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In the present talk we discuss specific features of plasma channels formed in gaseous media by a high power UV laser or electron beams. These features arise from strong nonequilibrium of an electron energy distribution function (EEDF) and possibility to form the EEDF with energy intervals characterized by population inversion that can be used for amplification of electromagnetic radiation of sub-THz frequency range in a plasma media with certain energy dependence of the electron transport cross section [1,2]. Different methods of producing population inversion in the electronic continuum are discussed.
One of them is based on the process of gas multiphoton ionization by femtosecond laser pulse. In this case the photoelectron energy spectrum consists of a number of peaks corresponding to absorption of a certain number of photons and is obviously characterized by energy intervals with population inversion [3,4]. In this situation the population inversion exist only within time intervals less than relaxation time of the initial energy spectrum. The EEDF with energy intervals characterized by population inversion can also appear to exist in the process of optical breakdown of gases with large low-lying excited states (electronic or vibrational) by laser radiation with quanta exceeding the threshold of excitation [5].
It is also demonstrated that low-temperature weakly-ionized nonequilibrium plasma sustained by a high-energy electron beam in gases (gas mixtures) with Ramsauer minimum in a transport cross section and effective attachment of slow electrons can be used as a medium for amplification and generation of electromagnetic radiation in subterahertz frequency band. Analysis of the EEDF for above mentioned situations is performed. A gain factor depending on an amplified radiation frequency and plasma parameters is analysed.
It is also worth to note that the EEDF with energy intervals with population inversion can also cause unusual refractive properties of a plasma channel. In particular, the real part of permittivity of such a plasma might be greater than unity, i.e. plasma turns out to be optically denser medium in comparison with the surrounding unionized gas [6]. Such a channel seems to be similar to the dielectric waveguide which can be used for both transportation and amplification of microwave radiation.
1. G. Bekefi, Y. L. Hirshfield and S. C. Brown, Phys. Fluids, 4, 173 (1961)
2. F. V. Bunkin, A. E. Kazakov, M. V. Fedorov, Sov. Phys. Usp., 15, 416 (1972)
3. A. V. Bogatskaya and A .M. Popov JETP Lett., 97, 388 (2013)
4. A. V. Bogatskaya, E .A. Volkova and A .M. Popov J.Phys.D, 47, 185202 (2014)
5. A. . Popov et al, High Temperature, 27, issue.1, 173 (1988)
6. A. V. Bogatskaya, I. V. Smetanin, E. A. Volkova, and A. M. Popov. Laser and Particle Beams, 33, 17 (2015)
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