飞秒强激光与固体靶相互作用静电激波加速离子的研究
文献类型：学位论文
| 作者 | 张晓梅 |
| 学位类别 | 博士 |
| 答辩日期 | 2008 |
| 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 圆偏振 固体靶 静电激波 离子加速 |
| 其他题名 | Ion Acceleration by Electrostatic Shock in Interaction of Femtosecond Intense Laser Pulse and Solid Target |
| 中文摘要 | 相对论强光光学是门正在迅速发展的新兴学科，它涉及非线性光学、等离子体物理、高能物理、加速器物理等学科，高能粒子的加热和加速是其中的核心问题。随着激光技术的飞速发展，人们已能获得聚焦强度超过1022W/cm2、单脉冲宽度小于10fs的相对论激光脉冲，这在实验和理论上大大推进了粒子加速的研究。强激光与固体靶相互作用中高能离子的产生，已得到很多的理论及实验研究，目前离子加速机制主要有前场离子加速、后场离子加速以及激波驱动的加速等。这些加速机制的效率和产生的离子能量随着所用激光强度、脉冲宽度和等离子体密度等参数的不同而有所不同。但无论是靶前加速还是靶后加速，很久以来，人们大多选用线偏振激光与等离子体作用，对于圆偏振激光的研究则相对较少。本论文主要进行了飞秒相对论圆偏振激光与等离子体靶相互作用过程中离子加速的研究工作，具体如下： 1、采用一维particle in cell粒子模拟程序研究了飞秒圆偏振激光脉冲与固体靶相互作用，通过得到的离子密度和离子速度分布图分析了离子随时间的运动演化。采用中等强度（2.74×1018W/cm2，相应的归一化矢势a=1）平顶形状的长脉冲作用在初始密度为5nc（nc为临界密度）的冷冻等离子体靶上时，靶前的电子离子分离层可以保持较长的时间，电场始终为正，离子持续加速。宏观上看，激光脉冲如同活塞一样可以比较长时间稳定加速离子，类比静电激波加速理论，离子呈现出不断反射的过程，相空间中形成速度平台，模拟结果与数值计算吻合较好。 2、中等强度的圆偏振平顶激光脉冲与初始冷冻的等离子体靶相互作用可以稳定加速离子，在离子相空间形成速度平台结构，当所有的离子都被加速到具有相同速度的单能态时便得到了单能离子源。此时，如若激光脉冲持续，靶前仍形成相同的静电场对离子进行二次加速到更高单能态。如果参数合适，离子会受到更多次加速。因而，利用这种多次加速机制，可以在某些特定时刻（可求）获得单能性较好的离子源。以峰值强度为1.096×1019W/cm2（a=2）的激光为例，经过二次加速后离子能量达到几兆电子伏特（MeV）。每一次加速后的离子速度可以通过离子反射模型求得，与模拟结果吻合的很好。 3、在以上基础上，采用数值解析和计算机模拟详细讨论了圆偏振激光作用下等离子体背景温度对静电激波产生及离子加速的影响。发现初始背景温度较低时，少数离子穿透到激波面后，其余被反射的离子速度比初始背景温度为零时稍大，这与理论解析结果相符。当初始背景温度较高时，由于离子速度分布较宽，满足反射条件的离子数量减少，尽管被反射的离子获得的能量更高，但大部分离子会穿过激波波面。这也就是说，较低的初始离子温度对应于更加单能的离子源，而较高的初始离子温度对应于被反射离子的较高能量。 4、采用超强激光脉冲（2.74×1022W/cm2，a=100）作用在初始密度为10nc的冷冻靶上时，对比圆偏振激光和线偏振激光脉冲作用效果，发现圆偏振激光作用时部分离子反射得到相对论能量，激光作用70个周期后激光能量转换效率达到近50%。当此种超强圆偏振激光与超薄靶（厚度为2倍激光波长）相互作用时，等离子体将被整体加速，产生吉电子伏特（GeV）能量的离子，且激光能量转换成离子能量效率趋于100%。 5、研究了圆偏振激光强度对离子加速和靶前动态平衡的影响，发现较强激光（1020W/cm2，归一化矢势a~10）作用到初始密度为10nc的冷冻靶上时，受离子运动影响，靶前电子所受光压和静电场的动态平衡比较明显，靶前静电分离场呈现周期性变化，离子被全反射，相空间不再是平滑的平台，而是包含很多弯曲状（curve）结构。 6、利用基于VORPAL的PIC模拟程序研究了圆偏振激光脉冲与包含两种离子（不同质量不同密度）的混合靶相互作用时的离子加速。我们发现在强度为1019W/cm2（a=2）的激光作用下，离子加速分成两个比较明显的过程，初始，由于重离子惯性大，质子先加速，效果与等离子体靶只含纯质子情况的加速相似。随后，重离子被加速，且赶上质子以相同的速度共同加速。随着质子比重的提高，重离子加速效果越好，速度值介于两种纯离子时被加速的两个速度值之间，这与数值解析结果吻合得较好。另外，还研究了等离子体电子初始温度和等离子体靶厚度等参数的影响。 |
| 英文摘要 | The relativistic optics is a developing new subject, which is related with nonlinear optics, plasmas physics, energetic physics, and accelerator physics and so on. The heating and acceleration of energetic particles are especially important. The rapid development of laser technology has made available ultraintense (1022W/cm2) ultrashort (<10fs) laser pulses, which has promoted the research of particle acceleration experimentally and theoretically. Ion acceleration in the interaction of intense laser pulse and solid target has been studied intensively and the efficiency and the ion energy are different for the different laser intensities, pulse widths and plasma densities for the well-known acceleration mechanisms, such as front side acceleration (FSA), target normal sheath acceleration (TNSA) and electrostatic shock acceleration and so on. The linearly polarized laser pulse is often used in previous works. In this paper, ion acceleration by a circularly polarized laser pulse is focused and some results are given as follows: 1、The interaction of a circularly polarized pulse with a cold solid target is studied with one dimensional particle-in-cell (PIC) simulation. The evolution of ion is analyzed from the distributions of ion density and ion velocity. When a long flat-top pulse with normal intensity (2.74×1018W/cm2, the corresponding normalized scale potential a=1) irradiates a cold target with density of 5nc, the charge separation inducing the positive electrostatic field can keep enough long time to accelerate ions, just acting as a piston. Ions are reflected by the steady laser-driven piston which is similar to ions reflection off an electrostatic shock front. A velocity profile with a flat-top in ion phase space is found and the simulation results agree qualitatively with the analytical results. 2、Ions can be accelerated stably by a normal intense circularly polarized laser pulse with a flat-top profile and thus a flat-top structure forms in ion phase space. After all the plasma ions are accelerated, the acceleration process is repeated by the same electrostatic field on the resulting monoenergetic ions. Under suitable conditions multiple repetitions can be realized and a high-energy quasi-monoenergetic ion beam can be obtained. Therefore, quasi-monoenergetic ion beams are produced at specific instants. As an example, MeVs ions can be obtained after twice accelerations by a laser pulse with intensity of 1.096×1019 W/cm2 (a=2) and the simulation results agree well with the numerical estimation. 3、Based on the above results, the effect of plasma temperature on electrostatic shock generated by a circularly polarized laser pulse in overdense plasma is studied by particle-in-cell (PIC) simulation. The results show that at low temperature, few ions are transmitted and the reflected ions have higher velocity than that in the cold target case, which is in agreement with the analytical results. At higher temperature, due to the wider velocity distribution, the number of ions satisfying the reflection condition decreases. Although the energy is higher, most ions are transmitted. That is to say, lower ion temperatures correspond to more monoenergetic accelerated ions, and higher ion temperatures correspond to higher maximum energy of the latter. 4、At a laser intensity of 2.74×1022 W/cm2 (a=100) and a cold target density of 10nc, it is found that ions can be reflected partly and obtain relativistic energy by a circularly polarized laser pulse and for the same laser and plasma parameters, ion acceleration by a circularly polarized laser pulse is more efficient than that by a linearly polarized laser pulse. PIC simulation results show that due to ion reflection, the efficiency of energy conversion from laser to ions is nearly 50% in the case of thick target after 70T and 100% in the case of ultrathin target (about 2 wavelengths) which is accelerated as a single entity, and GeV ions are produced. 5、The effects of laser intensities on the ion acceleration and dynamic equilibrium state at the front of the target have been investigated. Because of the influence of the ion motion, it is found that when the cold solid target with the density of 10nc is irradiated by a circularly polarized laser pulse with the intensity of 1020W/cm2, the dynamic equilibrium state of electrons between light pressure and electrostatic pressure is obvious and the electrostatic field changes periodically. Flat-top structures with curves in ion phase space are shown instead of the smooth flat-top structures. 6、Ion acceleration from the interaction of a circularly polarized laser pulse with a mixed solid target containing two species of ions is studied by 1D PIC code based on VORPAL. With the laser intensity of 1019W/cm2 (a=2), there are two obvious acceleration stages. At early time, the light ions are accelerated efficiently by the electrostatic field due to the charge separation between electrons and ions while the heavy ions don not have enough time to response. The results are similar with the pure protons case. At a later time, the heavy ions are accelerated efficiently and they catch up with the protons and thus the velocities of them tend to be equal. The acceleration effect of the heavy ions is better with the rising of the density ratio of the light ions and heavy ions. The maximum and minimum velocities correspond to these in the cases of pure protons and pure heavy-ion plasmas, which is in agreement with the analytical results. In addition, the influences of the parameters of the laser pulse and target on the ion acceleration are discussed in detail. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15221]  |
| 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 | 张晓梅. 飞秒强激光与固体靶相互作用静电激波加速离子的研究[D]. 中国科学院上海光学精密机械研究所. 2008. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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