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12Complex Systems of Charged Particles and their Interactions with Electromagnetic Radiation 2016
EXCITATION OF LANGMUIR WAVES BY NON-MONOENERGETIC ELECTRON BEAM I.N. Kartashov, M.V. Kuzelev
Faculty of Physics, M.V.LomonosovMoscow State University, Moscow, Russia, e-mail:
kartashov@ph-elec.phys.msu.ru
The problem of development of beam-plasma instability is considered for spatially homogeneous plasma and electron beam with various electron velocity distributions. Thermal effects in the beam are described by making allowance for gaskinetic pressure in the dielectric permittivity of a cold electron beam and by setting the velocity distribution function in the form of either Maxwellian or semi-Maxwellian distributions. A traditional approach consists in using the cold electron beam approximation that is valid for Sc >> kvTb, where Sc is growth rate of
excited wave with wavenumber k and vTb is electron velocity spread in the beam. However,
investigations in the area of the creation of electromagnetic radiation sources employing the phenomenon of beam-plasma instability showed [1] that obtaining a sufficiently monoenergetic beam is not always a simple technical task. If the condition mentioned above is violated, it is necessary to take into account a difference of the electron velocity distribution from S -shaped function. Since the exact form of the distribution function is unknown in most cases, we have performed a comparative analysis of various distribution functions and dielectric permittivities constructed on the basis of simple physical considerations. In particular, the dielectric permittivity of a cold electron beam has been modified by taking into account the gaskinetic pressure. In addition, we have fully thermalized electron beams with Maxwellian and semi-Maxwellian electron velocity distribution functions. The latter approximation is more adequate for the description of collisionless beam plasma systems.
In all approximations considered, it is possible to distinguish between two regimes of beam instability — the Raman and Compton regimes. The Compton regime of instability is observed for a small spread of electron velocities and is characterized by a wide range of wavenumbers (from zero up to resonance wavenumber) in which the instability is manifested. With increasing spread of velocities, the region of instability is localized near the resonance wavenumber, longwave perturbations are stabilized, and the instability transforms into the Raman regime. As the electron velocity spread increases, the instability growth rates tend to decrease. However, their dependences on parameters of the system can significantly vary for different distribution functions. In a fully thermalized electron beam, the instability growth rate decreases with increasing temperature much more rapidly than that in a beam with semi-Maxwellian distribution.
References
[1] Strelkov P S. et al, Plasma Physics Reports, 2015, 41, 6, 492-500
