飞秒激光等离子体中成丝现象的研究

采用光学阴影法和散射光成像,对飞秒激光等离子体中的局域密度涨落进行了,在实验中观察到了成丝不稳定性现象,从理论上对有质动力引起的局域密度涨落进行了分析,分析表明,这种成丝不稳定性主要是由有质动力造成的。     

超强线极化激光在e-p等离子体中传播的成丝不稳定性

基于超强线极化激光在正负电子对(e-p)等离子体中的传播方程,讨论了相互作用过程中的成丝不稳定性。得到了电磁波的非线性色散关系和不稳定性增长率,并讨论了传播过程中激光强度和等离子体温度对成丝不稳定性的影响。结果表明,成丝不稳定性的强弱主要由相互作用的非线性效应与受有质动力作用后等离子体密度的减小之间的竞争所决定。     

非局域自散焦克尔介质中空间光暗孤子成丝的理论与实验研究

对非局域自散焦克尔介质中的空间光暗孤子成丝进行了研究. 理论上从非局域非线性理论模型出发, 数值模拟研究了非局域程度和吸收系数对暗孤子成丝的影响. 当入射背景光强一定时, 非局域程度越大成丝起始点越远、成丝数量越少; 而当入射背景光强与临界光强之比一定时, 非局域程度基本不影响成丝起始点以及成丝数量, 且非局域下的成丝数量与局域下一样. 此外, 当入射背景光强一定时, 吸收系数越大成丝数量越少. 实验上通过改变染料溶液的浓度以及背景光斑的椭圆率, 分别研究了样品浓度和背景光斑椭圆率对暗孤子成丝的影响. 当入射背景平均光强一定时, 样品浓度越小成丝数量越少, 背景光斑椭圆率越小成丝数量越少; 而当入射背景平均光强与临界光强之比一定时, 样品浓度基本不影响成丝数量. 在实验中还观察到了光学冲击波现象.     

激光在高温等离子体中的成丝不稳定性数值模拟

简要介绍激光－靶耦合中,二维（ｙ,ｚ）、三维（ｘ,ｙ,ｚ）成丝不稳定性数值模拟方法及计算结果。该方法包括非局域热传导和Ｌａｎｄａｕ阻尼。二维（ｙ,ｚ）成丝过程的发展与一维双温单流体力学计算同步耦合,成丝的零级量由一维双温单流体力学计算提供。三维（ｘ,ｙ,ｚ）成丝的模拟采用固定时刻的零级量为背景进行计算。成丝过程的发展除与成丝背景有关外,还与扰动波长、入射激光强度、激光波长等有关。计算结果表明：成丝     

激光在高温等离子体中的成丝不稳定性数值模拟

简要介绍激光一靶耦合中,二维(y,z)、三维(z,y,z)成丝不稳定性数值模拟方法及计算结果。该方法包括非局域热传导和Landau阻尼。二维(y,z)成丝过程的发展与一维双温单流体力学计算同步耦合,成丝的零级量由一维双温单流体力学计算提供。三维(z,y,g)成丝的模拟采用固定时刻的零级量为背景进行计算。成丝过程的发展除与成丝背景有关外,还与扰动波长、入射激光强度、激光波长等有关。计算结果表明：成丝不稳定的产生和发展,将严重影响光束质量,府进一步深入、系统地研究。     

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由色散方程和流体方程推导得到了横等离激元和对等离子体相互作用的非线性控制方程,利用场论的方法构建非线性控制方程的拉格朗日密度函数并求得相互作用过程中守恒的等离激元数和总能量。通过标度变换发现横等离激元由于调制不稳定性将坍塌形成小尺度的局域结构。在坍塌过程中横等离激元产生的有质动力随场强的增强而增强。由于有质动力对粒子的排斥,对等离子体中将产生小尺度的密度空穴。     

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。     

强激光部分离化等离子体成丝不稳定性

从部分离化等离子体和通常的全离化等离子体的差异是存在束缚电子出发,分析强激光在部分离化等离子体中的传播和折射指数,其束缚电子加强成丝不稳定性的发展.对钕玻璃三倍频激光金靶等离子体的原子成丝不稳定性进行了计算和分析.结果表明强激光部分离化等离子体的原子成丝不稳定性显著高于相对论成丝不稳定性. 关键词： 激光等离子体相互作用 部分离化等离子体 成丝不稳定性     

相干与非相干激光成丝不稳定性的数值模拟研究

利用二维三温磁流体力学激光靶程序（ＣＡＳＴＯＲ）数值模拟了相干光和诱导空间非相干（ｉｎｄｕｃｅｄ ｓｐａｔｉａｌ ｉｎｃｏｈｅｒｅｎｃｅ ＩＳＩ）光的成丝不稳定性．用求解激光传播方程和简化流体模型的方法研究了随机相位板（ｒａｎｄｏｍ ｐｈａｓｅ ｐｌａｔｅ ＲＰＰ）光滑化技术对成丝不稳定性的影响，结果表明ＩＳＩ和ＲＰＰ去相干方法可以有条件地减弱或抑制成丝不稳定性． 关键词：     

等离子体中涡旋光束自聚焦与成丝现象的模拟研究

涡旋光束在低密度等离子体传输过程中出现的非线性物理效应与光子携带的轨道角动量(拓扑荷)存在密切关联.基于亥姆霍兹方程理论获得描述涡旋光束传输的旁轴近似方程,解析求出涡旋光束在低密度等离子体中传输的自聚焦临界功率表达式.利用分步傅里叶数值方法,针对不同参数条件下涡旋光束的自聚焦和成丝现象进行了模拟分析.研究结果表明,涡旋光束的拓扑荷数决定了自聚焦临界功率大小.涡旋光束发生成丝不稳定性现象的功率阈值和最大成丝数目与拓扑荷数存在密切关联.     

激光在等离子体中成丝特性的数值模拟

 用数值模拟方法研究激光在等离子体中成丝的发展过程及其特征参数对成丝的影响。模拟结果清晰地给出柱状成丝不稳定性的产生及发展。计算结果表明：入射激光强度、入射激光波长及等离子体背景密度是影响会聚强度的重要因素。该结果与国外报道的理论和实验研究吻合,可为研究抑制成丝提供参考。     

Spatially resolved measurements of laser filamentation in long scale length underdense plasmas with and without beam smoothing

The onset of filamentation, following the interaction of a relatively long (τ(L)?1 ns) and intense (I(L)?5×10(14) W/cm(2)) laser pulse with a neopentane filled gas bag target, has been experimentally studied via the proton radiography technique, in conditions of direct relevance to the indirect drive inertial confinement fusion scheme. The density gradients associated with filamentation onset have been spatially resolved yielding direct and unambiguous evidence of filament formation and quantitative information about the filamentation mechanism in agreement with previous theoretical modelings. Experimental data confirm that, once spatially smoothed laser beams are used, filamentation is not a relevant phenomenon during the heating laser beams propagation through typical hohlraum gas fills.     

FILAMENTATION INSTABILITY OF LASER BEAMS IN NONLOCAL NONLINEAR MEDIA

The filamentation instability of laser beams propagating in nonlocal nonlinear media is investigated. It is shown that the filamentation instability can occur in weakly nonlocal self-focusing media for any degree of nonlocality, and in defocusing media for the input light intensity exceeding a threshold related to the degree of nonlocality. A linear stability analysis is used to predict the initial growth rate of the instability. It is found that the nonlocality tends to suppress filamentation instability in self-focusing media and to stimulate filamentation instability in self-defocusing media. Numerical simulations confirm the results of the linear stability analysis and disclose a recurrence phenomenon in nonlocal self-focusing media analogous to the Fermi-Pasta-Ulam problem.     

On self-focusing of an ultrashort intense relativistic laser pulse in a plasma

The development of the spatiotemporal (filamentation) instability of a laser pulse upon excitation of a plasma wave is studied numerically and analytically. It is shown that first, as in a medium with inertialless cubic nonlinearity, the filamentation of radiation occurs and then filaments are attracted to each other. The following evolution differs weakly from the evolution of a smoothed wave beam in a medium with inertial nonlinear response.     

Density oscillations in laser produced plasma decelerated by external magnetic field

This paper presents the dynamics as well as the stability of laser produced plasma expanding across the magnetic field. Observation of some high frequency fluctuations superimposed on ion saturation current along with structuring in the pin hole images of x-ray emitting plasma plume indicate the presence of instability in the plasma. Two type of slope in the variation of x-ray emission with laser intensity in the absence and presence of magnetic field shows appearance of different threshold intensity of laser corresponding to each magnetic field at which this instability or density fluctuation sets on. This instability has been identified as a large Larmor radius instability instead of classical Rayleigh-Taylor (R-T) instability.     

Study of electron-beam propagation through preionized dense foam plasmas

The transport of an intense electron-beam produced by the Vulcan petawatt laser through dense plasmas has been studied by imaging with high resolution the optical emission due to electron transit through the rear side of coated foam targets. It is observed that the MeV-electron beam undergoes strong filamentation and the filaments organize themselves in a ringlike structure. This behavior has been modeled using particle-in-cell simulations of the laser-plasma interaction as well as of the transport of the electron beam through the preionized plasma. In the simulations the filamentary structures are reproduced and attributed to the Weibel instability.     

Filamentation of an annular laser beam in a short wavelength laser produced plasma

We report on the first observations of periodic structures caused by the filamentation instability. These structures were observed on 0.35 μm laser irradiated planar target experiments. Comparison of these observations with self-focusing theory indicates that even though thermal filamentation may initiate the instability, ponderomotive force effects eventually dominate the actual filamentation process.     

Experimental confirmation of high-stability of fluorescence in a femtosecond laser filament in air

Femtosecond laser filamentation is particularly interesting for remote sensing pollutant in the atmosphere. In this work, we investigate the local shot-to-shot stability of the filament induced fluorescence of nitrogen in air. It is found that the root-mean square fluctuation of the fluorescence signal is at least one order of magnitude lower than that of the linear propagation case. In practice, it would contribute to improve the robustness of long distance spectroscopic analysis of the fluorescence of pollutant molecules inside the filament. We further point out that this unique property of filament induced fluorescence spectroscopy is because of the intensity clamping, a profound phenomenon of filamentation.