粒子模拟程序的发展及其在激光等离子体相互作用研究中的应用

介绍针对超短超强激光脉冲与等离子体相互作用研究的多维粒子模拟程序KLAP.在其一维程序KLAP1D中,考虑场电离、碰撞离化及两体碰撞效应后,程序可以用于研究短脉冲激光与中性物质的相互作用.在其三维程序KLAP3D中,为了研究加速能量达GeV的长距离激光尾波场加速问题,程序采用移动窗口技术,使得模拟尺度可以达到厘米量级.同时介绍了利用KLAP程序得到的有关THz辐射、激光与中性气体相互作用中的脉冲及离化波前演化、激光固体靶作用中表面电子加速及激光尾波场加速的研究实例.     

超短超强激光与薄膜靶相互作用中不同价态碳离子的来源

研究了超短超强激光与不同厚度薄膜Al靶相互作用中靶背法线方向碳离子的最初来源. 通过对比分析碰撞电离率和场致电离率所起的作用, 发现C⁴⁺ 及更低价态的碳离子主要由场致电离产生, 而高价态的C⁵⁺和C⁶⁺ 离子主要来自于超热电子与靶表面的碰撞电离. 关键词： 超短超强激光与等离子体相互作用 离子加速 场致电离 碰撞电离     

激光等离子体相互作用的局域振荡电子加热机制

 用2.5维粒子模拟程序模拟了超强激光与等离子体的相互作用过程，发现超强激光可以通过J×B加热机制加速电子并引起电荷分离，从而产生很强的静电场并形成电场势阱，电子在静电场势阱中振荡，被多次加速，使得高速电子被甩出势阱，进而增强电荷分离，然后静电场结构被破坏，静电势能传给电子。在此过程中，电子在此阱中作局域振荡，并且被J×B机制多次加速，激光的能量会有效地传给电子，使电子能量高达10MeV。这是一种新的电子加热机制，称之为局域振荡电子加热机制。     

超强激光场物理学

首先,评述了超强激光场的理论结果;其次,描述了超短光脉冲在密的气体和光学介质传播中的自作用;第三,评述了强场离化无碰撞等离子体中高度离化的离子产生,及高功率超短激光脉冲巨大强度的电场可用于电子加速;第四,分析了超短声脉冲和无线电脉冲的产生和应用,讨论了在不同条件下,激光辐射谐波和 Ｘ 射线激光的产生,进一步评述了超强激光脉冲与凝聚靶相互作用可以产生接近星体物质参数的高温、超密、强磁场和巨大压力等离子体;最后,简要叙述了激光激发核、核反应,高能电子─光子相互作用的可能效应及可能进行的实验。     

超强激光场物理学

首先,评述了超强激光场的理论结果;其次,描述了超短光脉冲在密的气体和光学介质传播中的自作用;第三,评述了强场离化无碰撞等离子体中高度离化的离子产生,及高功率超短激光脉冲巨大强度的电场可用于电子加速;第四,分析了超短声脉冲和无线电脉冲的产生和应用,讨论了在不同条件下,激光辐射谐波和 Ｘ 射线激光的产生,进一步评述了超强激光脉冲与凝聚靶相互作用可以产生接近星体物质参数的高温、超密、强磁场和巨大压力等离子体;最后,简要叙述了激光激发核、核反应,高能电子─光子相互作用的可能效应及可能进行的实验。     

金激光等离子体冕区电离态特性研究

 提出一种测量金激光等离子体电荷态分布与平均电离度的X射线光谱学诊断方法。该方法基于稳态碰撞-辐射近似，考虑电子离子直接碰撞激发与双电子复合两种激发态布居方式，建立了金M带5f-3d跃迁组辐射总强度与离子态分布的耦合方程。根据实验测量的金平面靶激光等离子体冕区辐射的5f-3d跃迁线系的强度分布，诊断得到了金激光等离子体的电荷态分布与平均电离度。此外，还分析了电子温度、电子密度以及双电子复合过程对电荷态分布及平均电离度诊断的影响，并将实验诊断结果与辐射流体力学理论模拟结果及离化平衡动力学计算结果进行了对比分析。结果表明：实验诊断结果与基于CRE近似的离化平衡动力学计算结果近似；当电子温度高于1.5 keV时，双电子复合过程对电离度的诊断结果影响较小。     

强激光锥型靶真空电子加速数值模拟研究

真空激光加速机制具有加速场梯度大、加速电子电量高的优点，目前制约真空加速机制研究发展的主要问题是如何产生具有一定初速度的电子并将其注入加速场。提出了一种利用强激光与锥型靶相互作用产生高能电子并实现真空加速的新方法，利用二维PIC(Particle-in-cell)粒子模拟程序对这一方法进行了研究。模拟结果显示，对于光强为1021 W/cm2量级的高斯激光脉冲，产生了能量为GeV量级、发散角约为1°的强流快电子束。此外还通过理论解析和参数模拟研究了靶半径对这种超热电子加速机制的影响。     

填充预等离子体通道靶激光电子加速研究北大核心CSCD

采用紧聚焦的超强短脉冲激光与固体通道靶相互作用是获得大电量、高准直相对论电子束的一种有效方式。实验中由于激光预脉冲烧蚀靶壁产生预等离子体会膨胀、填充到真空通道中,从而导致电子束品质发生变化。采用二维PIC粒子模拟程序研究了通道靶中填充预等离子体的电子加速过程。模拟结果显示,在功率密度为5.0×10^(20W/cm^(2))的超强短脉冲激光条件下,通道中填充一定密度的等离子体时激光场优先与低密度等离子体相互作用,激光脉冲与通道壁的相互作用减弱,电子加速机制由纵向场主导的真空电子加速转变为横向电场主导的等离子体电子加速,产生电子束具有更大的电荷量,但能量降低,发散角增大。     

激光与固体靶相互作用中低密度预等离子体对高能电子产生的影响

 对线极化、圆极化的超短超强激光脉冲与靶前有一段低密度预等离子体的固体靶的相互作用进行了理论和粒子模拟研究。激光通过有质动力加速机制加速预等离子体中的电子，研究了电子获得的最大能量随激光强度和预等离子体密度的变化。当激光脉冲与靶直接作用时，靶中的电子由于J×B机制而得到加速，所获得的能量比预等离子体中电子低。研究表明，在超短超强激光脉冲与固体靶相互作用中，预等离子体的存在有利于高能电子的产生。     

通道靶对超强激光加速质子束的聚焦效应

发散角过大是制约超强激光与固体靶相互作用加速产生高能质子束应用的一个重大物理难题.本文提出了一种结构化的通道靶型,与超强激光相互作用可提高质子束的发散特性,通道壁上产生的横向电荷分离静电场可对质子有效聚焦.采用二维particle-in-cell粒子模拟程序对激光通道靶相互作用过程进行了研究,分析了加速质子束的性能特点.模拟结果表明,与传统平面靶相比,通道靶可以在不过多损失能量的情况下产生具有更好准直性的质子束,尤其当通道靶的直径与激光焦斑尺寸和质子源尺寸相当时,横向静电场能够有效聚焦质子束,并且可保证相对较高的激光能量利用率.     

Formation of fast multicharged heavy ions under the action of a superintense femtosecond laser pulse on the cleaned surface of a target

It is found that the mean charge of tungsten ions in a solid tungsten target cleaned from the surface layer of hydrocarbon and oxide compounds and exposed to femtosecond laser radiation with an intensity exceeding 10¹⁶ W/cm² attains 22+, while the maximum charge is 29+. The maximum energy of such ions approaches 1 MeV. The corresponding values obtained on a dirty target with the same laser pulse parameters constitute 3+, 5+, and 150 keV. The results of numerical simulation show that such a large maximum charge of ions can be attained owing to the emergence of an electrostatic ambipolar field at the sharp boundary between the plasma and vacuum. The main mechanism of ionization of ions with maximum charges is apparently impact ionization in the presence of an external quasi-static field. In addition, direct above-threshold ionization by this field can also play a significant role. It is also shown that heavy ions in a clean target are accelerated by hot electrons. This leads to the formation of high-energy ions. The effect of recombination on the charge of the ions being detected is analyzed in detail.     

高密度平面靶等离子体中激光驱动冲击波加速离子的能谱展宽

本文对超短超强激光脉冲辐照高密度等离子体产生的静电冲击波加速离子的能谱展宽机理进行了数值研究.着重讨论了三种冲击波加速离子的能谱展宽机理:能量沉积到离子中使得冲击波前沿不断减速,被加速离子与背景粒子的碰撞,以及高能离子到达靶背面时受到鞘层场进一步加速.还研究了驱动激光脉冲宽度对冲击波加速离子能谱宽度的影响. 关键词： 激光等离子体 冲击波加速 能谱展宽     

超短超强激光脉冲辐照超薄碳膜电离状态研究

为了进一步理解极端条件下物质的电离特性, 特别是超短超强激光脉冲辐照超薄靶时等离子体的形成与分布, 本文以超薄碳膜为例, 细致研究了超短超强激光脉冲辐照下原子的离化过程. 分析和比较了强激光场直接作用电离和靶内静电场电离等两种场致电离形式, 在碰撞电离可以忽略的情况下, 发现更多的电离份额是来自靶内静电场的电离方式. 研究了激光脉冲强度对电离的影响, 发现激光脉冲强度越强, 电离速度越快, 产生的高价态离子所占比例也越高.当激光强度为1×10²⁰ W/cm²时, 尽管该强度高于电离生成C⁺⁶所需要的激光强度阈值, 但该激光脉冲并不能将整个靶电离成C⁺⁶离子, 对此本文进行了详细的分析. 在研究激光脉冲宽度的影响时, 发现激光脉宽越小, 电离速度越快, 但越小的激光脉冲电离获得的高价态离子越少.     

乙醚团簇在纳秒激光场中的多价电离及其电子能量分布的研究

用5ns，1064nm的脉冲Nd:YAG激光，研究了乙醚团簇与纳秒激光的相互作用.在10¹¹ W/cm²量级光强下，观察到价电子完全剥离的O⁶⁺，C⁴⁺，且这些高价离子的强度比值基本不随激光能量而变化.用阻滞电压方法测量了电离过程中溢出电子能量分布，在最大激光能量4.0×10¹¹ W/cm²，溢出电子的平均能量为56eV，最大能量为102eV.实验结果支持了高价离子产生的“多 关键词： 高价离子 电子能量 纳秒激光 乙醚团簇     

超短脉冲超强激光与固体靶相互作用中高能离子的产生

 用2D3V PIC粒子模拟方法得到了超短脉冲超强激光与固体靶相互作用中高能离子产生的图像，并对其机理进行了研究。在靶前后表面都观察到了高能离子的产生，并诊断了离子能谱。模拟结果表明，在靶前表面所产生的高能离子，角分布较大，在向靶内输运过程中会损失能量；在靶后表面产生的高能离子，定向性很好，能获得很高的能量。模拟得到的离子能量和实验观测结果在量级上相符。     

Non-equilibrium plasma production by pulsed laser ablation of gold

A gold target has been irradiated with a Q-switched Nd:Yag laser having 1064?nm wavelength, 9?ns pulse width, 900?mJ maximum pulse energy and a maximum power density of the order of 10¹⁰?W/cm². The laser–target interaction produces a strong gold etching with production of a plasma in front of the target. The plasma contains neutrals and ions having a high charge state. Time-of-flight (TOF) measurements are presented for the analysis of the ion production and ion velocity. A cylindrical electrostatic deflection ion analyzer permits measurement of the yield of the emitted ions, their charge state and their ion energy distribution. Measurements indicate that the ion charge state reaches 6⁺ and 10⁺ at a laser fluence of 100?J/cm² and 160?J/cm², respectively. The maximum ion energy reaches about 2?keV and 8?keV at these low and high laser fluences, respectively. Experimental ion energy distributions are given as a function of the ion charge state. Obtained results indicate that electrical fields, produced in the plume, along the normal to the plane of the target surface, exist in the unstable plasma. The electrical fields induce ion acceleration away from the target with a final velocity dependent on the ion charge state. The ion velocity distributions follow a “shifted Maxwellian distribution”, which the authors have corrected for the Coulomb interactions occurring inside the plasma.     

Analyzing the transition rates of the ionization of atoms by strong fields of a CO<Subscript>2</Subscript> laser including nonzero initial momenta

Here, the method of including nonzero initial momenta for ejected electrons in strong infrared laser fields is further developed [8]. It has been shown that, apart from being natural, including the nonzero initial momenta enables one to go into a deeper analysis of the process of tunnel ionization of atoms in strong laser fields (intensity up to 10¹⁶ W/cm²). This is due to looking closely at Fig. 2, which indicates that all electrons that could be ejected, under the circumstances, are ejected at a field intensity ～10¹³ W/cm², and that the effect of ionization after that is strongly diminished, which can be seen from the slope of the plates on Figs. 2 and 4. This also explains the saturation effect for fields up to 10¹⁶ W/cm² [1, 4, 5, 7], and probably this saturation goes on until the fields raising relativistic effects ～10¹⁸ W/cm² [7]. Opposite to what was believed earlier [7], the atomic field intensities could be increased to values over 10¹⁷ W/cm² only when more than 10 electrons are ejected from the atom, it is shown that the properly calculated ionization of 9 electrons increases the atomic field intensity to ～10¹⁸ W/cm².     

Tungsten ions produced by infrared pulsed laser irradiation

Energetic ions have been obtained irradiating a tungsten target with a Q-switched Nd:Yag laser, 1064?nm wavelength, 9?ns pulse width, 900?mJ maximum pulse energy and power density of the order of 10¹⁰?W/cm². The laser-target interaction induces a strong metal etching with production of plasma in front of the target. The plasma contains neutrals and ions with high charge state. Time-of-flight measurements are presented for qualitative analysis of the ion production. A cylindrical electrostatic ion analyzer permits measuring of the yield of emitted ions, the charge state of detected ions and the ion energy distribution. Measurements indicate that, at a laser fluence of the order of 100?J/cm², the charge state may reach 9+ and the ion energy reaches about 5?keV. The ion energy distribution is given as a function of the charge state. Experimental results indicate that an electrical field is developed along the normal to the plane of the target surface, which accelerates the ions up to high velocity. The ion velocity distributions follow a “shifted Maxwellian distribution”, which the author has corrected for the Coulomb interactions occurring inside the plasma.     

Experimental investigations of fast-proton production in a picosecond laser plasma

Results of experimental investigations of fast-proton production in a laser plasma are presented for the case where the intensity of laser radiation at the targets is 2 × 10¹⁸ W/cm². Three processes of fast-proton acceleration in laser plasma are investigated: (1) the acceleration of protons from the front surface toward the laser pulse, (ii) the acceleration of protons from the front surface of the target toward its interior, and (iii) the acceleration of protons from the rear foil surface in the outward direction. The activation procedure and CR-39 tracker detectors featuring a set of various-thickness aluminum filters were used to record fast protons. It turned out that the proton-acceleration process is the most efficient in the case of proton acceleration from the rear foil surface in the outward direction. Experimental results revealed that about N _p = 10⁷ protons of energy in the region E _p > 1.9 MeV that are accelerated from the target surface toward a laser ray, N _p = 4× 10⁷ protons of energy in the region E _p > 1.9 MeV that are accelerated fromthe front surface of the target toward its interior, and N _p = 4×10⁸ protons of energy in the region E _p > 1.9 MeV that are accelerated from the rear foil surface in the outward direction are generated at a laser-radiation intensity of 2 × 10¹⁸ W/cm² at the surface of aluminum, copper, and titanium targets. Experimental investigations aimed at optimizing the process of proton acceleration from the rear surface of aluminum foils were performed by varying the foil thickness over the range between 1 and 100 μm. The results of these experiments showed that there is an optimum foil thickness of 10 μm, in which case protons of maximum energy 5 MeV are generated.