Longitudinal electron waves and instability of collisional plasma formed by multiphoton ionization of gas atoms

Longitudinal collective modes in plasma produced by multiphoton ionization of inert gas atoms are studied. The high-frequency and the low-frequency modes in Xe plasma are considered in detail. It is shown that due to the Ramsauer–Townsend effect, photoelectron collisions with neutral atoms lead to a significant increase in decrement of high-frequency mode in the long-wavelength region where this mode frequency is close to the electron Langmuir one. In the region of wave numbers larger than the ratio of electron Langmuir frequency to photoelectrons average velocity, the dispersion law of high-frequency mode is close to the dispersion law of electron acoustic wave, the phase and the group velocities of which are close to photoelectrons' average velocity. In this case, the acoustic wave is weakly damped even for photoelectrons' collision frequencies comparable to electron Langmuir frequency. There is a low-frequency mode along with the high-frequency one. In the region of relatively small wave numbers, this mode describes aperiodic instability with the growth rate not exceeding half of the electron Langmuir frequency. The instability is caused by the Ramsauer–Townsend effect. As the wave number increases, the low-frequency mode transforms into a longitudinal wave, the amplitude of which increases with time. In the region of wave numbers greater than the ratio of photoelectron average velocity to electron Langmuir frequency, the low-frequency wave is strongly damped due to photoelectron collisions. Influence of the spread of photoelectrons in energy on the low-frequency mode is relatively small. In contrast, a finite width of photoelectron distribution over energy leads to significant collisionless damping of high-frequency wave in the short-wavelength region.