超短脉冲激光与稀薄等离子体相互作用的准静态粒子模拟研究

具体讨论准静态近似在粒子模拟技术中的实现方法，主要包括：简化物理模型的建立，数值模拟方法的实现，程序结构的建立.最后，给出了三个成功的数值模拟结果，短脉冲激光的自聚焦现象、激光尾流场的激发和自生磁场现象，并进行了物理分析. 关键词： 准静态 粒子模拟 激光尾流场 自聚焦     

舰船尾流气泡后向光学检测方法研究

为探索舰船尾流后向光学检测方法,研究了尾流气泡对水中激光脉冲后向散射特性的影响。首先基于Fournier Forand 体积散射函数,利用蒙特卡罗(Monte-Carlo)方法理论分析了近距离尾流气泡对激光脉冲后向散射特性的影响。然后,采用蓝绿激光脉冲作为光源,实验研究了模拟尾流气泡对激光脉冲后向散射信号的影响。研究表明,尾流气泡的存在会使得激光脉冲后向散射信号前沿位置在时域左移,后沿位置在时域右移,信号时域宽度增加,能量增强,峰值增大且位置在时域左移。最后根据研究结果提出了一种基于激光脉冲后向散射信号特征变化的舰船尾流气泡后向检测方法。     

短脉冲激光尾流场中的前向Raman散射

用212维粒子模拟分析了前向Raman散射对激光尾流场加速电子的影响.前向Raman散射使脉冲长度在传播方向上被拉长，脉冲后沿变陡，产生的尾流场相速度明显减小，而且超热电子的最大动能明显小于理论估计值.此外激光频率整体向低频移动.     

稀薄等离子体中激发尾波场的共振动条件

研究了在稀薄等离子体中强激光激发尾波场的情况，发现尾波场的激发与入射激光的脉冲宽度有共振现象。在光强很小情况下，共振所需要的入射激光脉冲宽度度为λp/2，随着光强的增大共振激光脉冲宽度减小。同时发现在稀薄等离子体中激发的尾波势场与等离子体的密度几乎无关，而激发的尾波场最大电场强度与等离子体的密度有关。     

尾流的全息成像方法研究

鉴于全息三维成像的独特优点,提出了一种新的尾流测量方法---全息成像测量.在对影响水下全息成像各种因素进行详细理论分析的基础上,自行设计了激光在尾流场的衰减实验和尾流全息成像试验.衰减实验表明,在气泡幕数密度一定的情况下,激光衰减率基本保持恒定;尾流全息成像利用双脉冲激光配合使用4F放大系统首次记录下了气泡幕的全息图像.实验表明,气泡幕全息成像效果良好,证明尾流参量的全息测量方法是可行的.     

等离子体尾流场加速进展迅速

<正>等离子体尾流场加速技术是1979年首次提出的。在等离子体中,用驱动器(如激光脉冲)将带负电的电子与带正电的离子分离开,产生一个电场,如果驱动器的有效尺寸小于等离子体的波长,便可形成电荷分离的稳定结构,即所谓的尾流场。尾流场中电场梯度可达     

尾流的全息成像方法研究

鉴于全息三维成像的独特优点,提出了一种新的尾流测量方法---全息成像测量.在对影响水下全息成像各种因素进行详细理论分析的基础上,自行设计了激光在尾流场的衰减实验和尾流全息成像试验.衰减实验表明,在气泡幕数密度一定的情况下,激光衰减率基本保持恒定;尾流全息成像利用双脉冲激光配合使用4F放大系统首次记录下了气泡幕的全息图像.实验表明,气泡幕全息成像效果良好,证明尾流参量的全息测量方法是可行的.     

稀薄等离子体中激发尾波场的共振条件

研究了在稀薄等离子体中强激光激发尾波场的情况 ,发现尾波场的激发与入射激光的脉冲宽度有共振现象。在光强很小情况下 ,共振所需要的入射激光脉冲宽度为λp 2 ,随着光强的增大共振激光脉冲宽度减小。同时发现在稀薄等离子体中激发的尾波势场与等离子体的密度几乎无关 ,而激发的尾波场最大电场强度与等离子体的密度有关。     

激光脉冲形状对弓形波电子俘获的影响

使用二维粒子模拟程序研究了电子弓形波注入机制中激光脉冲形状对电子俘获效果的影响. 研究结果表明, 激光脉冲时间上升沿陡峭的正扭曲脉冲激发的尾波场强度高, 加速区域分布广, 并且有利于电子获得更高的初速度, 从而推动更多的电子进入尾波场加速相位. 在其他条件相同的情况下, 正扭曲脉冲的电子俘获数目远高于激光脉冲时间分别为高斯形和负扭曲分布的情形, 使得电子束的品质得到改善. 研究结果对于理解尾波场加速中电子注入过程以及获得大电荷量高能电子束具有积极意义. 关键词： 尾波场 电子俘获 时间波形 粒子模拟     

近共振区超短强激光脉冲激发的等离子体尾波场

 用一维相对论粒子模拟研究了相对论超短强激光脉冲在等离子体中传播时激发的尾波场，初步获得了近共振区尾波场的峰值幅度随激光脉冲宽度变化的特点，发现在近共振区等离子体波激发出现增强。通过准静态近似下尾波激发的一维非线性方程数值求解，并与粒子模拟结果比较，得到了该非线性方程的适用范围：当激光脉冲宽度小于等离子体波波长的4倍时，该方程所得结果与粒子模拟结果一致；而当激光脉冲宽度大于该数值时，该方程不再适用。     

A ponderomotive guiding center particle-in-cell code for efficientmodeling of laser-plasma interactions

A novel particle simulation code is described that self-consistently models certain classes of laser-plasma interactions without resolving the optical cycles of the laser. This is accomplished by separating the electromagnetic field into a laser component and a wake component. Although the wake component is treated as in a fully explicit particle-in-cell (PIC) code, the laser component is treated in the high-frequency limit, which allows the optical cycles to be averaged out. This leads to enormous reductions in computer time when the laser frequency is much greater than all other frequencies of interest. This work is an extension of the work of Mora and Antonsen, Jr. (1996 and 1997), who derived the time-averaged equations coupling the laser with the particles and developed a code to solve these equations in the quasi-static limit. The code presented is distinguished by the fact that it is useful when the plasma length is much less than the laser pulse length. Also, it is already parallelized and should be straightforward to extend to three dimensions     

Ionization focusing of a short intense laser pulse and generation of wake plasma waves

The propagation of a short intense laser pulse is studied in a gas taking into account the ionization of gas atoms by the high-frequency electromagnetic field of the pulse. The conditions are found under which the ionization structures produced by the laser pulse cause the pulse focusing accompanied by a substantial increase in its intensity. It is shown that the leading edge of the pulse is subjected to ionization refraction at the ionization front, the temporal profile of the pulse becoming steeper. This results in the efficient generation of a wake wave at the ionization front, which is amplified during the development of self-modulation instability. The amplitude of the wake plasma wave achieves a substantial value already at small paths of the pulse in matter (smaller than the diffraction length of the pulse).     

Generation of a quasimonoenergetic electron beam using a single laser pulse

The results of experiments are presented for a single laser pulse interaction with a very low density gas target under conditions when the generated wake wave is below the wave-breaking threshold and the laser pulse power is lower than the critical power for relativistic self-focusing. A quasimonoenergetic electron beam is found to be stably generated for various laser pulse intensity values by controlling the acceleration length.     

优化脉冲间距的多脉冲尾流加速PIC模拟

多脉冲激光尾波场加速电子方法中限制尾波场振幅的主要机理是“相位失谐”,起源于非线 性效应导致尾波波长随振幅的增长而变大,从而后续脉冲逐渐偏离加速相位. 借助2D3V PIC 模拟方法优化各脉冲之间的间距,使之等于前面脉冲激发的尾波波长,模拟结果表明激发了 更大振幅的尾波场,同时激发了更强的“前向Raman散射”,它在限制尾波场进一步增长的 过程发挥了重要作用. 关键词： 多脉冲激光尾波加速 有质动力 相位失谐 前向Raman散射     

Imaging Laser wake fields by Thomson Scattering a Co-Propagating Pulse

Thomson scattering imaging(TSI) is proposed and experimentally demonstrated to observe the fine structure of the laser wake field. By Thomson scattering a co-propagating laser pulse, we obtain clear images indicating that the wake field is like an acaleph swimming behind the pump laser. The wavelength of the wake field observed at different electron densities agrees well with the theory. Since no mathematics transformation is involved, TSI could be potentially used as an online monitor for future 'tabletop' plasma accelerators.     

Electron Acceleration and Bunch Generation by Intense Femtosecond Laser Pulse in Preplasma of a Target

We present analytical studies of electron acceleration in the low-density preplasma of a thin solid target by an intense femtosecond laser pulse. Electrons in the preplasma are trapped and accelerated by the ponderomotive force as well as the wake field. Two-dimensional particle-in-cell simulations show that when the laser pulse is stopped by the target, electrons trapped in the laser pules can be extracted and move forward inertially. The energeticelectron bunch in the bubble is unaffected by the reflected pulse and passes through the target with small energy spread and emittance. There is an optimal preplasma density for the generation of the monoenergetic electron bunch if a laser pulse is given. The maximum electron energy is inverse proportion to the preplasma density.     

Generation of short electron bunches by a laser pulse crossing a sharp boundary of inhomogeneous plasma

The formation of short electron bunches during the passage of a laser pulse of relativistic intensity through a sharp boundary of semi-bounded plasma has been analytically studied. It is shown in one-dimensional geometry that one physical mechanism that is responsible for the generation of electron bunches is their self-injection into the wake field of a laser pulse, which occurs due to the mixing of electrons during the action of the laser pulse on plasma. Simple analytic relationships are obtained that can be used for estimating the length and charge of an electron bunch and the spread of electron energies in the bunch. The results of the analytical investigation are confirmed by data from numerical simulations.     

Generation of hard x rays by femtosecond laser pulse interaction with solid targets in atmosphere

X ray radiation as high as 50 keV, including K(α) of Ba and Mo, have been observed from a solid target during the interaction of low energy ~0.65 mJ, 1 kHz 40 femtosecond laser pulses focused in air at atmospheric pressure. Energetic electrons generating such x rays are possibly produced when the field strength in laser pulse wake exceeds the runaway threshold in air. Two dimensional particle-in-cell simulations that include optical field ionization of air and elastic collisions support this mechanism.