超强激光与等离子体相互作用中粒子加速相关物理问题的研究
文献类型：学位论文
| 作者 | 王文鹏 |
| 学位类别 | 博士 |
| 答辩日期 | 2012 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 徐至展 ; 沈百飞 |
| 关键词 | 相对论离子的稳定加速 光压加速 热电子角分布 |
| 其他题名 | Theoretical Research of Particle Acceleration via Interaction of Ultra Intense Laser with Plasmas |
| 中文摘要 | 随着激光技术的不断发展，实验中已经能够得到聚焦强度超过1022W/cm2、脉冲宽度小于10fs的相对论激光脉冲，这种超强激光脉冲与物质的相互作用其应用十分广泛(如粒子加速、激光驱动的X射线源等)。激光驱动的离子加速是倍受人们关注的研究热点之一，加速得到的高能离子可以用于研究高能密度物理、医用质子治疗、瞬时X射线诊断、在正电子发射X线断层摄影术中产生同位素、激光聚变中的离子束快点火方案等。与传统加速器相比激光驱动的离子加速有着巨大的优势，如结构紧凑、耗费低等。超强激光与等离子体相互作用加速离子已经得到了广泛的理论和实验的研究，目前主要加速机制有靶后鞘层法向加速、激波加速、光压加速以及其它混合加速机制等。为了能够更有效得到低能散的准单能粒子束（可期待用于医学治疗、质子成像和惯性约束聚变），本论文主要对光压加速机制做了深入研究。同时,为了更好的解释实验结果，我们从模拟上研究了p偏振的激光斜入射到靶面上产生热电子的角分布。主要成果如下： 1.通过解析模型和PIC模拟研究了陡上升沿的圆偏振超强激光光压加速,得到相对论离子的稳定加速。研究打洞过程中压缩电子层(CEL)和离子的动力学行为,发现在我们研究的超短脉冲参数条件下，初始处于靶中间的离子先于其它离子被加速到靶的后表面。对于给定密度和厚度的靶，找到合适的激光上升沿时间使得初始位于靶中间的离子和CEL同时到达靶的后表面,从而可以抑制不稳定性的发展，并有效得到稳定的单能离子束。使用振幅a0=200的激光，可以产生10 GeV的质子束。PIC模拟很好的验证了我们的理论预测。 2.建立解析模型，研究了陡上升沿圆偏振激光与高密度等离子体靶相互作用过程中压缩电子层（CEL）的动力学行为。研究了厚靶、薄靶两种情况下，建立CEL稳定状态所需的条件。对于厚靶（激光不透射），发现CEL在激光到达峰值振幅a0后还需要经历一段弛豫距离最终达到稳定；对于薄靶（激光部分透射），发现足够小的激光上升沿斜率a0/tup可保证CEL达到稳定状态，否则，CEL将被光压推离薄膜靶并离散。结合离子的运动，通过解析模型，找到了陡上升沿激光（任意上升沿形状）作用下CEL和初始位于靶中间的离子同时进入光帆阶段的靶和激光参数，为超强超短激光进行优化的光压离子加速提供了理论设计方法。 3.利用PIC模拟研究了振幅为a=0.5、1、2的p偏振激光分别以22.5o、45o、67.5o入射到固体靶表面上时热电子发射的角分布，模拟结果验证了由动量守恒得出的靶表面热电子角分布的理论结果。研究了靶面的形变、阿尔文电流极限、靶表面自生电磁场对热电子角分布的影响。发现高能量的热电子沿着入射激光的偏振方向出射。通过比较模拟结果与实验中利用60 fs、180 mJ的激光与铝靶相互作用的结果，发现两者符合得很好。 |
| 英文摘要 | With the development of the laser technology, the intensities of relativistic laser up to the order of 1022W/cm2 (<10 fs)can now be achieved in the experiments. The interaction of such pulses with matter is related to many applications, such as particle acceleration and ultra-short and coherent X-ray radiation sources. Especially, the laser-driven ion acceleration has attracted much attention due to its potential applications, such as high energy density physics, proton therapy, radiographing transient processes, generating isotopes in positron emission tomography, and ion beam fast ignition for laser fusion. Laser-driven ion accelerator is of low cost and compact compared to the conventional accelerators. Ion acceleration via the interaction of ultra intense laser pulse with plasmas has been widely studied experimently and theoretically. The dominant methods of ion acceleration include shock acceleration, radiation pressure acceleration (RPA), target-normal sheath acceleration (TNSA) and so on. In order to obtain the high quality ions with low energy spread for ion injectors and medical applications, the RPA mechanism has to be studied in detail. To explain the experiment results, the angular distribution of electrons emitting from a foil surface illuminated by a p-polarized laser pulse is also studied using partical-in-cell simulation. The main results are given as follows: 1.Stable acceleration of relativistic ions by the radiation pressure of a super-intense, circularly polarized laser pulse with sharp front is investigated by analytical modeling and particle-in-cell simulation. The dynamics of the laser-compressed electron layer and the ions in the hole-boring stage are investigated. In the case studied, the ions initially in the middle of the target are found to be accelerated to the back surface of the target ahead of the other ions. For foils with given density and thickness, the suitable steepness of the laser front is found to suppress instabilities and efficiently drive a stable monoenergetic ion beam by controlling the ions initially in the middle of the target to enter the compressed electron layer just at the end of the hole-boring stage. With a laser pulse of peak amplitude a0=200, a proton beam of energy about 10 GeV can be generated. The PIC simulation results agree very well with our theoretical expectation. 2.The dynamics of the compressed electron layer (CEL) is investigated for a circularly polarized laser pulse with sharp front irradiating a high-density foil. The opacity case where the laser pulse is totally reflected is investigated with a simple analytical model. For the case of thick foil, it is found that a relaxation distance is required to build the quasiequilibrium state of CEL after the CEL reaches the distance d at the laser peak. For the thin foil, it is found that the formation of the quasiequilibrium state of CEL can be formed if the laser front steepness a0/tup is small enough. Otherwise the CEL will be pushed away from the target by the laser pressure and disperses. Depending on the dynamics of ions and the analytical model, the proper laser and target are found to make the CEL and the ions initially in the middle of the target simultaneously arrive at the back surface of target. It provides a theoretical design to optimize the acceleration driven by the radiation pressure of an ultraintense ultrashort laser. 3.The angular distribution of electrons emitting from a foil surface illuminated by p-polarized laser pulses is studied using particle-in-cell simulation for incident angles (22.5o,45o,67.5o) and laser amplitudes (0.5,1,2). Theoretical prediction of the emission direction, based on canonical momentum conservation along the target surface, is verified. Surface ablation, the Alfvén current limit, as well as self-generated electromagnetic fields on the surface are numerically investigated and found to play important roles in the modulation of the angular distribution of the emitted electrons. Emitted electrons of higher energy are found to be directly accelerated to near the polarization direction of the incident laser light. The simulation results agree very well with the recent experimental results from Al targets irradiated by a 60-fs, 180-mJ laser pulse. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15677]  |
| 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 | 王文鹏. 超强激光与等离子体相互作用中粒子加速相关物理问题的研究[D]. 中国科学院上海光学精密机械研究所. 2012. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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