a method to increase the operation frequency in low-voltage oscillators with non-laminar electron beams (low-voltage vircators). These devices can be used as compact nonrelativistic microwave sources with relatively low output power
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. Earlier
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we have shown some preliminarily first results of higher harmonics generation in low-voltage vircator system with the nonlaminar electron beam. In this work, we consider the results of the detailed theoretical and experimental study of the structure of spatially localized clusters of space charge in low-voltage vircators. Studies have revealed that to generate higher harmonics in the spectrum of the output signal, it is necessary to form electron bunches of a quasi-hollow shape with a ratio of the current density in the center to the current density at the periphery of 0.1-0.2. The mentioned above structure of the clusters can be formed as a result of applying a magnetic periodic focusing system with a non-sinusoidal distribution of induction in the longitudinal direction. Moreover, the size of the region with the maximum current density in electron bunches should be from 10 to 30 percent of the longitudinal size of the bunch itself, and the distance between the peaks of the current density of the bunch of the space charge should be approximately equal to the width of the peak. The results of the harmonic analysis of spatiotemporal bunches in the non-laminar electron beams are also presented. It was shown that in order to increase the number and amplitude of higher harmonics in the spectrum of the output signal of low-voltage vircator, it is necessary to form a space-charge bunch consisting of 10–15 partial quasi-hollow sub-bunches. In this case, the number of harmonics that are observed in the spectrum of the output signal reaches 15–16.