Plasma Eigenmodes Research Articles

Propagation Effects in Magnetized Transrelativistic Plasmas

The transfer of polarized radiation in magnetized and nonmagnetized relativistic plasmas is an area of research with numerous flaws and gaps. The present paper is aimed at filling some gaps and eliminating the flaws. Starting from a Trubnikov's linear response tensor for a vacuum wave with |k| = ω/c in thermal plasma, the analytic expression for the dielectric tensor is found in the limit of high frequencies. The Faraday rotation and Faraday conversion measures are computed in their first orders in the ratio of the cyclotron frequency Ω0 to the observed frequency ω. The computed temperature dependencies of propagation effects bridge the known nonrelativistic and ultrarelativistic limiting formulae. The fitting expressions are found for high temperatures, where the higher orders in Ω0/ω cannot be neglected. The plasma eigenmodes are found to become linearly polarized at much larger temperatures than thought before. The results are applied to the diagnostics of the hot interstellar medium, hot accretion flows, and jets.

Eigenmode analysis of geodesic acoustic modes

Geodesic acoustic modes (GAMs) are studied as plasma eigenmodes when an electrostatic potential nearly constant around a magnetic surface is applied to collisionless toroidal plasmas. Besides the standard GAM, a branch of low frequency mode and an infinite series of ion sound wavelike modes are identified. Eigenfrequencies of these modes are obtained analytically and numerically from a linear gyrokinetic model. The finite gyroradius effect is found to enhance the collisionless damping of the standard GAM, while this enhancement is not monotonic as the safety factor varies. Moreover, additional damping due to higher-harmonic resonances becomes important when the safety factor increases. The mode structure of the GAM is also discussed.

Nonlinear electromagnetic plasma eigenmodes and their stability to stimulated Raman scattering

Transverse mode structure of nonlinear laser eigenmodes in underdense and overdense plasmas has been obtained by numerically solving the wave equation under relativistic and ponderomotive nonlinearities. The mode structure closely resembles a Lorentzian with half width scaling inversely as the axial intensity of the laser. The threshold condition for laser penetration in an overdense plasma turns out to be γ0≡(1+a02∕2)1∕2⩾2n0∕ncr−1, where n0 is the equilibrium electron density, ncr is the critical density at laser frequency, and γ0 is the electron Lorentz factor due to the laser of normalized axial intensity a02. The nonlinear laser eigenmode, in a low density plasma, is unstable to stimulated Raman backscattering off a copropagating space charge reactive quasimode. The growth rate increases with laser intensity as a0 rises up to a0∼1. Beyond this value, growth rate decreases with a0, due to the enhancement of electron mass and depletion of electrons from the axial region. Geometrical effects also reduce the growth rate.

Spectra of high-frequency waves in hot magnetized plasmas

On the basis of numerical solution of the dispersion equation, we obtain the spectra of weakly damped high-frequency waves in a hot magnetized plasma for the case where the electron cyclotron frequency ωHe is below the plasma frequency ωpe. It is shown that the longitudinal wave propagating at an angle to the magnetic field evolves into the slow extraordinary wave for the refractive index n ≤ 1. For n ≫ 1, the longitudinal-wave frequency increases with the refractive index, and the wave evolves into the wave with anomalous dispersion if the angle θ between the wave vector and the magnetic field is close to 90°. In the same range of θ angles, Bernstein modes appear in the spectrum of plasma eigenmode oscillations.

Plasmons at a hole in a screen

The field distribution and the spectrum of plasma eigenmodes are determined for a circular hole in a screen having the form of a hyperboloid of revolution. The spectrum of plasmons is obtained for a planar screen with a circular hole. The symmetric mode with m=1 can be excited in the field of a dipole oriented along the screen plane, whereas the antisymmetric mode with m=0 can be excited by a dipole perpendicular to the screen plane.

Covariant magnetoionic theory -- I. Ray propagation

Accretion on to compact objects plays a central role in high-energy astrophysics. In these environments, both general relativistic and plasma effects may have a significant impact upon the propagation of photons. We present a fully general relativistic magnetoionic theory, capable of tracing rays in the geometrical optics approximation through a magnetized plasma in the vicinity of a compact object. We consider both the cold and warm, ion and pair plasmas. When plasma effects become large the two plasma eigenmodes follow different ray trajectories, resulting in a large observable polarization. This has implications for accreting systems ranging from pulsars and X-ray binaries to active galactic nuclei.

Effects of nonlinear frequency shifts on certain induced scattering processes

The present paper is a followup to an earlier paper (accepted in Physics of Plasmas, 2002) in which the nonlinear frequency shifts of electrostatic plasma eigenmodes in an unmagnetized plasma were investigated, and in which a promise was made that the methodology employed in such a study will be employed to deal with certain induced scattering terms which contain apparent singularities. This paper implements the analytical technique developed in the first paper, and demonstrates how these singular terms can be regularized.

Nonlinear frequency shifts of plasma eigenmodes

In the present article, the classic problem of nonlinear frequency shifts of electrostatic plasma eigenmodes in an unmagnetized plasma (i.e., the Langmuir and ion-acoustic waves) is revisited. In the standard literature, only the frequency shift of Langmuir waves by the finite-amplitude Langmuir waves themselves is usually treated. In the present approach, the discussion is generalized to include the ion-sound waves. The significance of the present article is that the analytical approach employed in the present discussion can be utilized to resolve certain apparently singular terms in the induced scattering coefficients of the wave kinetic equations. The detailed discussion of such a problem will be reported in a forthcoming article.

Electron Heating and Acceleration by Alfvén Waves with Varying Phase Velocity

The influence of the Alfvén wave on the electron distribution function is studied by numerical simulations. A quasi-linear operator models the electron Landau damping of any plasma eigenmodes: kinetic Alfvén, whistlers, and lower hybrid waves. Weak collisions are taken into account via 2D in the velocity space nonlinear Fokker-Planck kinetic equation. It is shown that due to the variation of phase velocity, as seen by the Alfvén wave in the course of propagation in the nonuniform magnetic field, the noninductive current induced by the Alfvén wave and the dissipated wave power may be several times larger in comparison with the case when the phase velocity is constant. The results might be applied in coronal regions, magnetosphere, tokamaks, stellarators.

Instabilities in localized non-uniform charged dust streams in cold plasmas

Surface eigenmodes in a dusty plasma, where the dust is confined to a particular region, modelled as a uniform slab with non-uniform smooth boundary layers, are discussed. Inside this region, electron depletion occurs, while outside it, the plasma is just a normal uncontaminated plasma. In addition, the dust component can flow with respect to the background plasma, as a first approximation to cometary tails, where there is a notable difference between the fast flowing solar wind and the slow moving dust tail, viewed in the comet frame. The equilibrium densities and flow are non-uniform and the description involves a space-dependent dielectric function, which indicates the possibility of singular points, where eigenmodes resonate with local plasma oscillations. Surface eigenmodes on the slab are obtained analytically in the limit of long wavelengths, and these give rise to two distinct frequency domains, both having three different wave solutions. One of these refers to plasma surface waves, unaffected by the dust flow. The other two lead to convective surface modes, which become unstable for large flow speeds, akin to Buneman-type instabilities. A detailed study is made of the resonant processes in the boundary layers, including damping and growth rates.

Numerical simulation of heating problems for a weakly collisional plasma

The nonlinear effect of MHD waves on the generation of rf-driven current is studied numerically. A quasi-linear operator models the electron Landau damping of any plasma eigenmodes. For the description of weak collisional plasma processes, 2D nonlinear kinetic Fokker–Planck equation is used. The constructed difference schemes allow us to carry out numerical calculations in a large time interval without error accumulation.

Secondary-Electron-Emission Instability in a Plasma

A 200-eV electron beam is incident on an electrode in a laboratory plasma. The emission of secondary electrons produces a region of negative differential resistance in the current-voltage characteristic of the electrode. Spontaneous dynatron oscillations are driven by the negative differential resistance when a resonant circuit is placed in series with the electrode. The instability is driven by the beam energy, produces large amplitudes comparable to the beam voltage, modulates beam and plasma parameters, and excites plasma eigenmodes such as ion acoustic waves. [S0031-9007(98)08228-3]

Discrete charging model and parametric excitation of the so-called acoustic mode in dusty plasmas

A simple harmonic oscillator model has been considered for the statistical charging process of the dust grainlike impurity ions. Treating it as a driver eigenmode and the plasma eigenmodes under consideration (the acoustic modes and plasma oscillations) as the idler modes, a simple process of parametric excitation of the idler modes has been discussed. It is suggested that the proposed model could provide a seeding mechanism for resonant excitation of the electrostatic plasma modes.

Ultra-Low-Frequency Electrostatic Modes in a MagnetizedDusty Plasma

A study on the extremely low-frequency possible electrostatic modesin a finite temperature magnetized dusty plasma taking the chargeddust grains as the third component has been carried out using theappropriate Vlasov-kinetic theory for the dynamics of the electrons,ions and the dust particles. It is found that the inequalities ofcharge and number density of plasma species, and thefinite-Larmor-radius thermal kinetic effects of the mobile chargeddust grains, introduce the existence of very low-frequencyelectrostatic eigenmodes in the three-component homogeneous magnetizeddusty plasma. The relevance of the present investigation to space andastrophysical situations as well as laboratory experiments for dustCoulomb crystallization has been pointed out.

Nonlinear resonance of plasma waves in the thermal irregularities of ionospheric plasma

We study the effect of striction plasma density disturbances on the generation intensity of longitudional cold and plasma oscillations due to polarization of the magnetic field-aligned ionospheric plasma irregularities with δNo<0 by a powerful radio wave. It is assumed that the plasma density level inside the irregularity intersects the upper-hybrid resonance level, in the vicinity of which the cold oscillations excited directly by a powerful radio wave are transformed to shorter-wave plasma oscillations. We consider the short plasma wave limit to reduce the problem to a system of two coupled equations for the cold wave induction and plasma wave electric field. The first equation is supplemented by a local source equal to the integral of the plasma wave electric field in the resonance region. The second equation involves the cold wave induction at the resonance point and describes the electric field of interacting waves in the resonance vicinity. We use simplifications connected with the small absorption of plasma waves propagating inside the irregularity and weak radiation of these waves outside the irregularity. These conditions correspond to the generation of eigenmodes of plasma oscillations trapped in the irregularity. We have obtained a resonance-type nonlinear equation for the electric field intensity (or energy flux) of eigenmode plasma waves with allowance for striction disturbances of the plasma density profile in the resonance region. It is shown that the striction expulsion of plasma is responsible for the occurrence of coefficients describing the change in the intensity of excitation and radiation of plasma waves at the irregularity boundary. Such an expulsion leads to variations of the efficient generation band of plasma eigenmodes with the total phase increment of the wave in the irregularity. It also leads to a change in the phase shift of the plasma wave reflected from the resonance. These coefficients and the nonlinear phase shift are expressed in terms of real wave functions of the nonlinear Airy equation which describes the electric field of the excited waves in the resonance vicinity when the dissipation is absent.

Electron acceleration due to the synergetic effect of DC electric field and waves

The combined influence of two types of fast waves and a dc electric field, which is aligned with the ambient magnetic field, on electron distribution function is studied by numerical simulations. A quasi-linear operator models the electron Landau damping of any plasma eigenmodes: kinetic Alfvén, whistlers, and lower hybrid waves. For the description of weak collisional plasma processes, the 2D nonlinear kinetic Fokker-Planck equation is used. The distribution function structure, the electron current induced by waves, and the quantitative and qualitative aspects of runaway production are analyzed.

Anomalous electron precipitation near the Earth’s auroral zone induced by the wave–particle interaction

The results of a numerical simulation of anomalous electron precipitation in the Earth’s auroral zone induced by waves are discussed. In order to study the plasma electron–wave interaction, a two-dimensional quasilinear code has been used. A quasilinear operator models the electron Landau damping of any plasma eigenmodes: kinetic Alfvén waves, whistlers, lower hybrid waves, etc. The distribution function structure was analyzed. The electron current induced by waves, the density, and the plasma energy decay, which stimulates the electron precipitation, have also been investigated.

Nonlinear wave-particle interaction and conditions for the applicability of quasilinear theory

In this paper it is shown that the usual conditions of the broad wave packet and overlapping of the trapping intervals of neighboring harmonics are not sufficient for the applicability of the quasilinear theory. The additional important condition is phase randomization in wave-wave interaction. In order for this randomization to occur during a time of wave evolution, i.e. of the inverse growth rate, the mismatch between the frequency of the three-wave nonlinear driver and that of the plasma eigenmode must exceed significantly the growth rate. If this condition is not fulfilled, quasilinear theory is applicable only for wave spectra smoothed by a procedure of averaging over neighboring harmonics inside the trapping interval. Experimental tests done with a warm beam moving through a traveling tube qualitatively support this point of view. At the same time, as it is shown in the paper, nonlinear interaction of waves has the tendency for steepening and modulation of the spectral energy density at a typical k scale of the order of inverse trapping length.

Diagnostics of α-particles using CO 2 scattering from spontaneously excited ion Bernstein waves

The possibility of measuring fusion α-particle features using collective scattering from self-excited plasma eigenmodes is discussed. It is shown that the proper choice of the modes, namely the high-order ion Bernstein waves, allows us to determine density and mean energy of the hot ion minority with the use of present day CO 2 lasers.

Reply to the comment 'Influence of inhomogeneity on the resonance interaction' by Ya.N. Istomin

The idea of the generation of eigenmodes in plasma caused by motion of particles along the field lines, like a curvature radiation of the particles, was first considered by Blandford (1975) and then by other authors. The result was simple: if the particle drift is neglected, no wave generation takes place. Beskin, Gurevich and Istomin (BGI) revived this problem in their series of papers. It is proved in their papers that the weak curvature of the field lines provokes the appearance of the new eigenmodes and their generation. In the paper by Machabeli (1991) it was clearly shown that the statement of BGI is wrong. The same point of view was previously outlined in the papers by Larroche and Peliat (1987) and Nambu (1989). However, Istomin (1994) criticized the papers noted above and supports the reasoning of BGI. Istomin stresses that they represented a new correct method of taking into account the effect of the curvature of the magnetic field lines. The author has, in an earlier article, shown that this reasoning is incorrect and reiterates his position in this comment.