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9Complex Systems of Charged Particles and their Interactions with Electromagnetic Radiation 2019
FEATERS OF ELECTRON PLASMA WAVES AT ABOVE-THRESHOLD
IONIZATION OF GAS ATOMS
K.Yu. Vagin1, T.V. Mamontova12, S.A. Uryupin1,3
1P.N. Lebedev Physical Institute of RAS, Moscow, Russia, e-mail: vaginasci. lebedev. ru 2National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Moscow, Russia e-mail: mamomtova.tat.v@gmail.com, uryupin@,sci.lebedev.ru
In experimental studies of non-linear gas atom ionization during the absorption of several photons of laser radiation it was found that the energy spectrum of forming photoelectron distribution consists of a set of individual peaks, corresponding to the absorption of a certain number of photons (see, e. g., [1]). The first peak corresponds to multi-photon ionization (MPI) and the higher peaks correspond to above-threshold ionization (ATI). The properties of non-equilibrium plasmas formed in MPI and ATI of gases differ from the properties of plasmas with an electron distribution close to the Maxwellian. Dispersion and dissipative properties of longitudinal waves in plasma formed by MPI of gases, when photoelectron distribution function consists of one peak, were studied in the paper [2].
In this report we considered the case of relatively weak ionizing laser pulse with intensity ~1012W/cm2, when the number of energy peaks in the photoelectron distribution in the ATI regime is not large. It is shown that the number of longitudinal waves in such plasma coincides with the number of peaks in the distribution function. When peaks practically don't overlap, the dispersion law of each wave in the region of short waves is determined by electrons from the corresponding peak. In this case the phase and group velocities of the waves are close to the electron velocity, which corresponds to the peak maximum. It is possible to talk about such waves as an electronic sound, since the perturbations of the electron density mainly arise due to pressure perturbations. When the peaks are narrow, but having a finite width, the Cherenkov damping of waves is exponentially small. Calculations of the dispersion laws for the two and four waves in photoionized xenon plasma, in which the electron distribution function consists of two or four narrow peaks are given.
Fig. 1. The dependence of the dimensionless frequencies of plasma waves on the dimensionless wavenumber in photoionized plasma with four energy peaks in the photoelectron distribution. The dotted, dash-dotted, dashed and solid lines correspond to the peak with number j=0 (MPI) and j=1, 2, 3 (ATI) respectively.
References
[1] G. Petite et al. 1987 J. Opt. Soc. Am. B 4 765.
[2] K.Yu. Vagin et al., 2018 Contributions to Plasma Physics 58 276.
