High-frequency collective modes in plasma with strongly anisotropic velocity distribution of photoelectrons formed by multiphoton or above-threshold ionization of gas atoms are studied. In the case ...of multiphoton ionization, along with the usual electromagnetic wave, there are two additional modes. In the region of large wavelengths, the higher-frequency mode is similar to the electron Langmuir wave. Its group velocity is mainly determined by the average photoelectron velocity, and Cherenkov damping is due to small velocity dispersion of photoelectron distribution. In the region of short waves, but with a small Cherenkov damping, the group and phase velocities of this wave are close to the average electron velocity. The second mode, which has lower frequency, in the region of wavelengths smaller than the ratio of average electron velocity to the plasma frequency, corresponds to quasi-potential wave. Its dispersion law is close to the linear one. In contrast, in the region of large wavelengths, this mode corresponds to aperiodic instability, the maximum growth rate of which is comparable to the plasma frequency. The distribution of photoelectrons formed during above-threshold ionization is characterized by the large number of energy peaks, which is accompanied by increasing in the number of collective modes. In particular, in plasma with photoelectron distribution which has two energy peaks, in addition to the electromagnetic mode four extra modes are possible. In the shortwave region, all four modes correspond to the waves that are damped due to Cherenkov interaction with photoelectrons. Two of these modes in the region of relatively long wavelengths are unstable. One of these unstable modes corresponds to quasi-potential wave whose amplitude aperiodically increases with time. The reason for the instabilities is the presence of counter streams of photoelectrons.
Collective electromagnetic modes in weakly ionized plasma formed by multiphoton ionization of inert gas atoms, in which the Ramsauer–Townsend effect takes place, are studied. It is shown that at a ...relatively low energy of photoelectrons of the order of 1 eV, typical for multiphoton ionization, amplification of electromagnetic waves is possible. Amplification is possible both in the case of rare collisions of photoelectrons with neutral atoms and for collision frequencies higher than electron plasma frequency. At photoelectron energies somewhat higher than 1 eV, aperiodic instability can develop with growth rate whose value is comparable to electron plasma frequency. Detailed analytical and numerical analysis of the effect of collisions of photoelectrons with neutral atoms on the dispersion law of electromagnetic wave and the growth rates of instabilities is presented.
The absorption of radiation by inhomogeneous plasma formed during multiphoton ionization of inert gas atoms has been studied. We considered conditions under which the incident radiation frequency is ...close to the plasma frequency in the depth of the plasma, where the photoelectron density is constant. Under these conditions, due to the expansion of the field penetration region into the plasma, the absorption coefficient increases significantly. The dependence of the absorption coefficient on the effective frequency of photoelectrons collisions with neutral atoms and on the thickness of the layer in which the photoelectron density grows linearly was found.
We investigate the interaction between a test monochromatic wave and semibounded plasma formed by multiphoton ionization of gas atoms. Under conditions where photoelectron distribution is isotropic ...and has a narrow peak in energy, the field in the plasma is represented by two contributions. The first of them arises from a pole in the complex plane of wave numbers and decays exponentially deeper into plasma. The second contribution comes from banks of the cut in the same plane and leads to a power-law decrease of the field under conditions of the anomalous skin effect. We obtain analytical expressions for the field under conditions of high-frequency and anomalous skin effects and find the surface impedance and absorption coefficient.
The dispersion law and collisionless damping rate of quasi-potential waves in the plasma formed upon tunnel ionization of gas atoms in the field of a short pulse of circularly or linearly polarized ...radiation are found. It is shown how the frequency and damping rate of quasi-potential waves depend on the wave propagation direction relative to the symmetry axis of the photoelectron distribution. It is established that, in plasma with a toroidal photoelectron velocity distribution, weakly damped waves with a linear dispersion law and frequency above the electron plasma frequency can propagate in a wide range of angles. In the case of a bi-Maxwellian photoelectron distribution, the frequency of weakly damped waves is comparable with the electron plasma frequency and the anisotropy of electron motion manifests itself in relatively small corrections to the dispersion law.
•Longitudinal waves in plasma formed by above-threshold ionization were studied.•Dispersion law of short wave responds to electronic sound.•Number of waves corresponds to number of photoelectron ...distribution energy peaks.•Frequencies of short wave exceed electronic Langmuir frequency.
In the regime of above-threshold ionization of gas atom in the field of laser radiation, plasma with photoelectron distribution consisting of peaks at discrete energy values is formed. 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. Numerical calculations the dispersion laws for of the two and four waves in photoionized xenon plasma, in which the electron distribution function consists of two or four narrow peaks are given.
New dependencies of frequency and damping decrement of high-frequency longitudinal waves on the wave vector in photoionized plasma formed by tunnel ionization of atoms in the field of circularly ...polarized radiation are found. Weakly damped longitudinal waves with a frequency much higher than the electron Langmuir frequency are predicted.
•The high-frequency longitudinal waves in photoionized plasma is studied.•Dispersion properties of the waves differ from those inherent in equilibrium plasma.•Group velocity and wave damping have strong anisotropy.•Possibility of waves with frequency higher than Langmuir frequency is shown.
A new phenomenon – the amplification of short pulse during its passing through a thin layer of unstable plasma with anisotropic bi-Maxwellian electron velocity distribution is described.
The penetration of a monochromatic electromagnetic wave into the weakly ionised plasma formed by multiphoton ionization of inert gas atoms has been studied. It is shown how the type of dependence of ...the photoelectron collision frequency on their velocity determines transverse permettivity in plasma. Explicit expressions for the absorption coefficient in inert gases at different probe radiation frequencies are obtained. In the high-frequency skin effect mode, the possibility of the absorption coefficient increasing due to the Ramsauer-Townsend effect has been demonstrated with argon as an example.
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