激光等离子体拍频波加速器中泵浦倒空的消除

泵浦倒空是限制激光等离子体拍频加速器概念成为现实的严重问题。本文从电子等离子体波拍频激发的弱相对论性方程出发,在光脉冲坐标中导出了对任意形状的泵浦脉冲消除倒空的条件。研究表明,只要泵浦光在脉冲中心有π位相突变,并且适当选取初始等离子体密度离谐,就可消除泵浦倒空。对于方形泵浦脉冲,本文数值计算了初始离谐随泵浦光强的变化关系,并证实了本文提出的消除泵浦倒空方法的正确性。 关键词：     

基于泵浦脉冲诱导等离子体通道的空心光束产生

利用飞秒脉冲诱导生成的具有渐变折射率分布的等离子体通道产生空心光束.当飞秒脉冲在非线性介质中传输时将电离产生等离子体通道,该等离子体通道具有渐变的折射率分布,等离子体通道中激光束只能在电子密度低于临界等离子体密度的区域传播,故当探测光束在等离子体通道边缘传输时将偏转形成空心光束.当泵浦光束功率为8mW时,探测光束转变为典型的空心光束结构;当泵浦光束的入射功率、重复率或等离子体通道中轴向电子密度改变时,等离子体通道中可供探测光束通过的区域将随之发生改变,这将导致空心光束中黑斑的面积发生变化.因此通过调节泵浦光束的入射功率和重复率可以控制产生空心光束的黑斑大小.     

（BaSr）TiO3晶体光诱导自泵浦位相共轭的实验研究

对（ＢａＳｒ）ＴｉＯ３晶体中光诱导自泵浦位相共轭现象进行了实验研究。发现诱导自泵浦的输出特性取决于诱导光束强度和信号光的入射条件，并在一定条件下呈现双稳性．作者认为，这是由于诱导光束改变晶体中信号光束Ｆａｎｎｉｎｇ散射强度分布的结果。     

双频强激光与等离子体相互作用中的双稳态效应

本文详细研究了双频激光驱动的朗缪尔(Langmuir)波所呈现的双稳态效应。完全相对论性地导出了郎缪尔波振幅和位相的演变方程,分别研究了朗缪尔波饱和振幅和稳态振幅对等离子体频率和泵浦光强的双稳态响应,并讨论了滞后效应在激光等离子体拍频加速器中的应用。此外,还研究了在双稳态临界点附近朗缪尔波振幅随时间的变化特性。 关键词：     

弱磁化等离子体中受激拉曼散射和受激布里渊散射抑制研究

本文针对典型激光聚变等离子体参数条件,利用动理学粒子模拟程序研究横向磁场和激光带宽在抑制受激拉曼散射(SRS)和受激布里渊散射(SBS)中的作用。模拟发现横向磁场对非均匀等离子体中SRS的非线性自共振增强有显著抑制作用,分析认为横向磁场作用于SRS激发的电子等离子体波(EPW)势阱中的俘获电子,使它们在横向上加速,对EPW造成非线性阻尼,同时减小EPW的非线性频移量,从而缩窄非均匀等离子体中SRS的自共振空间,极大降低SRS反射率。在此基础上利用横向磁场抑制SRS的特性,以及SBS增长对激光带宽的敏感性,提出了利用横向磁场和宽带激光将SRS和SBS同时抑制在低反射率水平的方案。在采用数十特斯拉横向磁场和实验中易于达到的千分之一量级的激光带宽时以及慢性约束聚变(ICF)相关参数下,SBS和SRS的反射率都得到了有效抑制。     

时空非相干光孤子的传播和相干特性

研究了时空非相干椭圆形高斯光束在饱和对数型非线性介质中的自陷行为,得到了孤子的形成条件并分析了其相干特性,讨论了光束自陷的变化形式,发现光束的初态和材料非线性对其有决定性的影响, 并进一步获得了光束半径和光束相干半径的解析表达式,发现光束的相干特性不但会随传播距离产生周期性的变化,且随组份频率的增大而减弱.     

等离子体中受激Raman效应引起的近简并四波混频

本文研究等离子体中由于受激Raman散射激发起来的电子等离子体波引起的近简并四波混频。文中解析地导出此情况下等离子体介质的三阶非线性极化率,并讨论其位相匹配条件。通过求解偶合波方程得到位相复共轭反射率,并详细研究该反射率与泵浦光和探针光的频率差以及与相互作用长度的关系。     

自加速光的调控及其新奇应用

实现自弯曲光乃至自回旋光一直是人们的梦想与科幻的题材。近年来,艾里光束以及推广的自加速光束因其无衍射、自弯曲传输以及自愈等奇异特性引起了人们极大的研究兴趣。这些光束的构想不仅得到了实验证实,而且具有广泛的应用前景,包括操控微纳颗粒、等离子体通道和表面等离子体激元、电子加速、精密成像、湍流传输、引导放电等,这些应用前景使得自加速光倍受青睐,成为一个广泛关注和激动人心的前沿热点课题。本文简单综述了基于相位调制的自加速光束的研究进展,包括自加速光束的产生和传输特性,以及其在空域和时域的推广与调控,并着重介绍自加速光在若干领域的新奇应用。     

泵浦光束质量对受激拉曼散射的影响

主要讨论泵浦光束质量对低气压拉曼种子源特性的影响。文中给出了一阶斯托克斯光泵浦阈值、输出能量和光束质量的实验数据。用Ｍ＾２因子分析了泵浦光和一阶斯托克斯光的光束质量。根据考虑泵浦聚焦和泵浦光束质量影响的近似拉曼散射理论计算了泵浦阈值的理论曲线，并与实验数据和有关文献进行了比较。     

双偏心椭圆高斯光束在一阶ABCD光学系统中的传输特性

通过求解时谐条件下的亥姆霍兹方程,得到一个特解双偏心椭圆高斯光束,该光束由两个偏心椭圆高斯光束叠加而形成的,可用于描述大功率激光二极管远场分布双峰特性.分析了该光束的光场模型,运用惠更斯-菲涅尔广义积分公式,得到了该光束在一阶ABCD光学系统中的传输场分布和解析表达式,在此理论基础上,数值计算得到光场分布和远场发散角,并运用该光束模型模拟了新型激光二极管光场分布,理论结果与实际结果吻合.     

Excitation of Electron Plasma Waves by Beating Two Self-Trapped Laser Beams

This paper presents the detailed studies of the focusing of pump beams and the excited electron plasma waves(EPW) in a PBWA(Paema-Beat-Wave-Accelerator) by beating two self-trapped laser beams. The equations governing the saturated amplitude of EPW in nonuniform pump beams have been derived analytically. With these the condition for self-trapping of pump beams has been studied self-consistently. The optimum frequency mismatch parameter and the corresponding optimum initial plasma density as well as the maximum amplitude of the EPW have been obtained analytically. The transverse distributions of the EPW for Gaussian pump profile have been calculated numerically and show peculiar characteristics for certain frequency mismatch parameters. In addition, the effect of the radial component of the EPW on the bunch of accelerated particles in PBWA has been briefly discussed.     

Electron Acceleration by Beating of Two Intense Cross-Focused Hollow Gaussian Laser Beams in Plasma

This paper presents propagation of two cross-focused intense hollow Gaussian laser beams(HGBs) in collisionless plasma and its effect on the generation of electron plasma wave(EPW) and electron acceleration process,when relativistic and ponderomotive nonlinearities are simultaneously operative. Nonlinear differential equations have been set up for beamwidth of laser beams, power of generated EPW, and energy gain by electrons using WKB and paraxial approximations. Numerical simulations have been carried out to investigate the effect of typical laser-plasma parameters on the focusing of laser beams in plasmas and further its effect on power of excited EPW and acceleration of electrons. It is observed that focusing of two laser beams in plasma increases for higher order of hollow Gaussian beams,which significantly enhanced the power of generated EPW and energy gain. The amplitude of EPW and energy gain by electrons is found to enhance with an increase in the intensity of laser beams and plasma density. This study will be useful to plasma beat wave accelerator and in other applications requiring multiple laser beams.     

Second Harmonic Generation of Self‐Focused Cosh‐Gaussian Laser Beam in Thermal Quantum Plasma by Excitation of an Electron Plasma Wave

This paper presents a scheme for second harmonic generation (SHG) of an intense Cosh‐Gaussian (ChG) laser beam in thermal quantum plasmas. Moment theory approach in W.K.B approximation has been adopted in deriving the differential equation governing the propagation characteristics of the laser beam with distance of propagation. The effect of relativistic increase in electron mass on propagation dynamics of laser beam has been incorporated. Due to relativistic nonlinearity in the dielectric properties of the plasma, the laser beam gets self‐focused and produces density gradients in the transverse direction. The generated density gradients excite electron plasma wave (EPW) at pump frequency that interacts with the incident laser beam to produce its second harmonics. Numerical simulations have been carried out to investigate the effects of laser parameters on selffocusing of the laser beam and hence on the conversion efficiency of its second harmonics. Simulation results predict that within a specific range of decentered parameter the ChG laser beams show smaller divergence as they propagate and, thus, lead to enhanced conversion efficiency of second harmonics. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of angular pump mismatch for the semi-linear oscillator

We investigate, analytically and numerically, the influence of an angular mismatch of counter-propagating pump beams on the threshold of oscillation for the photorefractive semi-linear oscillator driven by non-local non-linear response. Our analysis includes the frequency degenerate and non-degenerate regimes within a wide range of the variable input parameters—the coupling strength, the pump ratio, and the feedback mirror reflectivity. The limiting case of mirrorless oscillation is considered as well. The main outcome of our studies is the possibility of strong reduction of the oscillation threshold owing to the angular mismatch. Furthermore, increasing the mismatch induces pronounced bifurcations of the oscillation parameters, including transitions between degenerate and non-degenerate oscillation modes. The main findings are verified by direct simulations of the dynamic equations.     

Beam divergence effects on high power optical parametric oscillation

The beam divergence effects of the input pump laser on a high power nanosecond optical parametric oscillator （OPO） have been numerically simulated. The OPO conversion efficiency is affected due to the angular deviation of real laser beams from ideal phase matching conditions. Our theoretical model is based on the decomposition of the Gaussian beam and assumes each component has a single deviation angle and thus a particular wave vector mismatch. We take into account the variable intensity profile in the spatial and temporal domains of the Gaussian beam, the pump depletion effects for large-signal processes as well as the oscillatory effects of the three waves. Two nonlinear crystals β-BaB2O4 （BBO） and LiB305 （LBO） have been investigated in detail. The results indicate that the degree of beam divergence strongly influences the maximum pump intensity, optimum crystal length and OPO conversion efficiency. The impact of beam divergence is much more severe in the case of critical phase-matching for BBO than in the case of non-critical phase-matching for LBO. The results provide a way to choose the optimum parameters for a high power ns OPO such as the nonlinear material, the crystal length and the pump intensity, etc. Good agreement is obtained with our experimental results.     

Efficient phase conjugation by Brillouin enhanced four wave mixing

It is shown that frequency detuning and phase mismatch effects can strongly effect the reflection coefficient of Brillouin enhanced four-wave mixing. Extremely large reflectivities (R > 10⁶) can be obtained if the intensities and frequency of the pump beams and the length of the interaction region all take on certain critical values. This theory accounts for the high reflectivity which has been reported by another author.     

Stimulated raman scattering from plasma modes in magnetoactive semiconductors

Stimulated scattering off electron plasma mode is investigated analytically for the case when the pump wave is an intense circularly polarised electromagnetic wave propagating parallel to a homogeneous dc magnetic field in an isotropic semiconductor-plasma. The threshold electric field of the pump necessary for the stimulated Raman scattering and the growth rate of the parametrically unstable mode have been obtained for two cases (i)B ₀=0 and (ii) B₀ ≠ 0. It is seen that the magnetic field does not significantly affect the threshold electric field as well as the growth rate provided the cyclotron frequency is small compared to the frequency of the pump wave. The threshold conditions are also found to be insensitive to the electron thermal velocity.     

Waveguide propagation of laser beams in media with saturating non-linearity

The waveguide propagation of Gaussian laser beams in dielectric media with saturating non-linearity has been investigated analytically, using two profiles for the dependence of the dielectric constant on intensity. An expression for the radius of the uniform wave-guide, corresponding to a given power of the beam, has been obtained and the existence of a minimum radius, corresponding to an optimum power, has been predicted. For high power laser beams, oscillatory waveguides have been obtained; for very high powers the oscillations of the radius of the waveguide are small.Work supported by NOAA, USA.Work supported by CSIR, India.Work supported by NSF, USA; on sabbatical leave from IIT, Delhi.     

Enhanced Beat Wave Saturation Amplitude in an Ionizing Plasma

The excitation of a plasma wave by two laser beams, whose frequency difference is near the plasma frequency, is studied in a plasma with a density that is slowly increasing with time due to ongoing ionization as appropriate for experiments done in laser breakdown plasmas. Numerical integration of the relativistic equation for the evolution of the wave amplitude reveals that for a rate of increase of the plasma density of approximately 1017 cm-3/ns at a laser intensity I = 1014 W/cm2, the wave amplitude can rise considerably above the relativistic saturation limit of Rosenbluth and Liu which was obtained for a plasma of constant density. This increase in plasma density compensates the reduction in plasma frequency caused by the relativistic electron mass increase when the wave amplitude is large. The frequency and phase excursions of the plasma wave are reduced for an optimum time increasing density. We find that moderate damping can stabilize both the amplitude and the phase of the plasma wave with respect to the pump.