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Influence of neutral gas type on generation mode in a relativistic electron beam


A.M. Bashkirev – Post-graduate Student, Volgograd State Technical University

A.G. Shein – Dr.Sc. (Phys.-Math.), Professor, Volgograd State Technical University

One of the important and urgent problems of modern plasma physics and high-power electronics is the study of oscillatory and wave processes in intense beams of charged particles. In particular, such a problem is to study the effect of positive ions on the electron stream dynamics.

In case of the electron beam with a virtual cathode, the ionization processes of a neutral gas by an electron beam are of particular interest. The fact is that the electron velocities in the intense, and even more so in the relativistic beam substantially exceed velocities of the maximum ionization coefficient. However, when a virtual cathode emerges, the electron velocities in the virtual cathode area decrease substantially and can coincide with the maximum ionization coefficient. As a consequence, virtual cathode formation and neutral gas ionization processes are self-consistent, and the ionized gas distribution turns out to be significantly inhomogeneous.

Previously, occurrence of generation intensity oscillations with a stable frequency in a relativistic stream propagating in a gas-filled interaction space was shown. However, a number of questions remained open. In particular, the effect of different gas types on pulse repetition rate and amplitude, as well as conditions for appearance of various generation modes for various gases remained unexplored. In this paper, a series of numerical experiments was performed to identify these dependencies.

On the basis of the numerical simulation of the relativistic electron beam in a gas-filled interaction space, it was established that appearance of the pulsed generation mode in a relativistic electron stream mainly depends on the neutral gas type. This is due to a significant influence of the neutral gas type on the amplitude and time of the transient process. To ensure a pulsed mode with the maximum amplitude and stable frequency with minimum noise, the gas type shall be selected for a given pressure. Pressure selection, in turn, is determined by the required pulse frequency. It was also found that presence of various gases leads to appearance of different pulse frequencies. In addition, it is essential to have the opportunity of generating two-frequency generation mode for some gases.

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