Landau Damping Effects Research Articles

Extending magnetohydrodynamics to the slow dynamics of collisionless plasmas

A fluid approach aimed to provide a consistent description of the slow dynamics of a collisionless plasma, is presented. In this regime, both Landau damping and finite Larmor radius effects cannot be ignored. Two models are discussed; one retains the dynamics at sub-ionic scales, while the other is restricted to scales larger than the ion gyroscale. Special attention is paid to the capability of these approaches to accurately reproduce the properties of linear waves that are known to play an important role, for example, in the small-scale dynamics of solar wind turbulence.

Nonlinear dynamics of Landau damped kinetic Alfvén waves

In the present paper we have studied the nonlinear dynamical equation of Landau damped kinetic Alfven wave (KAW) to investigate the nonlinear evolution of KAW and the resulting turbulent spectra in solar wind plasmas. We have introduced a parameter g which governs the coupling between the amplitude of the pump KAW and the density perturbation. The numerical solution has been carried out to see the dependence on the parameter g in the nonlinear part of our equation. Our results reveal the formation of damped localized structures of KAW as well as steepening of the turbulent spectra by increasing g when damping is taken into account. The power spectra of magnetic field fluctuations indicate the redistribution of energy among the higher wave numbers. Each power spectrum with and without damping splits up into two different scaling ranges, Kolmogorov scaling followed by a steeper scaling. The steepening in the power spectra with Landau damping is more than without Landau damping case (for the same value of g). This type of steeper spectra has also been observed in the solar wind and is attributed to the Landau damping effects.

Turbulence Effects on Bremsstrahlung Emission in the Turbulent Solar Plasma

Abstract Turbulence effects on bremsstrahlung emission due to the electron–ion interaction were investigated in the turbulent solar plasma. The effective interaction potential, taking into account the correction factor to the nonlinear dielectric function due to plasma turbulence, and the impact-parameter method were employed in order to obtain the bremsstrahlung radiation cross section in a turbulent plasma as a function of the impact parameter, diffusion coefficient, radiation photon energy, projectile energy, and plasma parameters. It has been shown that the turbulence effects considerably enhance the bremsstrahlung radiation cross section. In addition, it was found that the turbulence effects of the solar atmosphere on the bremsstrahlung cross section increase with increasing radiation photon energy, but decrease with increasing thermal energy. It can be expected that the Landau damping effect on the bremsstrahlung spectrum from the solar corona is less important than that from the solar chromospheres due to temperature jumps. However, the effect of the plasma turbulence on the bremsstrahlung spectrum from the solar chromosphere would be smaller than that from the solar corona.

Collisionless turbulent transport and anisotropic electron heating in coronal flare loops

Context. One of the hypotheses about the generation of the hard X-ray emissions (HXR) of the sun is that a strong electron-beam is first accelerated near the looptop, and then propagated down to the chromosphere. There the HXR emissions are generated by the bombardment of electrons via thick-target bremsstrahlung. Recently, the beam-plasma model has been questioned because streaming instabilities make the beam propagation doubtful. Another open question in solar flare models is the generation of anisotropic electron distributions deduced from microwave emissions. The question is whether one can find a mechanism, in addition to the generally considered mirror motion, that may cause the electron anisotropy in flare loops. Aims. To understand the transport of coronal electron beams and the possible generation of electron anisotropic distribution in the course of the beam propagation, we simulated a beam-plasma return-current system. Our aim is to investigate the evolution of predicted streaming instabilities at the nonlinear stage and to determine the intensity of beam transport in coronal loops. Methods. The linear instabilities and wave coupling in the beam-plasma return-current system are investigated by a multi-fluid dispersion analysis. We performed a two-dimensional electromagnetic particle-in-cell simulation to understand the generation of turbulent transport and anisotropic electron heating at the nonlinear stage of the beam evolution. Results. The beam-plasma return-current system is stablized at a late stage of evolution by a combined effect of 1) a relaxation of electron bulk drifts; and 2) a fast thermalization of the beam and return-current electrons in the drift direction. As a result, the downward electron beam continues to propagate in the coronal loop. Distinguishable parallel and perpendicular electron heating is observed, which is caused by turbulent deflection of electron beams. The electron distribution becomes anisotropic through different heating rates. Conclusions. An electron beam injected from a solar flare looptop can continue to propagate stably despite a partial relaxation of its drift velocity. After drift relaxation and anisotropic electron heating, the slowed-down electron drifts and the ambient thermalized plasma create a stable beam-propagation environment because of the Landau damping effect. This electron beam can propagate stably at a modified drift speed down to the chromosphere, where it generates HXR radiation. We conclude that the beam plasma model is feasible for the HXR generation in a solar flare event. The observed anisotropic electron distribution is a direct consequence of turbulent deflections of the electron beams.

Equilibrium and transport for quasi-helical reversed field pinches

This paper presents the most recent results in theoretical/numerical studies on the physics of the quasi-helical regime in reversed field pinch (RFP) configurations. Such regime systematically characterizes RFX-mod experiments at high currents (Ip > 1.2 MA), producing clear internal electron transport barriers. Several approaches, ranging from a macroscopic (MHD) to a microscopic (transport) description, have been used to tackle the related complex physics. From the macroscopic point of view, we derive analytically the electrostatic velocity field consistent with a generic helical ohmic equilibrium. We also provide the first MHD initial-value simulation results in toroidal geometry obtained with the PIXIE3D code. Concerning transport, the effect of magnetic chaos healing by mode separatrix expulsion, believed to favour the formation of transport barriers, is discussed. Results indicate that helical equilibria originated by non-resonant modes are more resilient to chaos formation. Finally, gyrofluid and gyrokinetic tools have been used towards a first assessment of the role of microturbulence in the RFP. Concerning the electrostatic branches, ion temperature gradient mode stability is robustly improved in RFP with respect to tokamaks, due to stronger Landau damping effects, and the marginality condition is estimated to be only spottily reached in present experimental regimes, unless the effects of impurities are considered. Impurities, which in RFX-mod accumulate in the edge, may also significantly impact the stability of the impurity-driven modes. On the electromagnetic side, microtearing turbulence is found to probably play a role at the transport barriers.

Numerical simulations to study solar wind turbulence

Numerical simulation of coupled equations of kinetic Alfvén wave (KAW) and ion acoustic wave is presented in the solar wind. The nonlinear dynamical equations satisfy the modified Zakharov system of equations by taking the nonadiabatic response of the background density. The ponderomotive nonlinearity is incorporated in the wave dynamics. The effect of Landau damping of KAW is taken into account. Localization of magnetic field intensity and the wavenumber spectra (perpendicular and parallel) of magnetic fluctuations are studied in solar plasmas around 1 a.u. Our results reveal the formation of damped localized structures and the steeper spectra that are in good agreement with the observations. These damped structures and steeper turbulent spectra can be responsible for plasma heating and particle acceleration in solar wind.

Hamiltonian formulation of the effective kinetic theory for superfluid Fermi liquids

We present in a local form the time dependent effective description of a superfluid Fermi liquid which includes Landau damping effects at T≠0. This is achieved by the introduction of an additional variable, the quasiparticle distribution function, which obeys a simple kinetic equation. The transport equation is coupled with first order equations for the Goldstone mode and the particle density. We prove that a main feature of this formulation is its Hamiltonian structure relative to a certain Poisson bracket. We construct the Hamiltonian to quadratic order.

Raman scattering by LO-phonon–plasmon coupled modes inGa1−xInxAsySb1−y: Role of Landau damping

We present a detailed investigation of the phonons and longitudinal-optical-phonon--plasmon coupled modes in the ${\text{Ga}}_{1\ensuremath{-}x}{\text{In}}_{x}{\text{As}}_{y}{\text{Sb}}_{1\ensuremath{-}y}$ alloy by means of Raman scattering. A generalization of the dielectric Raman line-shape model to quaternary alloys is described and used to analyze the Raman spectra of a set of $n$-type ${\text{Ga}}_{1\ensuremath{-}x}{\text{In}}_{x}{\text{As}}_{y}{\text{Sb}}_{1\ensuremath{-}y}$ layers with $x=0.15$, $y=0.13$, and electron densities ranging from $2.8\ifmmode\times\else\texttimes\fi{}{10}^{17}$ to $3.7\ifmmode\times\else\texttimes\fi{}{10}^{18}\text{ }{\text{cm}}^{\ensuremath{-}3}$. Landau-damping effects are found to be essential to correctly describe the coupled modes that occur in the phonon-frequency range. Differences in the electron-density dependence of these modes between 80 K and room temperature are observed and attributed to different Landau-damping regimes. The plasmonlike ${L}^{+}$ mode is observed to shift to lower frequencies when the temperature is increased from 80 K to room temperature. This frequency shift is explained in terms of thermal population of the $L$ valleys.

Landau damped kinetic Alfvén waves and coronal heating

AbstractSome recent observations of solar corona suggest that the kinetic Alfvén waves (KAWs) turbulence may be responsible for electron acceleration in solar corona and coronal heating. In the present research, we investigate the turbulent spectra of KAW due to filamentation process in the presence of Landau damping and particle energization. We present here the numerical simulation of model equation governing the nonlinear dynamics of the KAW in the presence of Landau damping. When the ponderomotive and Joule heating nonlinearities are incorporated in the KAW dynamics, the power spectra of the turbulent field is evaluated and used for particle heating. Our results reveal the formation of damped coherent magnetic filamentary structures and the turbulent spectra. The effect of Landau damping is to make the turbulent spectra steeper. Two types of scalings k−3.6 and k−4 have been obtained. We have studied the turbulence with different initial conditions. Using the Fokker–Planck equation with the new velocity space diffusion coefficient, we find the distribution function of energetic electrons in these turbulent structures. Landau damped KAWs may be responsible for the acceleration of the energetic electrons in solar corona and coronal heating.

Effect of nonlinear Landau damping in plasma-based backward Raman amplifier

A plasma wave can mediate laser coupling in a plasma-based resonant backward Raman amplifier for high power amplification of short laser pulses. The resonant nature of amplification requires a long lifetime of the plasma wave. However, the plasma wave can be heavily Landau damped in warm plasma. On the other hand, Landau damping can be saturated in the presence of a strong plasma wave. We study backward Raman amplifier in the nonlinear regime of Landau damping using a simplified fluid model. We find the regime in which initially high linear Landau damping can be significantly saturated. Because of the saturation effect, higher temperatures can be tolerated in achieving efficient amplification. The plasma temperature can be as much as 50% larger compared to the case of unsaturated Landau damping.

ONSETS AND SPECTRA OF IMPULSIVE SOLAR ENERGETIC ELECTRON EVENTS OBSERVED NEAR THE EARTH

Impulsive solar energetic electrons are often observed in the interplanetary space near the Earth and have an attractive diagnostic potential for poorly understood solar flare acceleration processes. We investigate the transport of solar flare energetic electrons in the heliospheric plasma to understand the role of transport to the observed onset and spectral properties of the impulsive solar electron events. The propagation of energetic electrons in solar wind plasma is simulated from the acceleration region at the Sun to the Earth, taking into account self-consistent generation and absorption of electrostatic electron plasma (Langmuir) waves, effects of nonuniform plasma, collisions, and Landau damping. The simulations suggest that the beam-driven plasma turbulence and the effects of solar wind density inhomogeneity play a crucial role and lead to the appearance of (1) a spectral break for a single power-law injected electron spectrum, with the spectrum flatter below the break, (2) apparent early onset of low-energy electron injection, and (3) the apparent late maximum of low-energy electron injection. We show that the observed onsets, spectral flattening at low energies, and formation of a break energy at tens of keV is the direct manifestation of wave-particle interactions in nonuniform plasma of a single accelerated electron population with an initial power-law spectrum.

Stability threshold of ion temperature gradient driven mode in reversed field pinch plasmas

For the first time in the reversed field pinch (RFP) configuration, the stability threshold of the ion temperature gradient driven (ITG) mode is studied by linear gyrokinetic theory. In comparison with tokamaks, the RFP configuration has a shorter connection length and stronger magnetic curvature drift. These effects result in a stronger instability driving mechanism and a larger growth rate in the fluid limit. However, the kinetic theory shows that the temperature slopes required for the excitation of ITG instability are much steeper than the tokamak ones. This is because the effect of Landau damping also becomes stronger due to the shorter connection length, which is dominant and ultimately determines the stability threshold. The required temperature slope for the instability may only be found in the very edge of the plasma and/or near the border of the dominant magnetic island during the quasi-single helicity state of discharge

Patterns of sound radiation behind pointlike charged obstacles in plasma flows

The electrostatic potential and plasma density variations around a pointlike charged object in a plasma flow are studied. These objects can represent small charged dust particles, for instance. The radiation patterns can be interpreted as the result of sound waves being radiated by the obstacle. Two limits are considered: one where the electron-ion temperature ratio is large, Te>>Ti , and one where Te/Ti approximately 1 . The former limit can be described by a simple model based on geometrical optics, while the latter requires a kinetic model in order to account for the effects of ion Landau damping. The results are illustrated by numerical simulation using a particle-in-cell code, where the electrons are treated as an isothermal massless fluid, giving a nonlinear Poisson equation. The analytical results are in good agreement with the numerical simulations.

Plasmons, plasminos and Landau damping in a quasiparticle model of the quark-gluon plasma

A phenomenological quasiparticle model is surveyed for 2+1 quark flavors and compared with recent lattice QCD results. Emphasis is devoted to the effects of plasmons, plasminos and Landau damping. It is shown that thermodynamic bulk quantities, known at zero chemical potential, can uniquely be mapped towards nonzero chemical potential by means of a thermodynamic consistency condition and a stationarity condition.

Quantum wave kinetics of high-gain free-electron lasers

A wave kinetic equation equivalent to the Schrodinger-like equation for the electrons is derived from relativistic theory in the eikonal approximation. This equation is used to study the interaction of the relativistic electron beam in a wiggler field in the quantum regime, i.e., when the normalized quantum free-electron laser parameter ρ¯⩽1. A general quantum dispersion relation and an expression for free-electron laser instabilities are derived. Conditions for kinetic instability, which takes into account the Landau damping effect, are established.

Landau Damping Effects on SO Limited Mobility

Surface optical phonons scattering is essential in determning the channel mobility of high-k gate dielectric MOSFETs. Metal gates are believed to modulate the effects of high-k phonons to some extent in comparison to poly-Si gates. In this work nonideal metal gate model is used to study the impact of SO scattering on the nMOSFET channel mobility. Landau damping effects, which are found to be important in the coupled plasmon-phonon scattering, are studied from the perspective of different criteria in selecting the Landau limits.

Landau damping effects on collision-induced quantum interference in electron-hole plasmas

The Landau damping effects on the quantum interference in electron collisions are investigated in a quantum plasma composed of electrons and holes. The Born method and the total spin states are considered to obtain the scattering cross-section by using the effective screened potential model. It is found that the Landau damping effects enhance the scattering cross-section, especially, near the scattering angle θL = π/4.

Finite-size effects in surface-enhanced Raman scattering in noble-metal nanoparticles: a semiclassical approach

We study finite-size effects in surface-enhanced Raman scattering (SERS) from molecules adsorbed on small metal particles. Within an electromagnetic description of SERS, the enhancement of the Raman signal originates from the local field of the surface plasmon resonance in a nanoparticle. With decreasing particle sizes, this enhancement is reduced due to the size-dependent Landau damping of the surface plasmon. We show that, in small noble-metal particles, the reduction of interband screening in the surface layer leads to an additional increase in the local field acting on a molecule close to the metal surface. The overall size dependence of Raman signal enhancement is determined by the interplay between Landau damping and underscreening effects. Our calculations, based on a two-region model, show that the role of the surface layer increases for smaller nanoparticle sizes due to a larger volume fraction of the underscreened region.

Temporal signatures of radiation belt electron precipitation induced by lightning‐generated MR whistler waves: 1. Methodology

We present a novel technique designed to calculate the detailed differential number flux signature (as a function of energy and time) of precipitating radiation‐belt electrons, driven by a magnetospherically reflecting (MR) whistler wave, initiated by a single cloud‐to‐ground lightning discharge. Our model consists of several stages. First, we calculate the MR whistler wave characteristics at 1° latitude intervals along a given field‐line. This is accomplished using an extensive ray tracing and interpolation algorithm involving ∼120 million rays, and accounting for the effects of Landau damping, spatial, and temporal dispersion. We then use these wave characteristics to compute the pitch angle changes of resonant electrons by assuming that the interactions are linear, and independent between adjacent latitude and wave frequency bins. The pitch angle changes are transformed to precipitating flux using a novel convolution method and displayed as a function of particle energy and time at the feet of a given field line. We have calculated and compared the differential number flux signatures at the northern and southern feet of the L = 2.3 and L = 3 field lines, and found that precipitation onset and duration times increase with latitude (consistent with previous work). The precipitation consists of suprathermal Landau resonance electrons (E ≲ 10 keV) which are intense but contribute little to the total energy flux, a flux gap (10 keV ≲ E ≲ 80 keV) corresponding to a change in coupling mechanism from Landau resonance to gyroresonance, and a series of precipitation swaths (E ≳ 80 keV) corresponding to gyroresonance interactions. The swaths result in periodic maxima in the precipitated energy flux, which correspond to the equatorial traversals of the underlying MR whistler wave energy. Global precipitation signatures were computed for a number of lightning discharge latitudes and are presented in a companion paper (Bortnik et al., 2006).

Study of the gluon propagator in the large- Nf limit at finite temperature and chemical potential for weak and strong couplings

At finite temperature and chemical potential, the leading-order (hard-thermal-loop) contributions to the gauge-boson propagator lead to momentum-dependent thermal masses for propagating quasiparticles as well as dynamical screening and Landau damping effects. We compare the hard-thermal-loop propagator with the complete large- N f gluon propagator, for which the usually subleading contributions, such as a finite width of quasiparticles, can be studied at nonperturbatively large effective coupling. We also study quantitatively the effect of Friedel oscillations in low-temperature electrostatic screening.