Ion heating, burnout of the high-frequency field, and ion sound generation under the development of a modulation instability of an intense Langmuir wave in a plasma

Abstract

The development of one-dimensional parametric instabilities of intense long-wave plasma waves is considered in terms of the so-called hybrid models, when electrons are treated as a fluid and ions are regarded as particles. The analysis is performed for both cases when the average plasma field energy is lower (Zakharov's hybrid model -ZHM) or greater (Silin's hybrid model -SHM) than the plasma thermal energy. The efficiency of energy transfer to ions and to ion perturbations under the development of the instability is considered for various values of electron-to-ion mass ratios. The energy of low-frequency (LF) oscillations (ion-sound waves) is found to be much lower than the final ion kinetic energy. We also discuss the influence of the changes in the damping rate of the high-frequency (HF) field on the instability development.Reduced absorption of the HF field leads to the retardation of the HF field burnout within plasma density cavities and to the broadening of the HF spectrum. At the same time, the ion velocity distribution tends to the normal distribution in both ZHM and SHM.

On the Mechanisms of Formation of Density Cavities under Instability of Intense Langmuir Oscillations in a Plasma

“…Let us discuss hydrodynamical and hybrid models of Zakharov and Silin (hybrid models: electrons are represented by quasi-hydrodynamic equations, and ions are represented by particles), descriptions of the instability of intense Leggmuir oscillations in plasma. These models make it possible to see the formation of the Maxwellian velocity distribution of ions and evaluate the efficiency of their heating [7,9]. However, below we dwell on the consideration of the dynamics of plasma stratification -on the formation of its density caverns.…”

“…In addition, a dispersion term can be added to (2) and ( 5), which is proportional to , are determined by the contribution to the nonlinearity of the second harmonic, 0 n is the unperturbed plasma density, e T is the electron temperature, and the ions are assumed to be cold at the initial moment. Zakharov's hydrodynamic model (supersonic regime) under conditions ( [2], see also [7,9])…”

“…From the results of the linear theory [2] (see also [7,9]), it follows that in the Zakharov model, the correction to the frequency normalized to the Langmuir frequency can be written as…”

“…As is known [2], in the Zakharov model for a nonisothermal plasma, where the electron thermal energy density exceeded the field energy density, the growth rate of perturbations only increased with decreasing scale. In the Silin model of a cold plasma, where the field energy density was higher than the thermal energy density of the plasma, the increment of such instability was significant for perturbations whose scale was comparable to the amplitude of electron oscillations in the field of intense Langmuir oscillations [6,7]. In these two cases, with a large number of particles describing ions, the perturbation spectrum rapidly increased in the shortwavelength region, forming many small density caverns [8].…”

“…In addition, "tails" appeared -groups of energetic ions, and in cold plasma the energy of such "tails" turned out to be comparable with the energy of ions within the formed normal distribution. It is clear that the role of Landau damping during ion thermalization in hybrid regimes could hardly be dominant; rather, the Fermi scattering mechanism on inhomogeneities manifested itself here [7,9].…”