Low-frequency Density Perturbation Research Articles

2D kinetic simulations of whistler wave generation by nonlinear scattering of lower-hybrid waves in turbulent plasmas

Turbulent plasmas in space, laboratory experiments, and astrophysical domains can often be described by weak turbulence theory, which can be characterized as a broad spectrum of incoherent interacting waves. We investigate a fundamental nonlinear kinetic mechanism of weak turbulence that can explain the generation of whistler waves in homogeneous plasmas by nonlinear scattering of short wavelength electrostatic lower-hybrid (LH) waves. Two particle-in-cell (PIC) simulations with different mass ratios in two dimensions (2D) were performed using a ring ion velocity distribution to excite broadband LH waves. The wave modes evolve in frequency, and wavenumber space such that the LH waves are converted to whistler waves. The simulations show the formation of quasi-modes, which are low-frequency density perturbations driven by the ponderomotive force due to the beating of LH and whistler waves. These low-frequency oscillations are damped due to resonant phase matching with thermal plasma particles. By comparing the phase and thermal speeds, we confirm the nonlinear scattering mechanism and its role in the 2D evolution of the ring ion instability. Although the nonlinear scattering is theoretically slower in 2D than in 3D due to the absence of the vector nonlinearity, these simulations show that quasi-modes are an important diagnostic for nonlinear landau damping in PIC simulations that has not been utilized in the past. The nonlinear scattering mechanism described here plays an important role in the generation of whistler waves in active experiments, which will be experimentally studied in the upcoming Space Measurement of a Rocket Release Turbulence experiment.

Three-dimensional particle-in-cell modeling of parametric instabilities near the quarter-critical density in plasmas.

The nonlinear regime of laser-plasma interactions including both two-plasmon decay (TPD) and stimulated Raman scattering (SRS) instabilities has been studied in three-dimensional (3D) particle-in-cell simulations with parameters relevant to the inertial confinement fusion (ICF) experiments. SRS and TPD develop in the same region in plasmas, and the generation of fast electrons can be described accurately with only the full model including both SRS and TPD. The growth of instabilities in the linear stage is found to be in good agreement with analytical theories. In the saturation stage the low-frequency density perturbations driven by the daughter waves of the SRS side scattering can saturate the TPD and consequently inhibit the fast-electron generation. The fast-electron flux in 3D modeling is up to an order of magnitude smaller than previously reported in 2D TPD simulations, bringing it close to the results of ICF experiments.

Spontaneous magnetic field in the interactions of transverse plasmons with electron-positron-ion plasma

The magnetic field generated by transverse plasmons in three-component electron-positron-ion plasma is investigated based on a kinetic model. Nonlinear coupling equations, self-consistently describing the nonlinear behavior of high-frequency transverse plasmons, low-frequency density perturbation, and quasistatic self-generated magnetic fields, are derived considering the nonlinear wave-wave and wave-particle interactions. The governing equations can be easily reduced to the ones obtained in conventional electron-ion and electron-positron plasmas. There will be no spontaneous magnetic field in the pure electron-positron plasma due to the same mass of the electron and the positron. It is shown that the self-generated magnetic field is relevant to the mass difference of plasma particles, which is modulationally unstable. The increase in the amplitude of the pump wave field or the decrease in the ion concentration will lead to a faster growth of the self-generated magnetic field and higher intermittent magnetic flux.The magnetic field generated by transverse plasmons in three-component electron-positron-ion plasma is investigated based on a kinetic model. Nonlinear coupling equations, self-consistently describing the nonlinear behavior of high-frequency transverse plasmons, low-frequency density perturbation, and quasistatic self-generated magnetic fields, are derived considering the nonlinear wave-wave and wave-particle interactions. The governing equations can be easily reduced to the ones obtained in conventional electron-ion and electron-positron plasmas. There will be no spontaneous magnetic field in the pure electron-positron plasma due to the same mass of the electron and the positron. It is shown that the self-generated magnetic field is relevant to the mass difference of plasma particles, which is modulationally unstable. The increase in the amplitude of the pump wave field or the decrease in the ion concentration will lead to a faster growth of the self-generated magnetic field and higher intermittent magne...

X-mode coupling to Bernstein wave in a plasma embedded with clusters

Parametric coupling between a large amplitude X-mode laser, propagating perpendicular to the ambient magnetic field Bszˆ, through a plasma embedded with clusters with short-wavelength electron Bernstein wave and a lower hybrid wave is investigated. The coupling arises through the ponderomotive force by the high frequency waves and the low frequency density perturbation beating with the oscillatory velocity due to laser. Finite Larmor radius effects on plasma electrons are included while those on free electrons inside the clusters are neglected. The clusters response to nonlinear coupling is enhanced due to the Plasmon resonance. The cluster Plasmon resonance strongly modifies the nonlinear coupling between modes. For typical parameters, k1⊥2vth2/(2ωc2)=1,nc(4π/3)rc3=0.001,eA0/(mω0c)=0.1, the growth rate turns out to be maximum when ω/ωLH=1.4 and decreases sharply with ω/ωLH.

Analysis of chemiluminescence, density and heat release rate fluctuations in acoustically perturbed laminar premixed flames

Laser interferometric vibrometry (LIV) has recently been proposed as an alternative mean to obtain time-resolved density and heat release rate measurements at relatively low cost and experimental effort. This technique is sensitive to fluctuations of the refractive index of gases resulting from density and composition changes along the laser beam intersecting the reacting flow. It yields a line-of-sight integrated signal of the probed flow from which density and heat release rate disturbances may be inferred. The link between these signals with chemiluminescence is examined in the present study by first considering a theoretical analysis to determine the relationships between the LIV, density and heat release rate perturbation signals in a multi-species reactive mixture of gases. For air combustion systems interacting with sound waves, low frequency density perturbations in the flame zone, result mainly from heat release rate fluctuations below a certain frequency threshold. An experimental analysis is then conducted with confined conical laminar premixed flames submitted to harmonic flow modulations. Measurements are presented for methane–air mixtures at different equivalence ratios 0.8 ≤ ϕ ≤ 1.2 and thermal powers. It is shown that fluctuations of the chemiluminescence signal examined in different wavelength bands, including the OH*, CH* or the entire visible emission bands, always capture the same dynamics. This indicates that heat release rate fluctuations can be deduced without specific filters for the laminar premixed methane–air flames investigated. It is then shown that heat release rate measurements deduced from LIV and chemiluminescence data match well. A proportional relation is found that does not depend on the measurement position, modulation frequency and modulation level for fixed injection conditions. This linear relation slightly depends on the mean flow operating conditions partly due to the difficulty to interpret chemiluminescence emission for rich flames.

Nonlinear Behavior of Electromagnetic Waves in Ultra‐Relativistic Electron‐Positron Plasmas

AbstractA set of nonlinear equations which can self‐consistently describe the behavior of high frequency Electromagnetic (EM) waves in un‐magnetized, ultra‐relativistic electron‐positron (e‐p) plasmas is obtained on the basis of Vlasov‐Maxwell equations. Nonlinear wave‐wave, wave‐particle interactions lead to the coupling of high frequency EM waves with low frequency density perturbations which result from EM waves radiation pressure. The same as that in conventional electron‐ion (e‐i) plasmas, strong EM waves in e‐p plasmas will give rise to density depletion in which itself are trapped. But on the contrary to that in e‐i plasmas, there no longer exists electrostatic acoustic–like wave in e‐p plasmas due to the absence of mass difference. For linear polarized EM waves, a stationary EM soliton with a spiky structure will be formed. The possible relation of the localized field to pulsar radio pulse is discussed (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Well-posedness of the cauchy problem for the coupled system of the Schrödinger-KdV equations

This paper is devoted to the study of the Cauchy problem for the coupled system of the Schrodinger-KdV equations which describes the nonlinear dynamics of the one-dimensional Langmuir and ion-acoustic waves. Global well-posedness of the problem is established in the spaceH κ ×H κ (κ eZ +), the first and second components of which correspond to the electric field of the Langmuir oscillations and the low-frequency density perturbation respectively.

Electrostatic chains driven by nonuniform lower hybrid pump

We study the parametric interaction of a nonuniform long-wavelength lower hybrid (LH) wave propagating almost perpendicularly to the magnetic field lines, with low frequency density perturbations. Neglecting perturbations of the strong LH wave acting as a pump, we show that for a specific profile of its amplitude, a LH pump can excite electrostatic vortex chains propagating perpendicularly both to the magnetic field and the direction of the pump gradient.

Nonlinear lower hybrid vortices.

Using a two-fluid description, we derive a set of equations describing the nonlinear interaction of a lower hybrid pump wave, propagating almost perpendicularly to the external magnetic field, with low frequency density perturbations associated with a drift wave. In the strongly nonlinear regime, when the system is dominated by convective vector-product-type nonlinearities on the slow time scale, we find a stationary, localized dipole vortex solution driven by the lower hybrid ponderomotive force. These types of vortices can be driven both by a long-wavelength pump, in an oscillating two-stream parametric process, and by the modulation of a short-wavelength pump wave. In most cases, all vortex parameters (amplitude, radius, and velocity) are determined by the amplitude of the pump. \textcopyright{} 1996 The American Physical Society.

Lower-hybrid quasimode decay in a plasma cylinder

In the presence of an azimuthally symmetric lower-hybrid pump wave, a low-frequency density perturbation of frequency ω≊ωci having finite azimuthal mode number, generates a nonlinear current driving a lower-hybrid sideband wave. The sideband couples with the pump to produce a parallel ponderomotive force driving the low-frequency perturbation. For strongly ion cyclotron damped perturbation, the mode structure equation for the sideband wave has been solved using perturbation technique. The growth rate is strongly sensitive to the density and temperature of the plasma.

Driven Alfven vortices

Theory of plasma vortices is briefly reviewed, and their interaction with high-frequency waves and particle beams is discussed. It is shown that in the case of a large amplitude upper-hybrid pump wave interacting with low-frequency density perturbations, self-interaction on the low frequency scale leads to the formation of double vortex solitons. In the case of nonlinear shear Alfven perturbations, described by reduced MHD equations, evolution of the double vortex due to the interaction with resonant particles is studied, and it is demonstrated that in the presence of an electron beam a double vortex can emerge.

Upper-hybrid vortex solitons

A Zakharov-type set of two differential equations is derived, describing the interaction of upper-hybrid waves with low-frequency density perturbations in the case of Boltzmann distributed electrons. It is shown that low-frequency self-interaction terms lead to the formation of a traveling solution in the form of a solitary double vortex. This solution can be identified as a new nonlinear mode, related to the driven Hasegawa–Mima modon.

Nonlinear processes in the self-focused duct propagation of the whistlers in the magnetosphere

A finite amplitude whistler wave exerts a ponderomotive force on the electrons, creating a higher density duct and thus guiding itself to travel to long distances. This duct, however, is unstable to a short wavelength ( k, k z ⪢ k 0 z ) low frequency density perturbation, viz. oscillating two stream instability. In this process, the whistler pump wave excites a low frequency perturbation and two lower hybrid sidebands. The threshold for the instability is determined by the convection losses of sideband energy from the whistler duct. Threshold electric fields for the onset of the instability have been calculated in the magnetosphere for a range of pump frequency.

Nonlinear interaction of waves in sharply bounded plasmas

We consider the nonlinear interaction of three high-frequency waves and lowfrequency density perturbations in a sharply bounded plasma. The coupled equations are presented in a general and convenient form. To demonstrate the usefulness of our method we derive explicit expressions for the threshold of a number of possible instabilities in semi-infinite and cylindrical plasmas.

Nonlinear interaction of electrostatic surface waves in a semi-infinite plasma. Part 2. Time-dependent solutions to the coupled mode equations

The coupled mode equations which govern the nonlinear interaction of three electrostatic high-frequency surface waves and a low-frequency density perturbation are analysed considering time-dependent solutions only. We show the existence of a filamentation instability in the static limit for the low-frequency density perturbation. In the opposite case (density perturbation close to the lowfrequency surface wave resonance) we arrive at a decay instability where only surface waves take part. The parametric approximation (growth rate and threshold) as well as the nonlinear evolution of both types of instabilities are studied.

Nonlinear interaction of electrostatic surface waves in a semi-infinite plasma. Part 1. Derivation of the coupled mode equations

We have derived a set of coupled mode equations which govern the nonlinear interaction of three high-frequency electrostatic surface waves through a low-frequency density perturbation produced by them. The set is compared with that obtained when a similar problem is solved for bulk waves in an infinite plasma. Some differences are shown to exist caused by the specific features of surface waves such as the amplitude attenuation normal to the interface and their hybrid nature.

On the nonlinear development of the Langmuit modulational instability

We consider the nonlinear development of a long-wavelength finite-amplitude Langmuir wave. The wavenumber k0 of the initial Langmuir wave is chosen such that the three-wave decay is forbidden. We then describe the coupling of the initial Langmuir wave to Stokes and anti-Stokes Langmuir perturbations (with wavenumbers k0 ∓ ks) due to the presence of a low-frequency density perturbation of wavenumber ks. We then show that for a wide range of experimental conditions, the Stokes and anti-Stokes Langmuir waves are generated with wavenumbers well separated from k0. In order to describe the nonlinear evolution of these perturbations and the pump wave we make the static approximation for the ions and describe the high-frequency waves by three distinct wave envelopes. These coupled nonlinear differential equations are then solved exactly for a number of special cases. For the temporal evolution, we obtain periodic solutions and, when damping is included, we find a slow exponential decay of the amplitudes with a corresponding increase in the nonlinear period of oscillation. The stationary spatially varying solutions are shown to include four basic types of behaviour: periodic, solitary wave, phase jump and shock-like profiles. These latter solutions are of interest since they are obtained for zero dissipation and for a coherent wave interaction.

Nonlinear propagation of lower-hybrid waves in an inhomogeneous plasma

We investigate the problem of spatial depletion of propagating lower-hybrid waves in an inhomogeneous plasma. In particular, we consider the nonlinear evolution of a lower-hybrid pump which is coupled to a daughter lower-hybrid decay wave excited by low-frequency density perturbations. Our results show that pump depletion occurs when three-dimensional effects are included. This phenomenon competes with the filamentation of the pump caused by self- interaction. Implications to lower-hybrid heating experiments are discussed.