Waves in plasma formed by above-threshold ionization of gas atoms

Highlights: • 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. -- Abstract: 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.