Electron Langmuir Waves Research Articles

Grid-based Vlasov simulation of collisionless plasma expansion

Grid-based Vlasov simulations are carried out to re-evaluate the one-dimensional collisionless plasma expansion into vacuum. The grid-based method eliminates the inherent statistical noise in particle-based methods and allows us to extend the solution beyond the self-similar expansion region and resolve small electron timescale wave perturbations. It is shown that the expansion generates both an ion-acoustic rarefaction wave mode and an electron Langmuir wave mode that propagate into the unperturbed plasma upstream. The assumption used in the classical expansion solution that the electrons are an isothermal fluid is accurate within a quasi-neutral, self-similar expansion region but fails in both the upstream and downstream of that region due to electron timescale perturbations.

Collective modes of plasma formed by multiphoton ionization of rarefied gas

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.

Electron–Langmuir wave resonance in three dimensions

In the one-dimensional (1D) treatment of Langmuir wave generation by a particle distribution via the kinetic beam instability, there is a one-to-one resonance between the electron speed v and the phase speed vϕ of the wave. The 1D condition for wave growth is ∂f/∂v>0 with v=vϕ, and f(v) evolves due to quasilinear relaxation toward a plateau distribution ∂f/∂v=0. We show here that none of these results apply in a 3D treatment of the problem. For a wave with wavevector k and phase speed vϕ, there is a many-to-one resonance with all electrons with v>vϕ moving obliquely to k. Although growth requires a region with ∂f/∂v>0 below a peak in f(v), the growth can be driven primarily by particles above the peak where ∂f/∂v<0, and growth can even occur at phase velocities vϕ where ∂f/∂v|v=vϕ<0. Resonance at v≫vϕ favors diffusion of the particle distribution in angle, rather than plateau formation. These properties imply that intuition based on the 1D model can be seriously misleading, with far-reaching implications for modeling phenomena such as type III solar radio bursts.

Landau damping of electrostatic modes in nonthermal plasmas

The dispersion relation (DR) and Landau damping of basic electrostatic modes propagating in an electron-ion plasma modeled by a Cairn's distribution with a spectral index α are kinetically investigated. In the high-frequency range, i.e., ω/k>vte, the DR and Landau damping of the electron Langmuir wave (LW) are obtained. In the low-frequency range where vte>ω/k>vti, the DR of ion acoustic wave (IAW) is investigated. The phase velocity and damping of both the Langmuir and the IAWs are shown to be significantly influenced by the nonthermality parameter α.

Small-scale Langmuir wave instability in preflare chromosphere of solar active region

Necessary conditions have been investigated for the appearance of instability of high-frequency electron Langmuir waves in plasma of solar chromosphere near the foot-point of loop structure. We have considered the earliest stage of a flare process in solar active region.

Complex impedance of nonisotermic plasma capacitor

The capacitance of a planar capacitor filled with nonisothermic collisional plasma was calculated in the hydrodynamic approximation. Resonant properties of the complex impedance of the capacitor were studied. It was shown that resonant absorption in the capacitor is caused by excitation of ion-acoustic oscillations, the excitation of electron Langmuir waves, and the geometric plasma resonance phenomenon in the low-, high-, and intermediate frequency regions, respectively.

Nonlinear excitation of longitudinal photons and plasmons by high-power, short pulse lasers

Modulational excitation of longitudinal photons and electron Langmuir waves, as well as ion sound waves by an incoherent strong and superstrong radiation (high-power short pulse lasers, nonthermal equilibrium cosmic field radiation, etc.), in plasmas, is investigated. A simultaneous generation of longitudinal photons and plasmons is demonstrated. Furthermore, the kinetic instability is considered when low-frequency longitudinal photons are generated alone. The growth rates of these modes are obtained.

Escape of electron Langmuir waves from a magnetized plasma

A study is made of electromagnetic oscillations of a plasma in open field line geometry (open magnetic devices). The oscillations that propagate from the critical surface and are originally of the nature of the electron Langmuir waves are shown to continuously change their nature and to escape from the plasma into vacuum in the form of electromagnetic waves. This phenomenon may give rise to wave energy losses from a thermodynamically nonequilibrium (unstable) plasma, e.g., a plasma penetrated by charged particle beams.

ON THE RENORMALIZED DISPERSION EQUATION OF TURBULENT PLASMA

Starting from the kinetic equation in the K space, which is the Fourier space of velocity V and using the technique of operator calculation and transformation, the re-normalized dispersion equation of turbulent plasma has been deriv ed rather simply. The effect of finite correlation time of the turbulence has been calculated under the approximation of assuming a turbulence with effective correlation time τeff instead of the one with time and space correlation. It shows that the effect of finite τeff will reduce the effect of turbulence. Therefore, the expression obtained in the limiting case of short eorrelation time is the upper bound of the turbulence effect. The effects of turbulence on the ion sounds, electron langmuir waves and drift waves have also been discussed.

Two Langmuir Wave Parametric Decay Instability Driven and Trapped by a Self-Trapped Electromagnetic Pump

The conditions under which an incident, x - z polarized strong pump electro-magnetic wave propagating in the z -direction is self-trapped as an equilibrium filament in an initially uniform plasma are found for a Gaussian pump profile as exp [-( x - x m ) 2 /2 d 2 x ]. Coupled equations describing the parametric coupling of two electron Langmuir waves due to the spatially aperiodic inhomogeneity of the self-trapped pump filament amplitude are then derived. Moreover, an analysis of the eigenvalue problem arising from these equations demonstrates a possibility that decay Langmuir waves can be trapped as temporally growing waves in a density cavity created by the inhomogeneous pump when the pump amplitude exceeds the threshold rather less than that for the decay instability of Jackson-type.

Convective parametric decay instability in a laser plasma

It is shown that during the buildup of a convective parametric decay instability a high-intensity internal field can be generated throughout the spatial region in which excitation and propagation of electron Langmuir waves is possible. Under the conditions of a laser plasma, this region accounts for an appreciable proportion (~30%) of the plasma corona. A turbulence region of this size has the result that when the effective collision frequency exceeds the classical value, anomalous heating of the laser plasma corona may be found. An explanation of various experimental data on second harmonic generation and reemission is given.

Parametric Coupling of Two Langmuir Waves in an Inhomogeneous Plasma. III. Wave Trapping by a Density Cavity

On the basis of the W.K.B. method, it is shown that an inverse triangle density cavity can trap a pair of temporally growing, oppositely propagating electron Langmuir waves; these are parametrically coupled through a combined action of a density gradient inside the cavity and a high-frequency pump electric field which is uniform outside the cavity. The threshold value of the pump intensity for this decay instability is presented as a monotonously increasing function of the scale size of the trapping regions, the mode number, the Debye length, and the minimum threshold above which the decay instability of the convective-type sets in. Consideration of the electron ponderomotive force created by the intense pump field which is non-uniform inside the density cavity leads to a decrease of the density gradient, resulting in an appearance of the upper threshold above which the trapped waves are damped.

Parametric Coupling of Two Langmuir Waves in an Inhomogeneous Plasma, II. Stabilizing Effects Due to the Pondermotive Force Created by an Intense Pump Field

Spatial behaviors of two electron Langmuir waves parametrically coupled because of the combined action of a density gradient and a high-frequency pump electric field which is uniform outside the plasma are investigated. It is shown that the effective density gradient decreases due to the electron ponderomotive force created by the space-dependent intense pump whose space-dependence is produced by the density gradient, leading to stabilizing effects on the convective-type instability treated in I, 7) because it is excited more strongly, the greater is the density gradient. In a non-magnetized plasma with a monotonously varying density profile, any effective trapping of the instability is shown not to be expected under usual practical conditions. The scale size of the region in which the instability may spread, however, increases with the pump intensity from “smaller than the Airy characteristic length l ” to “much larger than l .”