飞秒强激光脉冲在等离子体中的自压缩

文章从激光等离子体相互作用的非线性薛定谔方程出发,理论研究了飞秒强激光脉冲在等离子体中的自压缩行为.结果表明在一定范围内随着激光脉冲宽度、激光强度的增大以及等离子体密度的减小,飞秒强激光脉冲在等离子体中传播的自压缩现象越明显.另外通过适当设定参量得到了近似稳定传播的基孤子.     

电子等离子体波对超短脉冲激光传播过程的作用

提出等离子体波对光脉冲产生相位调制的观点,把光脉冲在背景等离子体波中传播时的频率移动与脉宽压缩效应统一起来。给出了脉宽演化的方程,它类似于在势阱中运动的经典粒子的能量守恒方程;估算了脉宽压缩对应的等离子体波振幅阈值。数值计算进一步证实这些结论。最后指出实验上用等离子体波压缩短脉冲激光脉宽的可能性。 关键词：     

激光脉冲能量对激光诱导铝合金等离子体物理特性的影响

为研究激光脉冲能量对激光诱导等离子体辐射特性和膨胀过程的影响,采用ICCD相机对不同激光脉冲能量激发的铝合金等离子体进行快速成像,并利用Boltzmann斜线法和Stark展宽法分析等离子体的电子温度和电子数密度随激光脉冲能量的演化规律.实验结果表明,激光诱导等离子体呈现明显的分层结构,等离子体的激发阈值约为3mJ,等离子体不同区域的面积随激光脉冲能量变化呈现不同的特征.当激光脉冲能量低于10mJ时,等离子体的分层结构不显著.激光脉冲能量从10mJ增加到100mJ过程中,等离子体电子温度从4 980K升高到7 221K,等离子体的电子数密度在1017 cm-3量级并随激光能量增加而增大且趋于饱和.     

激光等离子体相互作用的数值模拟

 基于激光等离子体相互作用的复杂物理过程的数学模型，采用PIC方法分别研究了P极化和S极化非均匀短脉冲强激光入射均匀分布的稠密等离子体时引起的空泡、成丝等物理现象。模拟了激光脉冲在真空中的3维传播形貌。由3维密度分布图发现：激光产生的巨大的有质动力向两侧推动粒子，形成等离子体密度通道；当激光脉冲入射等离子体区域后，纵向加速的电子速度峰值出现在电流峰值处。     

激光脉冲对内层等离子体辐射源的影响

本文的基本思想是设计双层金铝薄膜靶以检测激光脉冲宽度与等离子体消融深度的关系,找出有效的等离子体加热方法以产生更强更亮的等离子体辐射源.由于有预脉冲激光的存在,表层金薄膜首先被消融,由主脉冲携带的大能量就能较易穿过表层金等离子体将能量聚焦在内层铝靶上,由此产生内层高温等离子体.又由于外层低温等离子体存在,其将有效的阻碍内高温等离子体因膨胀而引起的能量损失.对无预脉冲而言,直接入射激光能量都沉积在靶表层形成表层高温等离子体.但是激光直接入射而产生的等离子体辐射总强度只比由预脉冲情况下产生的金等离子体辐射强度增加15%.而预脉冲能量只占激光总能量的2%.实验结果显示Al光谱线主要来自类氢,类氦离子跃迁.Au等离子体光谱线主要来自它的N带,O带和P带谱.我们也观察到一个明显的软X射线短波发射极限.所有结果显示由于预脉冲的存在将对靶各层等离子体辐射产生极大的影响     

预脉冲激光对线状等离子体X射线发射特性的影响

报道了能量比为１：１０、间隔为２００ｐｓ、脉冲宽度为１２０ｐｓ的双脉冲激光驱动线状等离子体的Ｘ射线发射特性。通过与单脉冲驱动情况比较，获得了预脉冲作用下，等离子体轴向辐射增强的空间分布特性。并针对若干重要的辐射跃迁的增强情况，讨论了它们各自的物理原因。     

对超短脉冲激光与固体密度等离子体相互作用动力学过程的研究

结合Saha方程,建立了一个超短脉冲激光与固体密度等离子体相互作用的动力学过程中电子温度和电子密度随时间变化的简单模型.在降温过程中采用绝热膨胀模型,讨论了等离子体在绝热膨胀过程中的特殊性质,考虑了离子声速随温度的变化,推算了状态演化过程,得到了电离度随时间变化的演化规律. 关键词： 超短超强脉冲激光 固体密度等离子体 电离度     

超高亮度激光系统中光栅对脉冲整形器失调对输出脉冲时间特性的影响

在较全面地给出光栅对脉冲整形器失调产生附加相位项表达式的同时，详细地讨论了展宽器失调的附加相位放大系统传输过程中的变化，并与压缩器失调特性作了比较，分析了这些失调对超高亮度激光系统输出脉冲及其聚焦场时间特性的影响，得到了确定光栅对脉冲整形器失调容限的基本原则。     

中空等离子体通道中强激光脉冲对电子的加速

分析了中空等离子体通道内传播的强激光脉冲电磁场中注入电子的运动,发现当激光脉冲强度超过某个阈值时,电子能获得有效的纵向加速并自动与脉冲分离.所得结果为实验提供了参考和依据.     

激光脉冲重复频率对等离子体辐射特性的影响

为了提高激光诱导击穿光谱质量,采用Nd∶YAG激光器输出的纳秒脉冲激光激发产生土壤等离子体,采用光栅光谱仪和光电检测系统记录了元素谱线AlⅠ394.401 nm,BaⅠ455.403 nm,FeⅠ430.791 nm和TiⅠ498.173 nm的辐射强度和信背比,研究了激光脉冲重复频率(5,10和15 Hz)对等离子体辐射特性的影响。实验结果表明,在相同的激光输出能量条件下,当采用15 Hz的激光脉冲重复频率时,元素Al,Ba,Fe和Ti的谱线强度要比5 Hz时的分别提高50.94%,112.7%,107.46%和99.38%,光谱信背比分别提高15.16%,24.08%,40.26%和72.06%。通过测量等离子体参数,解释了激光脉冲重复频率对等离子体辐射特性的影响机理。     

Simulation of a chirped femtosecond relativistic laser pulse interaction with underdense plasma by using a hydrodynamic approach

A chirped laser pulse indicates that the laser frequency changes over the duration of the pulse: a positively (negatively) chirped pulse implies that the laser frequency increases (decreases) with time. In this paper, we use a simplified, fully relativistic hydrodynamic approach to simulate the influence of chirp on the propagation of a femtosecond relativistic laser pulse in underdense plasma. Based on this simplified cold‐fluid model, the influence of chirp on the main dynamics of the laser pulse, such as self‐steepening, red‐shift in the leading edge, variation of the frequency chirp, and the generated wakefields can be studied self‐consistently. The simulation results show that a pulse with a positive chirp results in a larger increment in the intensity parameter a₀ when propagating a certain distance into an underdense plasma compared with an un‐chirped and a negatively chirped pulse, which is largely because of a much greater forward shift of the peak amplitude and more severe pulse self‐steepening effect due to the frequency red‐shift at the leading edge when exciting a plasma wave. The ponderomotive force, which relates to the first‐order differential of the laser pulse intensity envelope, is expected to be stronger for a positively chirped pulse because of its steeper leading edge and larger intensity parameter a₀. As a result, the wakefield driven by the positively chirped laser pulse is more intense than that driven by an un‐chirped and a negatively chirped laser pulse, which is confirmed by our self‐consistent hydrodynamic simulation.     

尾波场与相对论效应对激光脉冲自相位调制及频移影响的比较研究

对共振和非共振情形下尾波场效应与相对论效应对强激光脉冲在稀薄等离子体中传输的影响 作了详细比较.尾波场效应导致脉冲自相位调制的不对称性，其大小与初始脉冲形状和脉宽 大小有关.在长脉冲极限下，尾波场效应远小于相对论效应，从而可以忽略；在短脉冲极限 下，尾波场效应可以抵消相对论效应.在共振情况下，尾波场效应导致脉冲内大部分光子减 速. 关键词： 脉冲传输 等离子体 相对论效应 尾波场效应 自相位调制     

Effects of relativistic and ponderomotive nonlinearities on the interaction of high-power laser beam with an inhomogeneous warm plasma

The influence of relativistic-ponderomotive nonlinearities and the plasma inhomogeneity on the nonlinear interaction between a high-power laser beam and a warm underdense plasma are studied. It is clear that the relativistic ponderomotive force and the electron temperature modify the electron density distribution and consequently change the dielectric permittivity of the plasma. Therefore, by presenting the modified electron density and the nonlinear dielectric permittivity of the warm plasma, the electromagnetic wave equation for the propagation of intense laser beam through the plasma is derived. This nonlinear equation is numerically solved and the distributions of electromagnetic fields in the plasma, the variations of electron density, and plasma refractive index are investigated for two different background electron density profiles. The results show that the amplitude of the electric field and electron density oscillations gradually increase and decrease, during propagation in the inhomogeneous warm plasma with linear and exponential density profiles, respectively, and the distribution of electron density becomes extremely sharp in the presence of intense laser beam. It is also indicated that the electron temperature and initial electron density have an impact on the propagation of the laser beam in the plasma and change the plasma refractive index and the oscillations' amplitude and frequency. The obtained results indicate the importance of a proper choice of laser and plasma parameters on the electromagnetic field distributions, density steepening, and plasma refractive index variations in the interaction of an intense laser beam with an inhomogeneous warm plasma.     

相位反射产生的激光场空洞现象及其与激光等离子体参数相关性研究

理论研究和数值模拟研究表明激光在等离子体中诱导的等离子体密度调制可以起类似布拉格反射镜的作用，使得后面的入射光在等离子体平均密度远低于临界密度的区域就对入射光产生相位反射.相位反射的发生会影响激光在等离子体中的传播，例如激光在等离子体中相对传播时会出现激光场空洞现象.进一步的理论和数值模拟研究表明，相位反射持续发生的时间以及反射率的高低与等离子体的密度、等离子体区域的长度、激光强度以及脉宽等因素密切相关，这些都会对激光在等离子体中的传输产生影响. 关键词： 相位反射 密度调制 激光等离子体 粒子模拟     

Characterization of relativistic electrons generated by a cone guiding laser pulse

We demonstrated the interaction of a gold cone target with a femto second(fs) laser pulse above the relativistic intensity of 1.37×10 18 μm 2 W/cm 2.Relativistic electrons with energy above 2 MeV were observed.A 25%-40% increase of the electron temperature is achieved compared to the case when a plane gold target is used.The electron temperature increase results from the guiding of the laser beam at the tip and the intense quasistatic magnetic field in the cone geometry.The behavior of the relativistic electrons is verified in our 2D-PIC simulations.     

相对论效应对激光在等离子体中的共振吸收的影响

采用一维粒子模拟(PIC)方法，研究了相对论效应对P偏振激光斜入射非均匀等离子体时产生的共振吸收的影响. 计算表明，弱相对论情况下，在临界面附近产生的电子等离子体波的相对论非线性效应占主要作用；随着入射光场的逐渐增大，吸收率逐渐降低. 当入射光强超过3.7×10¹⁷W/cm²时，由于超短激光脉冲本身在等离子体中产生相对论效应、等离子体波破裂效应，以及参量不稳定过程激发等，吸收系数随着激光强度又开始增加. 固定等离子体密度标长，取不同的激光入射角、电子初始温度，相对论效应对吸收系数的影响是一致的. 关键词： 激光等离子体 相对论效应 共振吸收 粒子模拟     

On the filamentation instability in degenerate relativistic plasmas

The filamentation instability of the electromagnetic (EM) beam in an underdense plasma with high level of degeneracy is examined by means of the momentum equation, continuity equation and Maxwell's equations. It has been demonstrated that the instability develops for weakly as well as strongly relativistic degenerate plasma and arbitrary strong amplitude of EM beams.     

The ion streaming effect on Weibel instability in the relativistic dense plasma

The Weibel instability plays an important role in stopping hot electrons and energy deposition mechanism in fast ignition of inertial fusion process. In this paper, the ion Weibel instability in counter propagating electron‐ion plasmas is investigate. The obtained results show that the growth rate of Weibel instability will be decreased about 40% with the anisotropy velocity as v_xe = 2v_ze = 20; the ion density ratio, b = n _{0i 1}/n _{0i 2}, and density gradient, are increasing 50 and 90% respectively. The ion streaming in density gradient of dense plasma leads to increasing the Weibel instability growth rate and its amplification through ion streaming in the large wavenumber. The maximum unstable wavenumber has been decreased with decreasing the ion beam density ratio. For fixed ion density ratio, increasing 90% of the density gradient in the near of fuel plasma corona leads to reducing growth rate and unstable wavenumber about 43 and 42% respectively.     

磁场中非广延相对论费米系统的稳定性

基于磁场中的非广延统计理论,运用理论解析与数值模拟方法,研究磁场中非广延极端相对论费米气体的力学稳定性,给出高温与低温下稳定性条件的解析式,分析非广延参数、极端相对论效应、磁场及温度对系统稳定性的影响机理     

Effect of electron inertia on the speed and shape of ion-acoustic solitary waves in relativistic plasma

Ion-acoustic solitary waves are investigated in a cold collisionless relativistic plasma. Electron inertia is also considered. The Sagdeevs pseudopotential is determined in terms of u, the ion speed and depends on v, the velocity of the wave. It is found that there exists a critical value of u ₁ ( 0), the value of u at which (u)² = 0, beyond which the solitary waves cease to exist. The critical value also depends on the relativistic parameter u ₀/c when u ₀ is the drift velocity of the ion and c is the speed of light.