Study on the interaction of intense femtosecond laser pulses with nanometre-sized hydrogen clusters

The interaction of intense femtosecond laser pulses with hydrogen clusters has been experimentally studied. The hydrogen clusters were produced from expansion of high-pressure hydrogen gas (backed up to 8\tm10⁶Pa) into vacuum through a conical nozzle cryogenically cooled by liquid nitrogen. The average size of hydrogen clusters was estimated by Rayleigh scattering measurement and the maximum proton energy of up to 4.2keV has been obtained from the Coulomb explosion of hydrogen clusters under 2×10¹⁶ W/cm² laser irradiation. Dependence of the maximum proton energy on cluster size and laser intensity was investigated, indicating the correlation between the laser intensity and the cluster size. The maximum proton energy is found to be directly proportional to the laser intensity, which is consistent with the theoretical prediction.     

Nuclear fusion from Coulomb explosions of deuterated methane clusters subjected to ultraintense femtosecond laser pulses

This paper reports that Coulomb explosions taken place in the experiment of heteronuclear deuterated methane clusters ((CD₄)₂) in a gas jet subjected to intense femtosecond laser pulses (170mJ, 70fs) have led to table-top laser driven DD nuclear fusion. The clusters produced in supersonic expansion had an average size of about 5nm in radius and the laser intensity used was 3×10¹⁷W/cm².The measured maximum and average energies of deuterons produced in the laser--cluster interaction were 60 and 13.5keV, respectively. From DD collisions of energetic deuterons, a yield of 2.5(±0.4)×10⁴ fusion neutrons of 2.45MeV per shot was realized, giving rise to a neutron production efficiency of about 1.5×10⁵ per joule of incident laser pulse energy. Theoretical calculations were performed and a fairly good agreement of the calculated neutron yield with that obtained from the present experiment was found.     

飞秒激光氘团簇库仑爆炸引发核聚变的机理研究

在超强飞秒激光与氘团簇的相互作用中，利用库仑爆炸模型，分析了可以引发核聚变的高能氘核产生的机理，提出了氘离子团簇膨胀尺寸与时间的关系式，计算了多种尺寸的氘团簇库仑爆炸时氘核的动能以及氘团簇的解体时间． 关键词： 飞秒强激光 氘团簇 库仑爆炸 核聚变     

High-intensity femtosecond laser absorption by rare-gas clusters

The energy absorption efficiency of high-intensity (～10^{16}W/cm^2) femtosecond laser pulses in a dense jet of large rare-gas clusters has been measured. Experimental results show that the energy absorption efficiency is strongly dependent on the cluster size and can be higher than 90%. The measurement of the ion energy indicates that the average ion energies of argon and xenon can be as high as 90 and 100keV, respectively. The dependence of the average energy of the ions on the cluster size is also measured. At comparatively low gas backing pressure, the average ion energies of argon and xenon increase with increasing gas backing pressure. The average ion energy of argon becomes saturated gradually with further increase of the gas backing pressure. For xenon, the average ion energy drops a little after the gas backing pressure exceeds 9 bar (3.2×10^5 atoms/cluster). The result showing the existence of a maximum average ion energy has been interpreted within the framework of the microplasma sphere model.     

SILEX-I装置上激光-团簇聚变实验中子产额测量

超短超强脉冲激光与CD4团簇相互作用可以产生高能量的氘离子,从而产生DD聚变中子,这种团簇聚变中子源有着非常广阔的应用前景。在SILEX-I 100 TW飞秒激光装置上,超短超强脉冲激光与CD4团簇相互作用实现了DD核聚变。团簇聚变的中子产额是实验中最为重要的物理量,在实验中采用高灵敏的中子探测器对团簇聚变产生的中子进行了测量,在现有实验条件下测得中子产额为103。     

飞秒强激光场中氢原子团簇的各向异性膨胀

利用LAMMPS程序研究了氢原子团簇在飞秒强激光场下的动力学行为, 讨论了引起小氢原子团簇各向异性膨胀的原因.通过对外电离过程中团簇内部电子的行为以及团簇各个方向上最外层质子距离团簇中心的距离随时间的变化情况的分析, 发现团簇的膨胀呈现各向异性,且引起这种各向异性的根源在于团簇内部电子的抖动以及逃逸.对氢原子团簇与强激光场相互作用过程中质子各能量分量以及各向异性程度随时间变化情况进行了研究,发现各向异性程度是随时间变化的, 这种各向异性程度首先随着激光电场的增强而增加,随后又逐渐减小,直到最后趋于某一大于1的稳定值.分析了激光脉冲结束后质子的平均能量与观测角之间的关系, 并将分析结果与Ditmire小组的实验结果进行了比较,发现我们的模拟结果在定性上与实验相符合.     

飞秒强激光场中大尺寸氩团簇爆炸机理的实验研究

利用飞行时间谱，研究了在飞秒强激光场（60 fs，2×1016 W/cm2）作用下，大尺寸氩原子团簇Arn（n—=3×103—3×106原子/每团簇）的电离爆炸过程，测量了 团簇爆炸所产生的氩离子的平均能量与团簇尺寸（气体背压）的关系.实验发现，随着气体 背压的升高（团簇尺寸增大），氩离子的平均能量也相应升高.通过分析两个不同几何尺寸 锥形超声喷嘴所产生团簇爆炸后的离子能量，结合Hagena团簇尺寸规律，发现在激光参数保 持不变的情况下，离子的平均能量由团簇尺寸唯一确定.分析表明，团簇尺寸在3×105原 子/每团簇以下时，团簇膨胀的主要机理是库仑爆炸.随着团簇尺寸的进一步增大，团簇膨胀 机理将由库仑爆炸向流体动力学膨胀过渡，在3×105关键词： 原子团簇 飞秒强激光 库仑爆炸 流体动力学膨胀     

Dynamics simulation on interaction of intense laser pulses with atomic clusters

Under classical particle dynamics, the interaction process between intense femtosecond laser pulses and icosahedral noble-gas atomic clusters was studied. Our calculated results show that ionization proceeds mainly through tunnel ionization in the combined field from ions, electrons and laser, rather than the electron-impact ionization. With increasing cluster size, the average and maximum kinetic energy of the product ion increases. According to our calculation, the expansion process of the clusters after laser irradiation is dominated by Coulomb explosion and the expansion scale increases with increasing cluster size. The dependence of average kinetic energy and average charge state of the product ions on laser wavelength is also presented and discussed. The dependence of average kinetic energy on the number of atoms inside the cluster was studied and compared with the experimental data. Our results agree with the experimental results reasonably well.     

Three-dimensional simulation on explosions of hydrogen atomic clusters irradiated by an intense femtosecond laser pulse

Using classic particle dynamics simulations, the interaction process between an intense femtosecond laser pulse and icosahedral hydrogen atomic clusters H_{13}, H_{55} and H_{147} has been studied. It is revealed that with increasing number of atoms in the cluster, the kinetic energy of ions generated in the Coulomb explosion of the ionized hydrogen clusters increases. The expansion process of the clusters after laser irradiation has also been examined, showing that the expansion scale decreases with increasing cluster size.     

Coulomb expansion of deuterated methane clusters irradiated by an ultrashort intense laser pulse

The simulations of three-dimensional particle dynamics show that when irradiated by an ultrashort intense laser pulse, the deuterated methane cluster expands and the majority of deuterons overrun the more slowly expanding carbon ions, resulting in the creation of two separated subelusters. The enhanced deuteron kinetic energy and a narrow peak around the energy maximum in the deuteron energy distribution make a considerable contribution to the efficiency of nuclear fusion compared with the ease of homonuelear deuterium clusters. With the intense laser irradiation, the nuclear fusion yield increases with the increase of the cluster size, so that deuterated heteronuelear clusters with larger sizes are required to achieve a greater neutron yield.     

强激光场中氘团簇双重膨胀引发核聚变

在超强fs激光与氘团簇的相互作用中, 分析了可以引发核聚变的高能氘核产生的原因,提出了团簇双重膨胀的机制,计算了氘核动能及团簇解体的时间, 为选取合适的激光脉冲宽度参数提供参考. Considering the Coulomb explosion induced by the interaction of a deuterium cluster target with ultra intensity femtosecond laser,the causation which generate energetic deuterium nuclei for the fusion has been analyzed. The mechanism for the dual explosion of deuterium cluster is proposed, and hence the velocity of deuterium nuclei and the expansion time of deuterium ion clusters have been estimated.     

强激光场中氘团簇的离子能谱与团簇尺度

在SILEX-Ⅰ装置上进行了超短超强激光与氘团簇相互作用的实验。采用法拉第筒测量了激光与氘团簇靶相互作用后发射出的氘离子能谱,并根据氘离子能谱计算出氘团簇的尺寸和分布。由于激光预脉冲的影响,得到了团簇的尺度比瑞利散射测量的结果小。这些结果对团簇源制备系统的改进和优化激光打靶有着重要的参考意义,而且可以为团簇尺度的测量提供一种新的方法。     

超短激光脉冲

概述了超短激光脉冲技术的发展进程、研究进展及其在液态分子研究方面、超快现象等领域的应用。     

超短电子脉冲在聚焦强激光场中的三维有质动力散射

采用三维测试粒子模拟(test-padicle simulation)的方法，研究了聚焦的飞秒脉冲强激光与超短电子束进行有质动力的散射过程.详细讨论了超短电子束经过不同激光延迟下有质动力散射的空间分布特征和能谱特征.提出了通过测量电子束的空间分布和能谱分布实现对超短脉冲电子束的脉冲宽度测量的方法. 关键词： 有质动力散射 聚焦的飞激光 飞秒电子束     

飞秒抽运随机激光输出波形的可控性研究

本文基于随机激光的时域理论,研究了飞秒脉冲抽运下二维随机激光的辐射特性,并着重讨论了抽运脉冲的峰值强度、脉宽和脉冲波形对辐射光时域波形的影响.结果表明, 辐射光的时域波形强烈依赖于抽运光脉冲的参数,通过调整抽运方式可以控制辐射光的输出波形.数值模拟结果为研究随机激光输出波形的可控性技术提供了理论依据. 关键词： 激光物理 随机激光器 飞秒抽运 脉冲波形     

高强度飞秒激光脉冲在空气中的自压缩

利用高重复频率（1kHz）、吉瓦级飞秒激光脉冲实验验证了高强度飞秒脉冲在空气中的自 压缩现象，研究了入射脉冲在不同初始啁啾情况下经空气中聚焦成丝后，时域及频域特性随 入射脉冲能量的变化规律.实验结果表明，在无需后继色散补偿情况下，高强度飞秒脉冲仅 通过在空气中的非线性传输过程就可以实现脉冲压缩；在入射脉冲为负啁啾情况下，实验观 察到脉冲光谱及时域宽度同时得到压缩，并可获得比激光源所能提供的更短的近双曲正割型 变换限脉冲. 关键词： 高强度飞秒激光脉冲 自压缩 自聚焦     

Modulation of ionization on laser frequency in ultra-short pulse intense laser--gas-target

Based on the dispersion relation of intense laser pulse propagating in gradually ionized plasma, this paper discusses the frequency modulation induced by ionization of an ultra-short intense laser pulse interacting with a gas target. The relationship between the frequency modulation and the ionization rate, the plasmas frequency variation, and the polarization of atoms (ions) is analysed. The numerical results indicate that, at high frequency, the polarization of atoms (ions) plays a more important role than plasma frequency variation in modulating the laser frequency, and the laser frequency variation is different at different positions of the laser pulse.     

SILEX-I装置上激光-团簇聚变实验中子产额测量

 超短超强脉冲激光与CD4团簇相互作用可以产生高能量的氘离子,从而产生DD聚变中子,这种团簇聚变中子源有着非常广阔的应用前景。在SILEX-I 100 TW飞秒激光装置上,超短超强脉冲激光与CD₄团簇相互作用实现了DD核聚变。团簇聚变的中子产额是实验中最为重要的物理量,在实验中采用高灵敏的中子探测器对团簇聚变产生的中子进行了测量,在现有实验条件下测得中子产额为10³。     

激光光斑形状和尺寸对扫描显示中散斑对比度的影响

计算了圆形、矩形激光光斑对于激光扫描显示系统中散斑对比度的影响.对于圆形光斑,当光斑尺寸大于散射表面相关长度时,得到的散斑对比度随着光斑尺寸变小而下降;当光斑尺寸接近表面相关长度大小时,由于光斑中包含的散射颗粒变少,得到的是非高斯散斑,散斑的对比度反而会变大;随着激光斑进一步变小,由于镜面反射效果散斑的对比度会很快下降.对圆形光斑部分结果给出了实验验证.为了保证光斑中有足够的散射颗粒,只在一个方向压窄光斑,分别计算了平行于扫描方向和垂直于扫描方向压窄光斑得到的散斑对比度.平行于扫描方向压窄光斑,散斑的时间相关性会下降,平均效果变明显,散斑的对比度变小;垂直于扫描方向压窄,散斑对比度变化不大.     

各阶克尔非线性折射率对强激光大气传输的影响

用数值方法研究了各阶克尔非线性折射系数对强激光传输的影响,结果显示第7阶克尔非线性系数和第9阶克尔非线性系数协同影响激光传输,当第7阶克尔效应起作用时,忽略第9阶克尔效应的作用会导致光强随传输距离震荡剧烈,传输非常不稳定。要得到稳定传输需要第7阶克尔非线性系数和第9阶克尔非线性系数同时起作用或同时不起作用。在自聚焦阶段第3阶非线性折射率对折射率改变起主要作用,在稳定光丝传输阶段第7阶克尔非线性系数,第9阶克尔非线性系数起主要作用。第5阶克尔系数在整个传输过程中对折射率的改变一直比较小。