超短相对论激光与固体靶相互作用 的等离子体动力学研究
文献类型：学位论文
| 作者 | 黄林根 |
| 学位类别 | 博士 |
| 答辩日期 | 2013 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 黄林根 |
| 关键词 | 激光等离子体相互作用 等离子体加热 电离 激光质子加速 particle-in-cell |
| 其他题名 | Plasma Dynamics in Ultrashort Relativistic Laser Pulse Interacting with Solid Target |
| 中文摘要 | 近30年来，高功率激光技术的迅速发展使得激光峰值强度高于10^18W/cm^2，脉宽小于10^-12 s ，从而使强场激光与物质相互作用进入到相对论领域，极大地推动了等离子体基础科学的研究。这样的超短相对论激光作用在固体靶上会触发非常丰富的物理过程：包括电磁场的吸收与产生，靶体的原子电离，表面热电子的产生与传输，带电粒子的碰撞，靶体电子的回流与加热，离子的加热与加速，反粒子的产生，流体不稳定性等等，这些动力学过程发生在 的极短时间量级内。当前，有大量的实验和理论模拟工作都努力试图理解这些超快基本物理过程，并应用到医学和能源领域，如肿瘤治疗和聚变等。尽管如此，由于其固有的复杂特性，仍有很多开放性的问题需要得到更深入和细致的研究。 本论文借助于超高性能计算机机群和particle-in-cell (PIC)程序从数值和理论上集中探索研究了超短相对论激光与固体靶相互作用过程中的离子加热，靶体电离和离子加速的物理过程，并探讨了利用第四代光源硬X射线自由电子激光器(XFEL)去探测相关的动力学现象。具体如下： 1. 在数值上详实研究了激光峰值光强为 10^18W/cm^2 ~ 5*10^20W/cm^2 范围内，脉宽为100fs ~ 500fs 范围内超短相对论激光作用在多层固体靶CD2-Al-CD2上靶体内部的离子加热动力学过程。研究发现，随着相对论激光作用在固体靶上，靶体内的电子会内电离加热，从而在Al-CD2界面处形成~10GPa的压强梯度。该压强梯度会使中间的Al层向外膨胀，压缩推动CD2塑料层。数值结果表明，在CD2压缩层的膨胀过程中，被膨胀波前静电场加速的离子束流动能会转换成其热能，使该层的氘离子在 时间量级被被加热至数百eV ， ~2倍于其他区域的离子热温度。文中基于XFEL的小角度散射技术(SAXS)和法拉第旋转技术，在理论上简要探讨了探测靶体内部等离子膨胀波，成丝现象和磁场产生的物理过程。 2. 基于Landau-Lifshitz场电离和Thomas-Fermi压力电离模型，利用PIC程序在数值上研究了超短相对论激光作用在固体Cu靶上的电离动力学。模拟结果表明，峰值光强为10^20W/cm^2激光作用在预等离子体特征长度大于趋附深度的Cu靶上，Cu离子的电荷分布出现横向长度为激光波长量级 的成丝结构。而对于无预等离子体的Cu靶，其电荷分布非常匀滑。该成丝结构不仅与预等离子体相关，也与入射激光强度有很大的关系。当入射激光较弱时，电离过程只发生预等离子体区域，且离子电荷分布匀滑。基于等离子体色散特性及Cu离子的 K-alpha吸收特性，文中提出了用XFEL探测固体靶中电离过程的方法，并给出了理论计算的XFEL探测图像。 3. 提出了利用圆偏振相对论激光作用在三明治靶上获得能谱发散度较小的质子束方案。其基本原理为：三明治靶受相对论激光辐射会在重离子层和质子层界面处形成横向静电场，该静电场会约束被圆偏振光加速的质子的横向运动，从而使之感受到空间分布更均匀的纵向加速静电场，获得相对来说较单一的加速，能谱发散度较小。PIC模拟结果显示，质子层的高度和重离子的电子密度对加速质子的能谱影响较大，而重离子的质核比A/Z则不会对质子能谱产生较大的影响。 文中也探讨了激光驱动质子加速的最优靶厚。 |
| 英文摘要 | With rapid development of the high power laser technique in last several decades, ultraintense ultrashort laser pulses with peak intensity higher than 10^18W/cm^2 and pulse duration shorter than 1ps have become available, which provides intense laser-plasma interaction accessible to relativistic and ultrafast regime. The technical development makes a great step forward for promoting the fundamental scientific research of plasma physics. Abundant dynamics of physics are triggered as long as such a high power laser interacting on solid target: such as atomic ionization, hot electron generation and transportation, collisions between the charged particles, return current, bulk electron heating, ion heating and acceleration, antiparticle generation, instability and so on. All of the dynamics processes happened in time scale. Motivated by the potential applications in proton diagnosis, cancer therapy, inertial fusion, etc., a number of studies have been done to try to understand the fundamental dynamics. However, due to the complexity of the subject in nature, there are still many open questions which require further and deeper investigation. In this work, particle-in-cell numerical simulations based on high performance computer cluster explored and evaluated the bulk ion heating dynamics, ionization dynamics and laser-driven proton acceleration in solid target irradiated by ultrashort laser pulses with relativistic intensities. We also proposed the potential techniques such as small angle X-ray scattering, Faraday rotation, and resonant coherent X-ray diffraction imaging to probe relative physical processes in high power laser plasma interactions using X-ray free-electron lasers. Following, part of the results obtained is listed: 1. We investigate bulk ion heating in solid CD2-Al-CD2 buried layer targets irradiated by ultra-short relativistic laser pulses with intensities ranging from 10^18W/cm^2 to 5*10^20W/cm^2 and pulse durations ranging from 100fs to 500fs using kinetic particle-in-cell simulations. We find enhanced deuteron ion heating in a layer compressed by the expanding aluminium layer. As long as the bulk target is ionized and heated, a pressure gradient with around ~0GPa difference created at the Al-CD2 interface pushes this layer of deuteron ions towards the outer regions of the target. Our simulations suggest that the directed collective outward motion of the layer is converted into thermal motion inside the layer, leading to deuteron temperatures to several hundred eV in ~ps time scale. This temperature is around 2 times higher than those found in the rest of the target. We also discuss the techniques of small angle X-ray scattering and Faraday rotation for probing the expansion process, filamentations and magnetic field inside the solid target. 2. The ionization dynamics in solid copper target irradiated by ultrashort relativistic laser pulses (I0>10^18W/cm^2,tau=40fs ) is numerically studied by particle-in-cell code implanted with Laudau-Lifshitz field ionization model and Thomas-Fermi pressure ionization model. We found that periodic filamentation structure of the copper charge state distribution with laser wavelength scale is triggered when the laser pulse with peak intensity 10^20W/cm^2 interacting on copper target with certain preplasma in front side. While the ion charge state distribution is quite smooth for the case without preplasma. The scaling of ionization dynamics on laser intensity shows that the structure of charge state distribution also strongly depends on the intensity. For the case of relative low laser intensity, i.e. 2*10^18W/cm^2 , due to the limitation of bulk electron heating, the ionization process only happened in the region of preplasma, and charge state distribution is quite smooth. The technique of resonant coherent X-ray diffraction imaging is proposed to probe the corresponding ionization dynamics using X-ray free-electron lasers. 3. Energetic proton acceleration with smaller energy spread from interaction of intense short circularly polarized laser pulse with a sandwich target is investigated using particle-in-cell simulation. The sandwich target consists of a hydrogen-plasma layer surrounded by carbon-plasma layers. It is found that the transverse electric fields generated at the plasma layer interfaces efficiently confine the longitudinally accelerated protons to within the hydrogen-plasma layer such that they are collimated and have smaller energy spread compared to a pure proton layer target. It is found that the width of the proton layer and the density of carbon layer can strongly affect the proton energy spectrum while the mass-charge ration of high Z layers does not affect too much. We also discuss the optimum target thickness on the proton acceleration. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15764]  |
| 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 | 黄林根. 超短相对论激光与固体靶相互作用 的等离子体动力学研究[D]. 中国科学院上海光学精密机械研究所. 2013. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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