Theoretical and experimental studies of virtual cathode microwave devices

The results of theoretical and experimental studies of microwave devices with virtual cathodes (VC's) are reviewed. It is shown that the basic factors affecting operation efficiency of these devices are phase separation of electron flux, diode impedance matching with the high-voltage transmission line, and the availability of two characteristic features in the output spectrum. Using nonlinear theory, we have observed the evolution of the electron velocity distribution function in the vicinity of the VC, have found the VC effective temperature, and have studied the fractal indices for various functions in time. This report also describes a series of experiments where 2-D features of the electron dynamics in the vircator are rather important, and 11% generation efficiency with a pinch-vircator was achieved. The understanding of the physical processes occurring in VC microwave devices gained in this work supports the proposal of several new highly efficient concepts for such devices.>