【作者】 林丹；

【导师】 霍裕昆； 孔青；

【作者基本信息】 复旦大学 ， 原子与分子物理， 2008， 博士

【摘要】 随着强激光技术的飞速发展,利用激光的超强电磁场来加速带电粒子(主要是电子)的研究受到了人们的广泛关注。本论文集中研究真空激光加速中的有质动力加速机制(PAS:Ponderomotive Acceleration Scenario)。PAS通常指如下的相互作用过程:在真空中强激光脉冲在焦斑区追上慢电子群,相互作用并加速其中的部份电子。本文系统的研究了PAS的电子动力学特性和加速特性,分析了PAS加速的物理机理。目前唯一证实了真空激光加速的实验即是通过PAS加速机制实现的。该加速机制方案设计和实验装置都相对比较简单,容易在实验上实现。PAS的研究是激光加速研究的一个重要分支,在理论和应用两方面都有重要的意义。PAS的加速机理在于被加速电子在激光场中所经历的加速阶段和减速阶段的不对称性:与脉冲上升沿相互作用的加速阶段发生在激光焦斑区附近的强光场区,而与脉冲下降沿相互作用的减速阶段发生在焦斑区后强度相对较弱的光场区(光束的衍射效应)。因此电子在与激光脉冲相互作用的过程中获得净能量增益。本论文通过三维计算机模拟程序求解相对论Newton-Lorentz运动方程来分析有质动力加速过程中电子在激光场中的动力学行为,探讨PAS的物理特性。我们的研究证明了PAS加速机制可以实现真空激光加速电子(在一定条件下真空中慢电子可以被加速到MeV量级,并得到与实验相符的结果)。结合CAS加速机制(该加速机制利用真空中传播的聚焦光束存在的低相速度区域,配合以较强的纵向电场,形成天然加速通道实现真空中的快电子加速),我们可以得到结论:自由电子(快电子或慢电子)与激光场发生净能量交换是完全可能的,有力的说明了所谓的Lawson-Woodward定理具有极大的应用局限性。同时我们从理论上分析了有质动力势模型,包括非相对论情况下和相对论情况下的有质动力势模型,并讨论了这些理论模型的适用范围;同时也基于有质动力势模型给出了PAS相关特性的物理解释。通过理论分析和系统的模拟计算,本论文研究了PAS的一系列加速特性:如电子束团在径向上的散射呈现各向同性的特点(在激光强度不是很大的条件下);出射电子的能量分布和散射角分布比较弥散;高能部分的出射电子产额比较低等。此外,在本论文中我们还重点研究了真空激光加速的一个关键问题:有质动力加速电子的定标关系,得出被加速电子的最大输出能量与激光强度和激光腰宽成正比,与激光脉冲宽度成反比。并基于光场的有质动力势模型对这一由模拟计算总结出来的定标关系给出了物理解释。我们还对PAS与CAS这两种真空激光加速机制进行了比较。此外,我们还研究了真空激光加速的另一个重要问题:被加速电子的出射能量和散射角度的关联。通过数值模拟发现了在PAS出射电子的能量-角度关联谱中存在分叉现象:即相同能量的出射电子可能分布在两个不同的散射角上。基于对模拟数据的分析及光场的有质动力势模型对该现象给出了物理解释。上述研究不仅可以帮助我们更加深入地了解PAS的物理特性,与我们对CAS的研究相辅相成完善了对真空激光加速的理论研究;而且也可以为将来进一步的实验研究提供直接的理论指导和有用的参考数据。更多还原

【Abstract】 Rapid development of intense laser technology has excited a lot of interest in studying the interactions of ultra intense lasers with matter, especially laser acceleration of particles. Among many laser acceleration schemes proposed, we focus on research of the Ponderomotive Acceleration Scenario (PAS) scheme. Normally PAS refers to an interaction process where an intense laser pulse catches non-relativistic free electrons in the focal region, then interacts with and accelerates the electrons. In this thesis, the electron dynamic characteristics and acceleration properties of PAS have been investigated systematically, and the physics underlying the PAS scheme is also analyzed. The PAS has been demonstrated by experiments producing MeV electrons in vacuum, which is the only experimental data available now on vacuum laser acceleration. The PAS scheme is simple for experimental design to test the vacuum laser acceleration scheme. Hence, as an important branch of the vacuum laser acceleration research, studies on PAS scheme are of significance to both theory and application development.The physics underlying the PAS scheme is the asymmetry in the fields experienced by the accelerated electron during its acceleration and deceleration stages. When a laser pulse catches an electron in the focal region, the intensity experienced by the electron in the ascending front (corresponding to acceleration stage) is greater than that in the descending part (corresponding to deceleration stage), owing to the diffraction effect of the focused laser beam. As a result, the electron can gain net energy in the interaction process with the intense laser pulse.To study the physical properties of PAS and the detailed electron dynamics in the laser field, a three-dimensional test-particle simulation is used to solve the relativistic Newton-Lorentz equations of motion. It has been confirmed by our studies that the PAS can accelerate slow electrons to energies up to MeV, and these data are in consistent with experimental observation. Together with the capture and acceleration scenario (CAS) scheme, which deals with vacuum laser acceleration of fast electrons using the low wave phase velocity region of a laser beam, it is clearly demonstrated that far-field laser accelerations are available for either fast or slow electrons without applying any additional condition. At the same time, we discussed the ponderomotive potential models, including models under the non-relativistic and the relativistic circumstances, as well as their applicability. And the ponderomotive potential models have been used to interpret the related characteristics of PAS.The acceleration properties of PAS have been investigated systematically based on physical analyses and large amount of calculations and simulations. It has been found that electrons are scattered isotropically in the radial direction for moderate laser intensity. The output bunch exhibit relatively wide angular and energy dispersions and the fraction of electrons with high outgoing energy are rather small. Besides, the scaling law for accelerated electrons, a key problem in vacuum laser acceleration, has been studied deeply. The maximal electron-energy gain in the PAS regime is found to be approximately proportional to the laser intensity and the laser beam width, and inversely proportional to the laser pulse duration. Physical interpretations based on the ponderomotive potential model are presented. We have also compared the scheme and acceleration properties of PAS with those of CAS.Still, another crucial subject of vacuum laser acceleration ~ the correlation between the outgoing energy and scattering angle of accelerated electrons has been investigated. Bifurcation phenomenon in the energy-angular correlation spectrum of the vacuum laser ponderomotive acceleration has been observed with computer simulation. It can be seen that for focused laser pulse the classical single-valued energy-angle correlation for a plane wave is not only broadened to a band, which means electrons with the same outgoing energy will have an angular spread, but is also bifurcated, with the classical value lying in between the two branches. Analytic expression to describe the correlation has been derived and physical explanations based on the ponderomotive potential model and Lorentz-Newton force analyses are presented.The above studies are not only helpful in better understanding of ponderomotive acceleration scenario and further improving the studies on vacuum laser acceleration together with CAS, but also of significance for experimental design to test the vacuum laser acceleration schemes by providing theoretical reference as well.更多还原