激光脉冲诱导的等离子体密度调制及其产生的相位反射

理论研究和数值模拟发现入射光和反射光在低密度等离子体中形成的干涉场可以产生深度的等离子体密度调制. 对于中等强度的入射光，譬如10¹⁵W／cm² ，产生密度调制的时间尺度在几十个光周期的范围. 这样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临界面以下的区域产生相位反射. 因为密度调制的周期是光在等 离子体中波长的一半，其产生的反射率可以接近100％. 相位反射也可以在不均匀的低密度 等离子体中产生，它可以极大地减少等离子体对光的吸收，因此在惯性约束核聚变中需要考 虑到它的影响. 关键词： 相位反射 密度调制 激光等离子体 粒子模拟     

Chirping and spreading of ultrashort laser pulses in underdense plasmas

A linear theory on the propagation of ultrashort pulses including only a few cycles in underdense plasmas is presented. It is shown that the dispersion in plasmas causes severe distortions in the pulse shape, including pulse chirping and spreading. The analytical calculations coincide very well with those obtained by particle-in-cell (PIC) simulations. The upper limit of the peak amplitude of the pulses, above which the linear theory breaks down due to the setting in of nonlinear effects of both the relativistic electron-mass increase and ponderomotive force, is also examined by PIC simulations. At certain high amplitudes, it is found that the ultrashort laser pulses can propagate like solitons.     

交叉传播激光脉冲与等离子体相互作用产生的等离子体密度光栅

理论上研究了两束交叉传播的激光束与等离子体相互作用产生的电子和离子密度调制. 用一维粒子模拟程序(particle-in-cell,PIC)研究了两束激光脉冲产生的干涉场激发的等离子体布拉格光栅. 研究表明等离子体初始密度、脉冲强度和宽度共同影响等离子体布拉格光栅的演化. 光栅的密度峰值可以达到初始等离子体密度的8倍以上,并且可以维持几皮秒的时间. 等离子体布拉格光栅可以囚禁由受激拉曼散射形成的电磁孤子,从而形成准稳态的孤子结构,很大程度上降低了形成电磁孤子所要求的激光脉冲强度. 关键词： 等离子体布拉格光栅 电磁孤子 交叉传播激光束 粒子模拟     

Ion heating and thermonuclear neutron production from high-intensity subpicosecond laser pulses interacting with underdense plasmas

Thermonuclear fusion neutrons produced by D(d,n)3He reactions have been measured from the interaction of a high-intensity laser with underdense deuterium plasmas. For an input laser energy of 62 J, more than (1.0+/-0.2)x10(6) neutrons with a mean kinetic energy of (2.5+/-0.2) MeV were detected. These neutrons were observed to have an isotropic angular emission profile. By comparing these measurements with those using a secondary solid CD2 target it was determined that neutrons are produced from direct ion heating during this interaction.     

Doppler shift of femtosecond laser pulses from solid density plasmas

Doppler-shift of short UV pulses (248 nm, 700 fs) reflected from aluminum, polystyrene and gold targets were measured at focused intensities up to 5×10¹⁵ W/cm². In spite of the fact that the Doppler-shifts are expected to be dependent on the atomic weight of the target material, experimentally similar shifts are found for all targets. A possible explanation is provided by the well known dependence of the Doppler shift on the plasma temperature, if different absorption coefficients are assumed for the different targets. This coefficient shows good agreement with that one deduced from a theory based on collisional absorption in the case of aluminum, but in the case of the other targets, however, the experimentally observed Doppler-shift suggests smaller absorption for lighter and larger absorption for heavier target materials. PACS 52.25.Mq; 52.25.Qt; 52.40.Nk; 52.50.Jm     

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     

Ion acceleration by superintense laser pulses in plasmas

Ion acceleration by petawatt laser radiation in underdense and overdense plasmas is studied with 2D3V-PIC (Particle in Cell) numerical simulations. These simulations show that the laser pulse drills a channel through the plasma slab, and electrons and ions expand in vacuum. Fast electrons escape first from the electron-ion cloud. Later, ions gain a high energy on account of the Coulomb explosion of the cloud and the inductive electric field which appears due to fast change of the magnetic field generated by the laser pulse. Similarly, when a superintense laser pulse interacts with a thin slab of overdense plasma, its ponderomotive pressure blows all the electrons away from a finite-diameter spot on the slab. Then, due to the Coulomb explosion, ions gain an energy as high as 1 GeV. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 2, 80–86 (25 July 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

超强短脉冲激光在不同密度分布等离子体中的自聚焦

讨论高斯型强激光束在具有初始柱对称密度分布的低密度冷等离子体中传播时，等离子体密度分布的不同对激光自聚焦的影响.推导出可以判断更有利于自聚焦发生的评价函数，这样通过比较不同密度分布的评价函数值就可以判断哪种密度分布更有利于自聚焦的发生.为了说明这种方法的有效性，对评价函数进行分析得出：在相同的激光场中等离子体柱的轴心密度给定时(以激光的光轴为轴)，离轴越远的地方密度越大及密度变化越陡，自聚焦越容易发生；相对论效应与有质动力共同作用比相对论的单独作用，自聚焦更容易发生.数值模拟证实了评价函数能准确的预测在不 关键词： 自聚焦 相对论效应 有质动力 评价函数     

强激光与等离子体相互作用产生的超强太赫兹辐射

超短强激光脉冲在等离子体中传播时会激发大振幅的等离子体尾波场，它是一种电子等离子体波．由于这是一种静电波，它一般不能转换成电磁辐射．我们发现在不均匀等离子体中激发的尾波场在一定条件下可以通过线性模式转换产生电磁辐射．由于用超短强激光脉冲尾波场可以达到的电场振幅达100GV／m，其振动频率在太赫兹（10^12Hz）附近，用这种方法可以产生电场强度达到GV／m的太赫兹辐射．     

Ion acceleration regimes in underdense plasmas

Acceleration of large populations of ions up to high (relativistic) energies may represent one of the most important and interesting tools that can be provided by the interaction of petawatt laser pulses with matter. In this paper, the basic mechanisms of ion acceleration by short laser pulses are studied in underdense plasmas. The ion acceleration does not originate directly from the pulse fields, but it is mediated by the electrons in the form of electrostatic fields originating from channeling, double layer formation and Coulomb explosion     

Intense laser beam guiding in self-induced electron cavitation channel in underdense plasmas

In underdense plasmas, the transverse ponderomotive force of an intense laser beam with Gaussian transverse profile expels electrons radially, and it can lead to an electron cavitation. An improved cavitation model with charge conservation constraint is applied to the determination of the width of the electron cavity. The envelope equation for laser spot size derived by using source-dependent expansion method is extended to including the electron cavity. The condition for self-guiding is given and illuminated by an effective potential for the laser spot size. The effects of the laser power, plasma density and energy dissipation on the self-guiding condition are discussed.     

Stimulated Raman backscattering from an ultrashort laser interacting with underdense plasmas

We present experimental results of the stimulated Raman backscattering instability (BSRS) in an ultrashort intense (45fs, 5.7×10¹⁷Wcm^-2) laser pulse interacting with an optically ionized helium gas. We have studied the stimulated Raman backscattering reflectivity and the phenomenon of the transition from strongly coupled BSRS to weakly coupled BSRS. We have obtained a good agreement between our experimental results and the theory of BSRS.     

The formation of relativistic electromagnetic solitons in plasma Bragg gratings induced by two counter-propagating laser pulses

The generation of relativistic electromagnetic solitons in plasma with spatiotemporal density modulation is investigated. When two counter-propagating laser pulses overlap in underdense plasma, the interaction between the pulses and plasma modulates the electron and ion densities resulting in localized, stable, long-living relativistic electromagnetic solitons. They are caused by the Stimulated Raman Scattering instability. The dependence of the formation of relativistic electromagnetic solitons on the ion motion, plasma parameters, and laser parameters is studied by particle-in-cell simulations as well.     

Excitation of multiple wakefields by short laser pulses in quantum plasmas

We present a theoretical investigation of the excitation of multiple electrostatic wakefields by the ponderomotive force of a short electromagnetic pulse propagating through a dense plasma. It is found that the inclusion of the quantum statistical pressure and quantum electron tunneling effects can qualitatively change the classical behavior of the wakefield. In addition to the well-known plasma oscillation wakefield, with a wavelength of the order of the electron skin depth (λe=c/ωpe, which in a dense plasma is of the order of several nanometers, where c is the speed of light in vacuum and ωpe is the electron plasma frequency), wakefields in dense plasmas with a shorter wavelength (in comparison with λe) are also excited. The wakefields can trap electrons and accelerate them to extremely high energies over nanoscales.