Self-focusing of Gaussian laser beam in collisionless plasma and its effect on stimulated Raman scattering process

This paper presents an investigation of self-focusing of Gaussian laser beam in collisionless plasma and its effect on stimulated Raman scattering process. The pump beam interacts with a pre-excited electron plasma wave thereby generating a back-scattered wave. On account of Gaussian intensity distribution of laser beam, the time independent component of the ponderomotive force along a direction perpendicular to the beam propagation becomes finite, which modifies the background plasma density profile in a direction transverse to pump beam axis. This modification in density affects the incident laser beam, electron plasma wave and back-scattered beam. We have set up the non-linear differential equations for the beam width parameters of the main beam, electron plasma wave, back-scattered wave and SRS-reflectivity by taking full non-linear part of the dielectric constant of collisionless plasma with the help of moment theory approach. It is observed from the analysis that focusing of waves greatly enhances the SRS reflectivity.     

Stimulated Raman scattering of gaussian laser beam in relativistic plasma

This paper presents an investigation of Stimulated Raman Scattering of gaussian laser beam in relativistic Plasma. The pump beam interacts with a pre-excited electron plasma wave and thereby generate a back-scattered wave. Due to intense laser beam, electron oscillatory velocity becomes comparable to the velocity of light, which modifies the background plasma density profile in a direction transverse to pump beam axis. The relativistic non-linearity due to increase in mass of the electrons effects the incident laser beam, electron plasma wave and back-scattered beam. We have set up the non-linear differential equations for the beam width parameters of the main beam, electron plasma wave, back-scattered wave and derived SRS back-reflectivity by taking full non-linear part of the dielectric constant of relativistic plasma with the help of moment theory approach. It is observed from the analysis that self-focusing of the pump beam greatly affects the SRS reflectivity, which plays a significant role in laser induced fusion.     

黑腔中高斯型激光束偏折的模拟研究

利用三维激光等离子体相互作用流体程序LAP3D数值模拟研究了高斯光束在均匀等离子体传播过程中的激光束偏折现象,分别考察了激光强度变化和等离子体横向流的流速变化对束偏折的影响。模拟结果表明:当流速接近离子声速时,激光束偏折效应最明显,此时激光偏折程度与激光强度成正比;当流速逐渐小于离子声速时,激光束偏折行为受抑制逐渐减弱;而当流速逐渐大于离子声速时,激光光强包络的变化以衍射效应为主。     

PHOTON ACCELERATION DRIVEN BY AN INTENSE LASER PULSE

Interaction of a laser field with a plasma wave is studied by metric optics. Analysis shows that the frequency upshifting of the laser pulse results from the plasma density gradient. A laser beam can be thought of as a packet of photons moving in a plasma and thus the laser frequency upshifting is equivalent to photon acceleration. Examination of the three-dimensional motion equations shows that a laser beam diffraction occurs in the presence of a radial variation of the plasma density. It is argued that the focusing mechanism originating from the plasma wave can curb laser diffraction so that photons may be trapped in the plasma wave and accelerated continuously.     

Enhanced Raman Scattering of Elliptical Laser beam in a Collisional Plasma

This paper presents the Enhanced Raman scattering of a elliptical laser beam in a collisional plasma. We have considered the mechanism of non‐uniform heating of carriers along the wave‐front, which is important in collisional plasma. The nonlinearity arising through non‐uniform heating leads to redistribution of carriers, which modifies the background plasma density profile in a direction transverse to pump beam axis. This modification in density effects the incident laser beam, plasma wave and back‐scattered beam. Non‐linear differential equations for the beam width parameters of pump laser beam, plasma wave and back‐scattered beam are set up and solved numerically. Numerical results predict the effect of self‐focusing of waves on the back‐scattered beam (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stimulated Brillouin scattering of elliptical laser beam in collisionless plasma

This paper presents an investigation of self-focusing of elliptical laser beam in collisionless plasma and its effect on stimulated Brillouin scattering. The pump beam interacts with a pre-excited ion-acoustic wave leading to Brillouin back-scattered process. The transverse intensity gradient of a pump beam generates a ponderomotive force, which modifies the background plasma density profile in a direction transverse to pump beam axis. This modification in density effects the incident laser beam, ion-acoustic wave and back-scattered beam. Non-linear differential equations for the beam width parameters of pump laser beam, ion-acoustic wave and back-scattered beam are set up and solved numerically. It is observed from the analysis that the focusing of waves enhances the SBS back-reflectivity.     

Allowable propagation of short pulse laser beam in a plasma channel and electromagnetic solitary waves

Nonparaxial and nonlinear propagation of a short intense laser beam in a parabolic plasma channel is analyzed by means of the variational method and nonlinear dynamics. The beam propagation properties are classified by five kinds of behaviors. In particularly, the electromagnetic solitary wave for finite pulse laser is found beside the other four propagation cases including beam periodically oscillating with defocussing and focusing amplitude, constant spot size, beam catastrophic focusing. It is also found that the laser pulse can be allowed to propagate in the plasma channel only when a certain relation for laser parameters and plasma channel parameters is satisfied. For the solitary wave, it may provide an effective way to obtain ultra-short laser pulse.     

激光支持等离子体爆轰波温度的实验测量

 高能量激光聚焦空气产生等离子体,等离子体进一步吸收激光能量会形成激光支持等离子体爆轰波。等离子体爆轰波温度是表征爆轰波的一个重要参数,研究等离子体爆轰波温度对于深入了解激光支持等离子体爆轰波形成机理有重要意义。分析了激光聚焦空气形成等离子体爆轰波过程和影响等离子体爆轰波温度的主要因素。采用多通道瞬态光学高温计,测量了不同激光发射能量下空气中形成的激光支持等离子体爆轰波的辐射强度,获得了一系列等离子体爆轰波温度动态变化曲线。测量结果表明：等离子爆轰波温度在随时间演化过程中出现3个峰,最高温度在7 000～10 000 K范围内;激光能量与等离子体爆轰波温度没有明显的相关性。     

Second Harmonic Generation of Self Focused Cosh‐Gaussian Laser Beam in Collisionless Plasma

This paper presents an investigation of self‐focusing of a Cosh‐Gaussian (ChG) laser beam and its effect on second harmonic generation in collisionless plasma. In the presence of ChG laser beam the carriers get redistributed from high field region to low field region on account of ponderomotive force as a result of which a transverse density gradient is produced in the plasma which in turn generates an electron‐plasma wave at pump frequency. Generated plasma wave interacts with the incident laser beam and hence generates its second harmonics. Moment theory has been used to derive differential equation governing the evolution of spot size of ChG laser beam propagating through collisionless plasma. The differential equation so obtained has been solved numerically. The effect of decentered parameter, intensity of ChG laser beam and density of plasma on self‐focusing of the laser beam and second harmonic yield has been investigated. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of a laser ripple in a magnetoplasma and its effect on plasma wave excitation

Summary This paper presents an investigation of the growth of a radially symmetrical ripple, superimposed on a Gaussian laser beam in a collisionless magnetoplasma. The effect of the magnetic field and the intensity of the laser on the growth of the ripple is presented in some detail. The effect of the presence of the ripple on the excitation of an electron plasma wave is also investigated. Coupling of a weak plasma wave with the main laser beam is through the modified background density. The combined effect of increased intensity of the laser beam and magnetic field is observed to suppress the growth of the ripple as well as the excitation of the plasma wave. The authors of this paper have agreed to not receive the proofs for correction.     

Dynamics of quadruple laser beams in collisionless plasmas

Dynamics of a quadruple laser beam propagating through collisionless plasma has been investigated. The dielectric function of the plasma has been obtained for ponderomotive nonlinearity. Moment theory has been used to find the solution of nonlinear Schrodinger wave equation for the field of laser beam. Particular emphases is put on the variations of both the beam width and longitudinal phase delay with distance of propagation through the plasma. Self-trapping of the laser beam has been also investigated.     

A New Scheme for High‐Intensity Laser‐Driven Electron Acceleration in a Plasma

We propose a new approach to high‐intensity relativistic laser‐driven electron acceleration in a plasma. Here, we demonstrate that a plasma wave generated by a stimulated forward‐scattering of an incident laser pulse can be in the longest acceleration phase with injected relativistic beam electrons. This is why the plasma wave has the maximum amplification coefficient which is determined by the acceleration time and the breakdown (overturn) electric field in which the acceleration of the injected beam electrons occurs. We must note that for the longest acceleration phase the relativity of the injected beam electrons plays a crucial role in our scheme. We estimate qualitatively the acceleration parameters of relativistic electrons in the field of a plasma wave generated at the stimulated forward‐scattering of a high‐intensity laser pulse in a plasma. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of channel alternating corrugation on a laser beam propagation in plasmas

The propagation of an intense laser beam in an alternating corrugated plasma channel, which has a wide region and a narrow region in one corrugated space period, is investigated. Compared with the usual corrugated plasma channel, it is found that there are many more resonance peaks and many more abundant beat-like wave phenomena for the laser beam in this extended channel. Moreover, the much narrower region in some special alternating corrugated channels can play the role like a plasma lens for some laser beams, i.e., it can obviously change the laser spot size from small (large) to large (small) amplitude when the laser passes through the narrow region. These results are well confirmed by the final numerical simulations of wave equation and particle-in-cell approach.     

线性偏振激光在相对论等离子体中的调制不稳定性

 从相对论等离体中电磁波的非线性色散方程出发,利用Karpman方法获得了线性偏振波模所满足的非线性控制方程,在非线性色散方程和非线性控制方程的基础上对线性偏振激光在相对论等离体中传播的调制不稳定性进行分析,给出了调制不稳定的时间增长率与扰动态波数之间的函数关系。     

强激光束在等离子体通道中传输的变分法研究

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Non‐linear interaction of a q‐Gaussian laser beam in a plasma channel created by the ignitor‐heater technique

This paper presents a theoretical investigation of the propagation characteristics of a q‐Gaussian laser beam propagating through a plasma channel created by the ignitor‐heater technique. The ignitor beam creates the plasma by tunnel‐ionization of air. The heater beam heats the plasma electrons and establishes a parabolic channel. The third beam (q‐Gaussian beam) is guided in the plasma channel under the combined effects of density non‐uniformity and non‐uniform ohmic heating of the plasma channel. Numerical solutions of the non‐linear Schrodinger wave equation (NSWE) for the fields of laser beams are obtained with the help of the moment theory approach. Particular emphasis is placed on the dynamical variations of the spot size of the laser beams and the longitudinal phase shift of the guided beam with the distance of propagation.     

由强激光产生的烧蚀等离子体随时间演化的数值模拟

采用单流体模型描述由强激光产生的烧蚀等离子体,并用具有五阶精度的广义Godunov差分格式——加权本质无振荡格式对该模型进行离散化,考虑激光与等离子体相互作用和能量耦合,数值模拟强激光与固体靶相互作用时产生的烧蚀等离子体随时间演化的物理过程,给出数值模拟结果,并对其进行分析和讨论.数值模拟结果表明,激光能量在靶面等离子体中被强烈吸收,激光支持LSD(Laser Supported Detonation)波速度约为理想LSD波速度的一半.     

黑腔中空间束匀滑光束成丝不稳定性的数值模拟北大核心CSCD

为深入研究黑腔内激光成丝不稳定性产生和发展,建立了空间束匀滑光束的传播模型,利用三维激光等离子体相互作用程序LAP3D模拟了黑腔物理条件下的大尺度成丝现象。模拟结果可正确描述成丝后光强的束发散现象及谱空间光强分布特征。从激光能量参数角度分析了导致成丝不稳定性发展的物理因素,并讨论了应用空间束匀滑技术抑制成丝不稳定性的局限性。研究表明:空间束匀滑光束发展成丝的能力取决于光斑内小焦斑的成丝表现,当具备成丝能力的小焦斑份额达到一定比例后,就会导致整个空间束匀滑光束成丝,因此对于光斑平均功率密度较高的光束,须考虑结合其他束匀滑手段来抑制成丝发展。     

Direct evidence for thermal mechanism of plasma influence on shock wave propagation

Shock wave propagation through a glow discharge is studied by a double beam laser Schlieren method. A pulsed discharge is used to separate electron and other plasma related phenomena from thermal effects. The results prove the pure thermal nature of the influence of a plasma on a shock wave.     

Numerical Simulation of Plasma Flows in the Interior Cavities of Laser Fusion Targets

This paper deals with a numerical simulation and theoretical investigation of the behavior of plasma flows produced when injecting a laser beam into a target for inertial thermonuclear fusion and absorbing laser radiation in the target internal cavity. Results of two-dimensional numerical calculation are presented for laser beam interaction with the sub critical plasma contained in the gap between two coaxial shells of a laser greenhouse target. The calculations confirm the possibility of initiating a supersonic wave of electron heat conduction in the absorber of the laser greenhouse target.