飞秒强激光与固体靶相互作用的数值模拟和理论研究
文献类型：学位论文
| 作者 | 金张英 |
| 学位类别 | 博士 |
| 答辩日期 | 2009 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 沈百飞 |
| 关键词 | 飞秒激光脉冲 固体靶 圆偏振 等离子体通道 离子加速 |
| 其他题名 | Numerical and theoretical studies on the interaction of Femtosecond Intense Laser Pulse with Solid Target |
| 中文摘要 | 本博士论文首先介绍了激光与等离子体相互作用的一些基本物理现象、研究方法以及主要研究方向等内容。然后就飞秒强激光与固体靶相互作用的几个问题展开了以下几方面工作： 1. 在考虑相对论和有质动力非线性，以及全局电量守恒的前提下，分析了强激光在冷等离子体窄通道中稳定传播的情况。采用较为简化的二维理论模型，给出了一系列描述激光光场与等离子体密度的方程，从而可以准确地确定激光在通道中传播的状态和通道产生的变化。当存在预通道时，即使等离子体通道的密度远大于临界密度很多且通道很窄（激光波长量级），激光仍然可以在通道中传播；激光稳定传播所需要的最低激光强度要比没有预通道时小，这个激光强度可以通过数值求解得到。当通道越宽，通道密度越小，激光强度越大时，激光越容易传播。在同样的通道状况和传播系数时，TE1模的激光需要更高的激光强度。这一工作有助于确定脉冲是否能够在通道里顺利传播，这对于“快点火”，以及激光等离子体电子加速、x射线激光、谐波的产生等的研究都有帮助。 2. 利用二维（2D）粒子模拟（PIC）程序研究了超强圆偏振的激光脉冲与高密度离子体通道中的高密度小靶相互作用的情况。等离子体通道可以将激光限制在其中，因而激光可以在更长时间保持较高的强度。在我们的模拟参数下，即使当激光已经离开通道15个激光周期后，激光的横向尺度仍然比没有通道时的小；激光强度也要比没有通道时的高28%。相应的，有通道时，小靶从激光中获得的能量较没有通道时提高了约26%。而且，由于激光强度更大，有质动力也更大，对小靶横向的约束也更强；因此被加速的离子束的准直性更好。在有通道的情况下，即使在小靶离开通道15个激光周期后，小靶中的大部分质子的横向动量与纵向动量比值小于0.18。由此可见，等离子体通道不但有助于提高离子加速的效率；而且可以使得被加速的离子具有更好的准直性。 3. 在考虑相对论和库仑爆炸的静电场随时间变化的情况下，研究分析圆偏振超强超短激光脉冲与薄膜靶相互作用时光压与库仑爆炸相结合的离子加速过程，解析给出了最大离子速度随时间变化的表达式。离子加速的过程大致上可以分为两个连续的加速阶段：首先，薄膜靶在第一阶段被光压加速，在激光传播方向上获得一定的速度 。由于激光脉宽非常短，并且光压对薄膜靶有一定的约束作用；因此当激光脉冲离开靶的时候，虽然薄膜靶已经获得了一定的速度并且薄膜靶中的电子温度很高，热压也很大，但是薄膜靶并没有向前移动很远，也尚未来得及剧烈膨胀。随后，位于靶后的质子经过第二阶段的库仑爆炸，能量进一步提高。通过二维PIC模拟比较，发现模拟结果与解析结果符合的较好。在第一阶段光压加速结束后，最大离子能量达到302 MeV；经过第二阶段库仑爆炸的加速，最大离子能量可达到 以上。两种机制对于离子的加速都起到了非常重要的作用。对于一定的激光参数，等离子体靶的参数决定了哪一种机制起主要作用。 4. 我们利用二维PIC模拟程序研究了激光与纳米尺度的微结构靶（在碳靶的背面掺入少量的质子）相互作用的过程。位于靶后的质子层既可以被激波产生的静电场加速，又可以被靶后鞘层产生的静电场加速。同时由于质子层很薄，密度也不高，可以获得准单能的质子束。与线偏振情况相比，虽然线偏振时的电子加热更为剧烈，TNSA效率更高，但是圆偏振情况时的激波加速效率远比线偏振时的高，可以弥补TNSA效率的不足。因而质子束可以被加速到比线偏振情况时更高的能量。在我们的模拟参数下，可以获得中心能量约为11 MeV（为LP情况中的两倍），能散度约为0.18（不到LP情况的二分之一）的准单能离子束。我们考虑的激光和等离子体靶参数是在目前技术条件已经可以实现的范围内，因此这一工作对于在实验上研究准单能高能离子束的产生是有帮助意义的。 |
| 英文摘要 | This thesis is composed of six chapters. Some basic physical phenomenon, research methods and the attractive research fields in the interaction of ultra-fast and ultra-intense laser pulse with plasma are briefly reviewed in Chapter 1. In other chapters we researched several problems in the interaction of femtosecond laser pulses with solid targets, and the main results in this thesis are given as follows: 1. Considering relativistic and ponderomotive nonlinearities and global charge conservation, the steady propagation of a high-intense circularly polarized (CP) laser pulse in preformed a cold plasma channels is investigated. A simplified 2D model is derived to describe the structures of the laser pulse and the channel and exact analytical solutions describing the cross-sectional structures of the laser amplitude and the plasma density are presented. It is proved that if there is a preformed channel, the laser pulse can propagate in the channel of pulse-duration order, even the plasma density is much higher than the critical density. If the channel is wider, the plasma density is lower or the laser intensity is higher, the laser pulse will propagate more easily. In the same channel condition, higher laser intensity is needed to propagate for the TE1 mode at the same group velocity as that for the TE0 mode. This work is helpful to certain whether the laser pulse can easily propagate in a preformed channel, which is important to the fast igniting scheme of inertial fusion, electron accelaration in laser-plasma interaction, x-ray lasers and high-harmonic generation. 2. Two dimensional (2D) particle-in-cell (PIC) simulations are taken to study the interaction of an ultra-intense, CP laser pulse with a preformed overdense plasma channel in which there is a slice of micron size. Because the laser pulse can be confined in the channel, it can keep higher intensity on a longer time scale than the case without a channel. In our simulation, comparing to the case of no channel, after the laser pulse has left the channel for 15 laser periods, the transverse dimension of the laser pulse is smaller, and the intensity of laser pulse is 28% higher, as a result, the acceleration of the slice is more efficient and the ion energy getting from the laser energy is 26% higher. Moreover, because of the higher intensity of the laser pulse in the case of channel, the ponderomotive force is higher and the effect of the radial (ponderomotive) confinement is stronger. And hence, the ions may have better collimation comparing to case of no channel. After the slice has left the channel for 15 laser periods, the ratio of transverse and longitudinal momentum of most of the ions is samller than 0.18 in the case of channel. In a word, the channel can not only help to acceleate ions more efficiently but also help to produce ions with better collimation. 3. Taking the temporal evolution of the electrostatic field due to the Coulomb explosion into consideration, we studied a scheme of generating energetic ions. We combine the effects of radiation pressure and Coulomb explosion u using an ultra-intense, CP laser pulse interacting with a thin solid foil. The maximum ion velocity varying with time is predicted theoretically. The ion acceleration regime can be described by two subsequent stages on a whole. Firstly, the foil gains a velocity as a whole along the direction of laser propagation due to the light pressure on it. Because the laser pulse is very short and the heated foil is confined by light pressure, when the laser pulse leaves the target, the foil has not moved forward very much and the foil is not able to expand apparently despite the foil has gain a high velocity and the electrons have already been heated a lot. Then, the ions located at the rear side of the target are further accelerated by the Coulomb explosion. 2D PIC simulations are performed to verify the theory, and the theoretical results match the simulation quite well. After the first acceleration stage, the maximum proton energy is about . After the second acceleration stage, the maximum proton energy is above . Both mechanisms play important roles in ion acceleration. For a given laser pulse, the plasma parameters determine which acceleration mechanism is dominant. 4. 2D PIC simulations are carried out to study the interaction of laser pulse with a nano-scale micro-structured target (a carbon target with protons doped in the rear side of it). The protons located at the rear side of the target can be affected by the both shock acceleration (SA) and target normal sheath acceleration (TNSA). Because the proton layer is very thin and its density is lower than the foil, quasimonoenergetic proton bunch can be obtained. The electron heating is more tempestuous and the TNSA is more efficient for the linearly polarized (LP) case, however, the shock acceleration in the CP case is much more efficient and SA acceleration can well compensate the comparatively lower efficiency of the TNSA energy comparing to the LP case. Consequently, with CP laser pulse, the protons can be accelerated to a higher velocity than with a LP laser pulse. With our simulation parameters, a quasimonoenergetic proton bunch is produced with central energy about 11 MeV (twice as that in the LP case) and energy spread about 0.18 (less than the half of that in the LP case). The simulation parameters of laser pulse and plasma target is available for nowday the experimental techniques. So, this work is helpful to the experimental researches on how to generation quasimonoenergetic proton bunch. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15267]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 金张英. 飞秒强激光与固体靶相互作用的数值模拟和理论研究[D]. 中国科学院上海光学精密机械研究所. 2009. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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