不均匀等离子体中电磁波与Langmuir波的相互作用

通过理论研究与数值计算,不均匀等离子体中Langmuir波与电磁波的相互作用及其线性模式转换规律得到了充分的展示.导出了不均匀等离子体中的电磁色散关系,研究了入射电磁波或Langmuir波在通过不均匀等离子体的过程中发生转换的物理过程,以及波的传播矢量随空间坐标变化的关系,并对电磁波与Langmuir波相互作用的机理进行了讨论.研究结果对密度梯度所驱动的等离子体波产生电磁辐射的研究具有重要意义. 关键词： 电磁波 Langmuir波 不均匀等离子体 线性模式转换     

Trapping of plasmons in ion holes

We present analytical and numerical studies of a new electron plasma wave interaction mechanism, which reveals trapping of Langmuir waves in ion holes associated with nonisothermal ion distribution functions. This Langmuir ion hole interaction is a unique kinetic phenomenon governed by two second nonlinear differential equations in which the Langmuir wave electric field and ion hole potential are coupled in a complex fashion. Numerical analyses of our nonlinearly coupled differential equations exhibit trapping of localized Langmuir wave envelops in the ion hole, which is either standing or moving with sub-or super ion thermal speed. The resulting ambipolar potential of the ion hole is essentially negative, giving rise to bipolar slow electric fields. The present investigation thus offers a new Langmuir wave contraction scenario that has not been rigorously explored in plasma physics.     

The Nonlinear Langmuir Waves in a Multi-ion-Component Plasma

We investigated the nonlinear Langmuir waves in a multi-ion-component low-temperature plasma. Beginning with the fluid theory of plasma, and taking fully nonlinear response of the low-frequency ion motion into account, we derived a set of equations governing the nonlinear coupling of the amplitude of the Langmuir wave and the low-frequency perturbation density. Using the Sagdeev potential method, we analyzed the characteristics of solitary wave. In the limit of small amplitude, the envelope soliton was found. Our investigation demonstrates that the properties of soliton in a multi-ion-component plasma are different from those of soliton in an electron-ion plasma.     

Two-wave interaction through the same resonant particles

The possibility of mode-coupling, in a plasma, waves being in resonance with the sample particles is considered. The physical mechanism of this phenomenon is discussed, using as an example two monochromatic finite-amplitude waves: a Langmuir wave and an extraordinary wave. It is shown that the resonant electrons can be an effective channel for energy exchange between the waves.     

等离子体拍频波加速器中三波相互作用的时间发展

计算了等离子体拍频波加速器中双频抽运光与Langmuir波的相互作用及其时间发展。指出Langmuir波出生成到饱和的过程,与三波位相由匹配到失配密切相关。三波位相差作为描写三波耦合程度的量对于双频激光激发Langmuir波至关重要。 关键词：     

High-frequency emissions during the propagation of an electron beamin high-density plasma

A relativistic annular electron beam passing through a high-density plasma excites Langmuir waves via Cerenkov interaction. The Langmuir waves are backscattered off ions via nonlinear ion Landau damping. At moderately high amplitudes these waves are parametrically up-converted by the beam into high-frequency electromagnetic radiation, as observed in some recent experiments. A nonlocal theory of this process is developed in a cylindrical geometry. It is seen that the growth rate of the Langmuir wave scales as one-third the power of beam density. The growth rate of parametric instability scales as one-fourth the power of beam density and the square root of beam thickness     

多光子非线性Compton散射对激光等离子体中强朗缪尔湍动谱的影响

 基于电子与多光子集团非线性Compton散射模型,研究了多光子非线性Compton散射对激光等离子体中强缪尔湍动谱的影响,提出了将入射光和散射光作为形成强缪尔湍动的新机制,给出了横等离激元、强朗缪尔激元和离声激元之间相互作用满足的修正方程,并进行了数值模拟.结果表明:Compton散射使横等离激元和朗缪尔激元间的碰撞频率大大增加,随着时间的演化,横等离激元和朗缪尔激元的能量由小波数区向大波数区的转移比散射前要快得多,同时产生剧烈的坍塌.坍塌后期,等离激元的强非线性作用激发出高次共振谐波,使能量从一个谐波转移到另一个谐波,形成无限高次谐波,引起波的破碎,出现由调制不稳定性控制的强朗缪尔湍动、较强的激光成丝和能量均分现象.研究结果为进一步研究强朗缪尔湍动的加速机制、反常碰撞、激光加热实验及快点火实验提供了理论支持.     

Two-stream instabilities in degenerate quantum plasmas

The quantum effects on the plasma two-stream instability are studied by the dielectric function approach. The analysis suggests that the instability condition in a degenerate dense plasma deviates from the classical theory when the electron drift velocity is comparable to the Fermi velocity. Specifically, for a high wave vector comparable to the Fermi wave vector, a degenerate quantum plasma has larger regime of instability than predicted by the classical theory. A regime is identified, where there are unstable plasma waves with frequency 1.5 times of a normal Langmuir wave.     

Plasma-maser interaction of langmuir wave with kinetic Alfvén wave turbulence

A theoretical study is made on the generation mechanism of Langmuir mode wave in the presence of kinetic Alfvén wave turbulence in a magnetized plasma on the basis of plasma-maser interaction. It is shown that a test high frequency Langmuir mode wave is unstable in the presence of low frequency kinetic Alfvén wave turbulence. The growth of the Langmuir wave occurs due to direct and polarization coupling terms. Because of the universal existence of the kinetic Alfvén waves in large scale plasmas, the results have potential importance in space and astrophysical radiation processes.     

Effect of random density irregularities on nonlinear evolution of Langmuir waves in interplanetary medium

The evolution of nonlinear Langmuir waves in the interplanetary medium is investigated by appropriately accounting for the random density irregularities of the medium. A pair of modified Zakharov equations, which describe these waves, is solved numerically as an initial value problem for large scale (≫ 10² km) initial pertubations. For an ion acoustic-Langmuir solitary wave, the random irregularities damp the Langmuir wave by way of scattering and let the ion density perturbation radiate away in a few days. However an initial solitary or shock-like Langmuir wave excites the ion density perturbations within a fraction of a second, and then itself gets damped. These effects will strongly decelerate the collapse of large scale Langmuir waves. The possibility of detecting these processes, by means of interplanetary scintillation, is discussed. The authors felicitate Prof. D S Kothari on his eightieth birthday and dedicate this paper to him on this occasion.     

Frequency upshifting of electromagnetic radiation via obliqueincidence on an ionization front

Frequency upshifting of electromagnetic radiation impinging on a relativistically moving ionization front is theoretically investigated. Unlike previous works in this field treating the case of normal incidence and qualitatively similar case of oblique incidence of a transverse electric polarized wave, oblique incidence of a transverse magnetic polarized wave on the front is considered. The peculiarities of the case under consideration are connected with the generation of Langmuir waves behind the front and Brewster's phenomenon. We present a complete analysis of the incident wave transformation including analysis of the frequencies and amplitudes of the waves excited ahead of and behind the front. Special emphasis is made on energy transformation in the case when a wave packet is incident on the front. In particular, we show that even for negligible angles of incidence, energy losses via transformation into Langmuir waves may be very high (up to ~60%). In general, generation of Langmuir waves may play a significant role in the plasma-based radiation sources with relativistic ionization fronts     

Scattering of light waves by electron electrostatic waves in laser produced plasmas

The propagation of light waves in an underdense plasma is studied using one-dimensional Vlasov-Maxwell numerical simulation.It is found that the light waves can be scattered by electron plasma waves as well as other heavily and weakly damping electron wave modes,corresponding to stimulated Raman and Brilluoin-like scatterings.The stimulated electron acoustic wave scattering is also observed as a high scattering level.High frequency plasma wave scattering is also observed.These electron electrostatic wave modes are due to a non-thermal electron distribution produced by the wave-particle interactions.The collision effects on stimulated electron acoustic wave and the laser intensity effects on the scattering spectra are also investigated.     

Electron exchange-correlation effects on dispersion properties of electrostatic waves in degenerate quantum plasmas

Based on the quantum Magnetohydrodynamic (QMHD) model, the obliquely propagation of electrostatic waves in degenerate magnetized quantum plasmas with electron exchange-correlation effects are theoretically investigated. The modified linear dispersion relations of electrostatic waves are obtained and discussed in some specific cases. The analytical results clearly show that the dispersion properties of the high frequency electron waves (including the Langmuir wave and upper-hybrid wave) and the low frequency ion acoustic wave are modified by the quantum effects together with the electron exchange-correlation effects. The numerical results depict that the Langmuir wave and upper-hybrid wave can be unstable in the presence of the electron exchange-correlation effects, and it is also evidently indicated that the electron exchange-correlation effects can reduce the phase velocity of the waves, especially in the high wave number region. The corresponding results should be of relevance for identifying electrostatic fluctuations which transport in an inhomogeneous and magnetized quantum plasmas.     

Theory of cavitons in complex plasmas

Nonlinear coupling between Langmuir waves with finite amplitude dispersive dust acoustic perturbations is considered. It is shown that the interaction is governed by a pair of coupled nonlinear differential equations. Numerical results reveal the formation of Langmuir envelope solitons composed of the dust density depression created by the ponderomotive force of bell-shaped Langmuir wave envelops. The associated ambipolar potential is positive. The present nonlinear theory should be able to account for the trapping of large amplitude Langmuir waves in finite amplitude dust density holes. This scenario may appear in Saturn's dense rings, and the Cassini spacecraft should be able to observe fully nonlinear cavitons, as presented herein. Furthermore, we propose that new electron-beam plasma experiments should be conducted to verify our theoretical prediction.     

Propagation dynamics of relativistic electromagnetic solitary wave as well as modulational instability in plasmas

By one-dimensional particle-in-cell(PIC) simulations, the propagation and stability of relativistic electromagnetic(EM) solitary waves as well as modulational instability of plane EM waves are studied in uniform cold electron-ion plasmas.The investigation not only confirms the solitary wave motion characteristics and modulational instability theory, but more importantly, gives the following findings. For a simulation with the plasma density 10²³ m^-3 and the dimensionless vector potential amplitude 0.18, it is found that the EM solitary wave can stably propagate when the carrier wave frequency is smaller than 3.83 times of the plasma frequency. While for the carrier wave frequency larger than that, it can excite a very weak Langmuir oscillation, which is an order of magnitude smaller than the transverse electron momentum and may in turn modulate the EM solitary wave and cause the modulational instability, so that the solitary wave begins to deform after a long enough distance propagation. The stable propagation distance before an obvious observation of instability increases(decreases) with the increase of the carrier wave frequency(vector potential amplitude). The study on the plane EM wave shows that a modulational instability may occur and its wavenumber is approximately equal to the modulational wavenumber by Langmuir oscillation and is independent of the carrier wave frequency and the vector potential amplitude.This reveals the role of the Langmuir oscillation excitation in the inducement of modulational instability and also proves the modulational instability of EM solitary wave.     

Hydrodynamic analysis of electrostatic counter-streaming instability in a spin-polarized electron–positron–ion plasma

In this study, we present linear analysis of electrostatic counter-streaming instability in spin-polarized electron–positron–ion (e-p-i) plasma. With the aid of the separate spin evolution-quantum hydrodynamic (SSE-QHD) model, we derive the dispersion relation of counter-streaming instability. We numerically solve the dispersion and find four wave solutions: Langmuir wave, positron acoustic mode, and two electron and positron spin-dependent waves. It is noted that coupling of streaming and spin effects excites Langmuir instability and positron acoustic mode instability. However, in the absence of spin effect, only Langmuir instability will survive in e-p-i plasma. We have also discussed the effects of positron concentration, streaming speed, and spin polarization on the real frequency of waves and the growth rate. The present study may be helpful for understanding longitudinal wave propagation and instabilities in dense magnetized environments.     

Evolution of a langmuir wave in a weakly inhomogeneous plasma with a positive concentration gradient

Spatial evolution of a Langmuir wave excited by external sources in a weakly inhomogeneous electron plasma without external sources is considered for a small positive gradient of the plasma concentration in the direction of propagation of the wave. At the first state of the evolution, the dispersion of the wave is close to linear. When the phase velocity is doubled, the second stage of the evolution begins. The wave loses its individuality and becomes a hybrid of two waves. Its profile acquires the shape of an alternating sequence of fragments of these waves. The wave dispersion is determined by the dispersion of each fragment. In the course of evolution, the spacing between the equilibrium values of the wave fragments increases; as a result, the wave decays into two waves, which are also loaded by trapped electrons. Prior to decay, the humps of the wave become steeper; as a result, at the instant of the decay, the wave is transformed into a sequence of solitons with different polarities.     

Particle Simulation of Electrostatic Waves Driven by an Electron Beam

A one-dimensional electrostatic particle-in-cell simulation is performed to study electrostatic wave excitation due to an electron beam in a plasma system. The excited fundamental and harmonic waves are analyzed with the fast Fourier transformation and the wavelet transformation. The second harmonic is suggested to be generated by wave-wave coupling during the nonlinear evolution, which involves forward propagating and backward propagating Langmuir waves. Furthermore, the background electrons may be heated and accelerated by the electrostatic waves.     

Resonant Joule phenomena in a magnetoplasma

The present state of research of resonant Joule interactions of collisional plasmas with electromagnetic waves, including both the problems of plasma heating and wave dynamics, is reviewed. The controlled development of non-linear wave processes in gaseous and solid-body plasmas in radiowave and microwave ranges via resonant heating is discussed. The series of thermal bistability effects, produced by electron-temperature hysteresis near Langmuir and cyclotron resonances, is considered. The geometrical resonances of absorption in layered structures and bounded volumes are illustrated. Localization of dissipation phenomena near resonant regions in heterogeneous and anisotropic magnetoplasmas is analyzed. Relativistic and quantum effects in resonant collisional attenuation of waves in a plasma are shown. Some analogous tendencies in Joule wave phenomena are marked in plasmas characterized by very different physical conditions-from laboratory devices up to cosmic objects.     

Laboratory modeling of physical processes in the ionosphere modified by powerful radio emission

We present the results of the laboratory modeling of physical processes occurring in the ionosphere during active experiments on the ionospheric modificaton by powerful radio emission. The process of nonuniform thermo-diffusion of a magnetoplasma due to local electron heating is studied under the conditions modeling the ionospheric F layer. It is revealed by direct measurements that thermo-diffusion and diffusion are accompanied by excitation of macroscopic eddy currents. In this case, electrons and ions diffuse along and across the magnetic field, respectively, and the eddy current is carried by particles of the background plasma. As a result, a magnetic-field-aligned density depletion rapidly forms in the plasma. The possibility of trapping and guided propagation of Langmuir waves in such a plasma inhomogeneity is demonstrated. Conditions are found under which the wave trapping and the formation of the inhomogeneity occur in a self-consistent regime, i.e., Langmuir waves are trapped in a small-scale inhomogeneity which, in turn, is formed due to local plasma heating by the field of the trapped waves. Such nonlinear wave trapping takes place only above a certain threshold, which significantly increases in the vicinity of gyroharmonics. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 8, pp. 731–746, August 2007.