一维动理学数值模拟激光与等离子体的相互作用

利用一维(1D3V)、显式、全电磁、相对论粒子模拟代码研究动理学范畴内激光与等离子体相互作用中的受激拉曼散射,给出了粒子代码的控制方程及其数值离散的详细方案,研究表明:动理学效应在受激拉曼散射不稳定性中十分重要;时问平均的反射率在阈值强度处跃升,在更高的激光强度处达到饱和;受激拉曼背向散射周期性地在次皮秒内爆发,离子效应延迟背向拉曼散射的发生;电子俘获导致了背向拉曼散射出现爆发;Langmuir波的非线性频移使得背向散射达到饱和。     

利用一维Vlasov和Maxwell方程模拟激光等离子体相互作用

应用一维Vlasov和Maxwell耦合程序详细研究了激光等离子体相互作用中的基本问题——受激拉曼散射(SRS). 通过研究发现, SRS的产生与电子速度分布函数在相空间中的结构密切相关, 当电子速度分布函数形成相空间涡旋时,背向SRS光大幅增加,而当电子等离子体波相速度附近的电子速度分布函数曲线变平坦后, 背向SRS光基本停止发生. 在模拟中观测到了SRS的爆发、电子速度分布函数形成相空间涡旋、电子俘获等清晰的物理图像. 关键词： Vlasov-Maxwell模拟 受激拉曼散射     

激光等离子体受激散射的线性理论研究

利用包含两种阻尼(Landau阻尼和碰撞阻尼)成分的受激拉曼散射(SRS)、受激布里渊散射线性分析程序LIP,在给定的等离子体状态下分析了阻尼成分、激光强度以及等离子体组分对点火尺度等离子体中受激散射不稳定性发展的影响.详细评估了碰撞阻尼的效应,发现碰撞阻尼在接近1/4临界密度处能有效抑制受激散射的增长,从而造成SRS光谱上的"缝"现象.同时,电子温度的升高会在特定的密度区域促进SRS的发展.研究结果可为点火实验的设计提供参考.     

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。     

用染料激光增益降低二元混合物中少量化合物的受激拉曼散射可探测浓度

在二元混合物构成的圆形谐振腔中,用染料的激光增益增强了少量化合物(乙醇溶液中的甲醇、水溶液中的乙醇)的受激拉曼散射（SRS）光谱,将少量化合物在悬垂液滴中的瞬态SRS探测极限提高了近一个数量级. 用受激拉曼散射理论,解释了增强少量化合物SRS的机理. 关键词： 受激拉曼散射 染料激光增益 二元混合物 最小可探测浓度     

三角激光脉冲尾波加速粒子模拟

电子俘获是激光尾波场加速电子的主要机理，增大电子的初速度可以使更多的电子被尾波场俘获.提出三角脉冲激发尾波加速电子的方案，三角脉冲平缓上升沿激发受激Raman散射，用以初步加速电子，三角脉冲陡峭下降沿激发尾波场，将更多的电子加速到接近光速.2D3V粒子模拟结果证实了这一点.同时表明：脉冲长度为几个等离子体波长的超强激光在稀薄等离子体中传播时，还激发侧向Raman散射.在侧向受激Raman散射中，静电波增长最快的波矢模式为k_p=(2ω_p/ω₀ 关键词： 有质动力 电子俘获 前向受激Raman散射 侧向受激Raman散射     

小能量激光等离子体的受激Raman散射

研究了激光平面靶耦合时受激Raman散射(SRS)的时间分辨光谱-在小能量激光入射平面靶时，SRS主要受激光等离子体电子密度分布的影响-在激光靶耦合初期，等离子体电子密度呈指数分布，SRS因对流阈值较高而被抑制，耦合后期平面靶被烧穿后，等离子体电子密度呈高斯分布，SRS就在均匀密度处绝对增长起来-多种材料构成的靶以及成丝存在，加剧了等离子体电子密度平台的出现，也增加了SRS光的发射- 关键词：     

黑腔等离子体中SRS与SBS过程的散射光谱分析

针对惯性约束聚变中激光通过等离子体时,在光路上激发的受激Raman散射(SRS)与受激Brillouin散射(SBS)过程进行了研究。介绍了用于分析两种不稳定性过程背向散射光谱特征的程序模型,数值模拟了不同等离子体状态参数条件下的SRS与SBS过程,讨论了黑腔内等离子体参数条件对两种不稳定性过程及其散射光谱特征的影响,提出了有益于抑制两种不稳定过程的等离子体参数条件。     

非Maxwell电子速度分布对受激散射的影响

从考虑动理学效应的受激散射不稳定性的线性理论出发,对于n=2的Maxwell分布函数直到n=5的饱和情况的超高斯分布函数,计算了电子等离子体波和离子声波的频率和阻尼率。对受激喇曼散射和受激布里渊散射进行分析,结果表明,在激光高Z等离子体中,或者在具有激光热斑的中等Z等离子体中,电子等离子体波的阻尼率降低很多,离子声波的频率比Maxwell分布情况升高约15%。这些结果可和实验进行比较。     

多色非相干光入射ICF黑腔受激布里渊散射分析

为研究多色非相干激光入射ICF黑腔的受激布里渊散射（SBS）和受激拉曼散射（SRS）发展情况,建立了一维受激散射稳态谱分析模型,并发展了相应的数值模拟程序。分析了不同频率激光激发的受激散射光通过共用等离子体波耦合的物理图像以及影响背散光谱的物理因素。针对波长差0.3 nm的等强度双色光入射封底金腔的SBS进行了模拟分析,结果表明:采用双色光有效抑制了SBS;SBS光谱劈裂成间距为0.3 nm的两个峰;波长较长的入射光对应的SBS光获得了较大的增益;如果固定激光总强度和总带宽,则存在抑制SBS的最优光束数目。     

Saturation of backward stimulated scattering of a laser beam in the kinetic regime

Stimulated Raman (SRS) and Brillouin scattering (SBS) are examined in the kinetic regime using particle-in-cell simulations. Wave front bowing of electron-plasma waves (ion-acoustic waves) from trapped particle nonlinear frequency shift is observed in the SRS (SBS) regime for the first time. Self-focusing from trapped particle modulational instability (TPMI) is shown to occur in 2D and 3D SRS simulations. The key physics of SRS saturation is identified as a combination of wave front bowing, TPMI, and self-focusing: Bowing marks the beginning of SRS saturation and self-focusing terminates SRS. Ion-acoustic wave bowing also contributes to SBS saturation. Velocity diffusion by transverse modes and rapid loss of hot electrons in regions of small transverse extent formed from self-focusing dissipate wave energy and increase Landau damping, despite trapping that reduces Landau damping initially.     

Transient development of backward stimulated Raman and Brillouin scattering on a picosecond time scale measured by subpicosecond thomson diagnostic

The excitation and the relaxation of the plasma waves and ion acoustic waves (IAW), respectively, driven by stimulated Raman (SRS) and Brillouin (SBS) backscatterings have been experimentally investigated with short-pulse lasers. The spectra have been obtained with a 0.3 ps time resolution. It is shown that SRS develops before SBS and suddenly decays around the peak of the pump, as the IAW reaches saturation. On this short time scale, electron kinetic effects play a major role for SRS saturation, contrary to ion dynamics. These results are supported by particle-in-cell simulations.     

Transient enhancement and detuning of laser-driven parametric instabilities by particle trapping

Kinetic simulations of backward stimulated Raman scattering (BSRS), where the Langmuir wave coherence time is greater than the bounce time for trapped electrons, yield transient reflectivity levels far above those predicted by fluidlike models. Electron trapping reduces the Langmuir wave damping and lowers the Langmuir wave frequency, and leads to a secular phase shift between the Langmuir wave and the BSRS beat ponderomotive force. This phase shift detunes and saturates BSRS and a similar effect, due to ion trapping, is the saturation mechanism for backward stimulated Brillouin scattering. Competition with forward SRS is discussed.     

Langmuir decay instability cascade in laser-plasma experiments

Thomson scattering has been used to investigate the nonlinear evolution of electron plasma waves (EPWs) generated by stimulated Raman scattering (SRS). Two complementary diagnostics demonstrate the occurrence of the cascade of Langmuir decay instabilities (LDI). The EPW wave-number spectrum displays an asymmetric broadening towards small wave numbers, interpreted as a signature of the secondary EPWs produced in the LDI cascade. The number of cascade steps is in agreement with the broadening of the associated ion-acoustic-waves' spectra. The total energy transferred in the EPWs cascade is found to be either less than or of the same order of magnitude as the energy of the primary EPW.     

Observation of a transition from fluid to kinetic nonlinearities for langmuir waves driven by stimulated Raman backscatter

Thomson scattering is used to measure Langmuir waves (LW) driven by stimulated Raman scattering (SRS) in a diffraction limited laser focal spot. For SRS at wave numbers klambda(D) less similar 0.29, where k is the LW number and lambda(D) is the Debye length, multiple waves are detected and are attributed to the Langmuir decay instability (LDI) driven by the primary LW. At klambda(D) greater similar 0.29, a single wave, frequency-broadened spectrum is observed. The transition from the fluid to the kinetic regime is qualitatively consistent with particle-in-cell simulations and crossing of the LDI amplitude threshold above that for LW self-focusing.     

First observation of ion acoustic waves produced by the langmuir decay instability

Thomson scattering measurements are presented which demonstrate conclusively the occurrence of the Langmuir decay instability (LDI) in a laser-produced plasma experiment. Both products of the instability, the ion acoustic wave and the electron plasma wave, were simultaneously observed and identified with their spectral characteristics. The secondary decay of the LDI-generated electron plasma wave, into another Langmuir wave and an ion acoustic wave, has been observed for the first time. The connection with growth and saturation of the stimulated Raman instability is discussed.     

Nonlinear nature of kinetic undamped waves induced by electrostatic turbulence in stimulated Raman backscattering

The influence of low-frequency waves of kinetic nature induced by electron trapping in backward Stimulated Raman Scattering (SRS) is investigated. Semi-lagrangian Vlasov-Maxwell simulations are carried out not only for periodic boundary conditions but also in the case of an open plasma with parabolic shape, in optical mixing. We provide a numerical example of generation of KEEN (kinetic electrostatic electron nonlinear) waves nonlinearly induced from the SRS through a mechanism we first here elucidate. In particular we identify a process of backward scattering of the SRS probe light from the so generated KEEN waves, which may provide a mechanism for the possible experimental observation and measurement of such nonlinear structures.     

水热法掺杂的Yb^3＋对NiYbxFe2-xO4铁氧体磁性和形貌的影响

本文以研究稀土元素镱掺杂对镍铁氧体磁性能的影响用为目的,采用水热法,去离子水作为溶剂,用硝酸镍（Ni（NO3）2·6H2O）,硝酸铁（Fe（NO）3·9H2O）,硝酸镱Yb（NO3）3·5H2O,氢氧化钠（NaOH）为原料,制备出镍铁氧体NiYbxFe2-xO4纳米粉体.利用X射线衍射（XRD）,X射线能谱仪（EDS）,扫描电镜（SEM）,透射电子显微镜（TEM）,振动样品磁强计（VSM）在成分,定性定量和形貌等方面进行了表征.结果表明,稀土元素镱掺杂将会降低镍铁氧体纳米粉体的晶粒尺寸.稀土离子磁矩的磁稀释作用导致镍铁氧体纳米磁性材料的饱和磁化强度降低,并且随着掺杂量的逐渐增加,样品的饱和磁化强度和矫顽力逐渐降低.