Studies on the Interaction of Intense Femtosecond Laser Pulses with Hydrogen (Deuterium) and Methane (Deuterated Methane) Clusters

近十几年来飞秒强激光脉冲与团簇的相互作用研究吸引了研究者很大的关注，成为激光与物质相互作用研究的一个热点，取得了许多重要研究成果，如X射线、高能离子（MeV）和高能电子（keV）的产生。特别值得一提的是飞秒强激光脉冲与氘团簇喷流相互作用产生的台式激光核聚变反应的实现，为台式中子源的实现提供了一种可能的途径。此后，国内外开展了大量相关的理论和实验研究，重点放在如何提高台式激光核聚变的中子产额。 本论文开展了飞秒强激光脉冲与氢（氘）团簇和甲烷（氘代甲烷）团簇喷流相互作用的理论和实验研究，对如何提高台式激光核聚变的中子产额进行了探讨。主要研究成果包括： 1. 根据气体经喷嘴向真空绝热膨胀的理论和对应喷流（corresponding jets） 概念推演了Hagena参量，计算了一些惰性气体的成团系数K，获得了与文献一致的结果。同时利用Saenger模型探讨了气体超声脉冲团簇喷流达到稳态所需要的最小喷流时间宽度，并与有限的实验结果进行了比较。这些研究结果对于团簇源的设计和评价是有意义的。 2. 飞秒强激光脉冲与氢团簇喷流相互作用的理论和实验研究。实验上研究了激光－氢团簇相互作用产生的质子平均能量和最大能量随气体背压的变化趋势和特征。理论上建立了一简化的库仑爆炸模型，在考虑了团簇喷流中氢团簇的尺寸分布和飞秒强激光脉冲在氢团簇喷流中传输时的能量衰减之后，对实验结果进行了分析。同时本工作首次提出了一种团簇尺寸的标定方法，即在纯库仑爆炸近似下，通过对具有对数－正态（log-normal）尺寸分布的氢团簇爆炸得到的离子能谱与实验得到的离子能谱的拟合，给出团簇的平均尺寸和尺寸分布宽度。 3. 提出在团簇喷流边缘区域由预电离激光脉冲产生等离子体通道的新方案以提高激光－氘团簇相互作用的聚变中子产额。计算表明，利用该等离子体通道方案，主激光脉冲的大部分能量可以沉积到团簇喷流的中心区域，从而使激光－氘团簇核聚变的中子产额可望提高约3倍。 4. 研究了飞秒强激光脉冲与氘团簇喷流相互作用过程中，喷流中聚焦的光斑尺寸对激光－氘团簇核聚变中子产额的影响。研究发现，对于给定的飞秒激光脉冲和氘团簇喷流，存在一个与最大聚变中子产额对应的最优光斑尺寸，该光斑尺寸与激光脉冲参数和团簇源参数紧密相关。 5. 提出并建立了一个经典静电模型，分别详细研究了单一尺寸和具有尺寸分布的甲烷团簇(CH4)n和氘代甲烷团簇(CD4)n的纯库仑爆炸。研究发现，当碳离子价态超过+4价后，氘代甲烷团簇纯库仑爆炸产生的氘离子平均能量随碳离子价态的升高而趋于饱和。这一发现对于如何提高激光－氘代甲烷团簇核聚变中子产额给出了重要提示，即在确保激光脉冲能够将碳离子电离到+4价的前提下，应该增加激光与气体喷流相互作用的体积，以提高相应的氘离子数目，并非改善聚焦以获得更高的激光强度使碳离子电离到+6。同时本工作的研究还表明，团簇纯库仑爆炸产生的轻离子获得的动能与他们的初始静电库仑势能之间的比值与团簇的尺寸大小无关。据此我们提出，氘代甲烷（以及异核甲烷）团簇的纯库仑爆炸产生的轻离子获得的动能可以根据这一比值和其初始的静电库仑势能简单给出，无需复杂的分子动力学模拟。 6. 实验上成功实现了飞秒强激光脉冲与氘团簇、氘代甲烷团簇喷流相互作用的台式激光核聚变。对于氘团簇和氘代甲烷团簇喷流，每焦耳激光能量分别获得了3.7´104和1.5´105个中子。与国际同类实验结果[103]相比，我们的氘代甲烷团簇库仑爆炸的聚变反应具有更高的中子产生效率。For more than a decade, the interaction of femtosecond (fs) intense laser pulses with clusters has attracted much attention and become a field of fundamental scientific interest in the laser interaction with matter. Many important findings have been reported, including the generation of x-ray, energetic ions (MeV) and energetic electrons (keV). Perhaps, one of the most remarkable achievements is the observation of the intense fs laser driven nuclear fusion from Coulomb explosions of deuterium clusters, from which a potential table-top neutron source for practical applications would be expected. Since then, many efforts have been made experimentally and theoretically aiming at mainly the increase of the fusion neutron yield. In this thesis, some theoretical and experimental investigations, which are related to the improvement of the laser-driven table-top nuclear fusion yield from the explosions of gaseous clusters in jets, have been carried out, including the interaction of intense fs laser pulses with hydrogen, deuterium, methane, and deuterated methane clusters. The main work is given as follows: 1. Based on the theory of gas adiabatic expansion from a nozzle into vacuum and the concept of “corresponding jets”, the Hagena parameter and the gas specific constants K for some rare gases are obtained, which are in agreement with those reported in the literature. Meanwhile, by employing the model suggested by Saenger, et al., the minimum gas pulse duration required for a pulsed supersonic cluster jet to reach the fully developed steady state of cluster formation is investigated, and some calculated results were compared with the available experimental results. The results may provide useful information for the design and estimation of a gas cluster source. 2. Theoretical and experimental studies on the interaction of intense fs laser pulses with hydrogen clusters are carried out. The dependence of the maximum and the average proton energies on the gas backing pressure is investigated. With a simplified Coulomb explosion model developed and by taking into account both a log-normal cluster size distribution and the laser energy attenuation in the hydrogen cluster jet, the experimental results are analyzed. Meanwhile, a method is proposed for the calibration of the cluster size and the size distribution width by fitting the calculated ion energy spectrum of hydrogen clusters with a log-normal size distribution in the CVI (Cluster Vertical Ionization) domain to that obtained experimentally. 3. A novel scheme is proposed in order to improve the table-top laser-driven fusion neutron yield from the explosions of deuterium clusters in which a preformed cylindrical plasma channel in the outer region of the cluster jet is created by the laser prepulses. With the aid of this plasma channel, the main laser pulse will be allowed to deposit its energy into the central region of the deuterium cluster jet and the fusion neutron yield would be expected to increase a factor of about 3 under the present calculation conditions. 4. The effect of the laser spot size on the neutron yield of table-top nuclear fusion from explosions of an intense fs laser pulse heated deuterium clusters is studied. It has been found that there exists a proper laser spot size for the maximum fusion neutron yield for a given laser pulse and a specific deuterium gas cluster jet. The proper spot size is dependent on the laser parameters and the cluster jet parameters. 5. Pure Coulomb explosions of methane clusters (CA4)n (light atom A=H or D) have been studied by an electrostatic model for both a single cluster and an ensemble of clusters of a given size distribution. It is found that the average energy of deuterons from the explosion of (Cq+D4+)n tends to saturate as the charge q of the carbon ions becomes larger than 4. This implies that when the laser intensity is sufficiently high for the (CD4)n to be ionized to a charge state of (C4+D4+)n, the neutron yield from Coulomb explosion of deuterated methane clusters could be increased significantly by increasing the interaction volume, i.e., increasing the number of deuterons, rather than by increasing the laser intensity to produce the higher charge state (C6+D4+)n. Meanwhile, it is revealed that the ratio of the average kinetic energy of the light ions to their initial average electrostatic potential energy before cluster expansion is irrelevant to the cluster size. This finding means that as far as the ratio is given, the average kinetic energies of the light ions from the pure Coulomb explosion of (Cq+A4+)n can be simply estimated from their initial average electrostatic potential energies, rather than from the time-consuming simulations. 6. Nuclear fusion of deuterium clusters (D2)n and deuterated methane clusters (CD4)n driven by intense fs laser pulses was experimentally demonstrated. The efficiency of about 3.7×104 and 1.5×105 fusion neutrons per joule of incident laser energy was achieved for the (D2)n and the (CD4)n clusters, respectively. Compared with the results reported in the literature [103], the neutron production efficiency of the Coulomb exploded deuterated methane clusters in this work was improved