-Results are presented from experimental investigations of oscillatory phenomena in an electron beam with a virtual cathode in a diode gap with a decelerating field. Experiments have revealed a stochastic broadband generation of the microwave oscillations of a virtual cathode in a decelerating field. Numerical simulations based on a simple one-dimensional model have shown that the onset of the stochastic generation and the broadening of the oscillation spectrum with increasing beam deceleration rate are governed by the processes of regrouping of the electrons in a beam with a virtual cathode.
The appearance of the chaotic synchronization regimes has been discovered for the coupled spatially extended beam-plasma Pierce systems. The coupling was introduced only on the right bound of each subsystem. It has been shown that with coupling increase the spatially extended beam-plasma systems show the transition from asynchronous behavior to the phase synchronization and then to the complete synchronization regime. For the consideration of the chaotic synchronization we used the concept of time scale synchronization described in work [Chaos, 14, 3 (2004) 603-610] and based on the introduction of the continuous set of phases of chaotic signal. In case of unidirectional coupling the generalized synchronization regime has been observed in the spatially extended beam-plasma systems. The generalized synchronization appearance mechanism has been analyzed by means of the offered modified system approach [Phys. Rev. E, 71, 6 (2005) 067201].
The effect of mobile ion background on the dynamics of a planar virtual cathode with retarding field is investigated. The ions appear in the diode gap due to ionization of a neutral gas by the electron beam. Detailed examination of the self-consistent processes of the virtual cathode dynamics, gas ionization, and ions motion is performed using the particle-in-cell simulation. It is shown that the presence of ions leads to the virtual cathode neutralization, its displacement, and subsequent anew formation. The microwave generation has the form of a sequence of pulse packets, whose duration depends on the value of the retarding potential and the kind of gas.
621.385We numerically simulate the influence of positive ions on characteristics of the microwave oscillations in a nonrelativistic electron beam with the virtual cathode formed in a decelerating field (low-voltage vircator). A numerical scheme allowing for ionization of a residual gas by an electron flow is proposed. It is shown that the residual-gas ionization in the operating chamber of a low-voltage vircator leads to a forcing of the virtual cathode out of the transit gap and to a cutoff of microwave oscillations. The obtained numerical data are confirmed by an experimental study using a low-voltage vircator model.
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