基于相对论强激光的聚变中子产生及离子加速研究
文献类型：学位论文
| 作者 | 张辉 |
| 学位类别 | 博士 |
| 答辩日期 | 2015 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 李儒新 |
| 关键词 | 相对论强激光脉冲，气体团簇，库仑爆炸，靶后鞘层加速，无碰撞静电激波加速 |
| 其他题名 | Generation of Fusion Neutrons and Ion Acceleration Based on Relativistic Intense Laser |
| 中文摘要 | 随着激光技术的快速发展，尤其是啁啾脉冲放大技术，激光聚焦功率密度在过去的十多年里已经提高了7-8个量级，实验室条件下能够达到1022 W/cm2量级，激光单脉冲的宽度小于10 fs。这种超短超强的激光能为人类提供全新的实验手段和极端的物理条件，所以相对论激光与物质相互作用的研究受到广泛的关注，如激光驱动加速粒子、高次谐波的产生及激光惯性约束核聚变等。超强激光与固体或气体相互作用加速离子是激光与物质相互作用的重要组成部分，在理论和实验上得到广泛的研究，加速的离子束可以用于医用离子治疗、传统离子加速器的注入级和激光聚变中离子束快点火方案等。同时，激光与团簇相互作用近年来也取得了很大的进展，尤其是飞秒强激光与含氘团簇喷流相互作用引发氘氘核聚变实验的成功，为台式化中子源的实现提供了一个可能的途径。本文围绕相对论激光与物质相互作用进行核聚变中子产生和离子加速及相关物理问题展开实验研究。研究取得的结果如下： 1．搭建了相对论强激光与团簇相互作用的实验研究平台，开展了台式激光驱动含氘团簇喷流异核库仑爆炸引发氘氘核聚变的实验研究。为了提高聚变中子的转化效率，使用低温冷却CD4团簇与120 mJ, 60 fs激光脉冲相互作用。通过研究聚变中子产额随等离子体通道中氘离子密度和氘离子平均动能的时间演化关系，优化了聚变中子产额。在50 bars背压下，当CD4团簇的滞止温度降低到200 K时，获得了高达5.6 105的中子产额，这比相同背压、296 K时得到的中子产额提高了一个量级。实验分析表明，中子产额随温度降低而显著增加主要有两方面原因。一方面，当背压保持不变时，氘离子密度与滞止温度呈反比关系，这基本符合热力学预期。另一方面，CD4团簇尺寸和团簇温度存在强非线性关系，Hagena参量为： ，表明CD4团簇尺寸和温度的非线性关系比单原子或者双原子团簇更强，最终导致库仑爆炸中氘离子平均动能和中子产额的显著增加。为了获得更高的中子转化效率，增加团簇背压到80 bars，在217K时获得了1.9 107中子/J中子转化效率，这个结果比以前的实验结果提高了近一个量级。同时，根据实验中得到的氘离子密度和氘离子能谱以及等离子体通道的几何结构对中子产额进行理论估计，获得理论估计值与实验测量结果基本相符。 2．搭建了相对论强激光与固体靶相互作用进行离子加速的实验平台，系统的研究了靶后法线鞘层加速机制下预脉冲对质子加速能量和发散角的影响。首先，开展了高对比度 (1011)、相对论聚焦强度 (~1018 W/cm2) 激光与不同厚度的薄膜靶相互作用实验研究，在最优靶厚0.8 μm时获得了低发散角 (10o)、MeV量级的质子束。采用二维PIC模拟研究靶的厚度、密度和密度梯度对质子加速的影响，发现质子束的截止能量和发散角主要由靶后的密度梯度决定的。其次，利用高对比度 (1011)、更高聚焦强度 (2 1019 W/cm2) 激光与不同厚度的铝薄膜靶进行相互作用，研究了纳秒预脉冲对质子加速的影响，并且在最优靶厚2.5 μm时获得了截止能量为7 MeV的质子束。二维PIC模拟表明纳秒预脉冲能弯曲靶材，使得靶后法线方向的质子束能量降低。 3．开展了800 nm飞秒强激光 (1.7-3.5 1019 W/cm2)与碳薄膜靶相互作用进行无碰撞激波加速离子的实验研究。通过调节激光能量，30 nm厚的碳薄膜靶在激光主脉冲到达前被预脉冲加热并且扩散成近临界密度的等离子体。在等离子体密度稍大于临界密度时，加速获得的碳离子能谱在7.5 MeV处有一个明显的峰。二维PIC模拟表明在激光与略大于临界密度的等离子体相互作用时，主脉冲产生的热电子会激发无碰撞激波，使得碳离子束反射。这是在实验上第一次用800 nm飞秒激光证实无碰撞激波加速。 |
| 英文摘要 | With the rapid development of the laser technique, especially chirped pulse amplification (CPA), the laser peak intensity has been enhanced 7-8 of magnitude, reaching 1022 W/cm2 under laboratory conditions, and the laser duration is also less than 10 fs. Since this ultra-short and ultra-intense laser provides us new experimental means and extreme physical conditions, the laser-matter interaction has attracted much attention, such as laser-driven particle acceleration, high-order harmonic generation and inertial confined fusion. Ion acceleration based on the interaction of intense laser and solid/gas has become an important part of the laser-matter interaction and been widely studied theoretically and experimentally due to its potential applications, such as ion therapy, injector for conventional ion accelerators and ion beam fast ignition for laser fusion. Meanwhile, the laser-cluster interaction has also been made much progress and development. Especially, the experimental demonstration of DD nuclear fusion based on the interaction of femtosecond intense laser and deuterium clusters in gas jets illuminates a promising way for the development of a high-flux table-top fusion neutron source. In this thesis, experimental investigations on fusion neutron generation and ion acceleration via the interaction of the relativistic intense laser and matter have been carried out. The results are shown as follows: 1.An experimental platform on relativestic intense laser-cluster interaction has been built up to investigate the DD nuclear fusion from heteronuclear Coulomb explosion of deuterium clusters in gas jets driven by a table-top laser. Cryogenically cooled heteronuclear deuterated methane (CD4) clusters irradiated by 120 mJ, 60 fs laser pulses are used to improve the conversion efficiency of fusion neutron. The fusion neutron yield is optimized by investigating the evolution relationship of the fusion neutron yield and the deuteron density in the plasma filament and the average kinetic energy (KE) of deuterons. The yield of as high as 5.6 105 neutrons per pulse is obtained by lowering the stagnation temperature of CD4 gas to 200 K under a backing pressure of 50 bars, which is an order of magnitude higher than that of 296 K under the same backing pressure. Experimental analysis indicates that the marked increase of the fusion yield induced by lowering the temperature mainly comes from two contributions. On the one hand, the deuteron density increases inversely with the temperature when the backing pressure remains the same, which basically agrees with the predictions of the empirical thermodynamics. On the other hand, a strong nonlinear relationship of Hagena parameter is revealed. This nonlinear relationship between the clulster size and the stagnation temperature for CD4 clusters is much stronger than that for monoatomic and diatomic clusters, resulting in marked increase in the average KE of deuterons and the fusion neutron yield. To obtain higher conversion efficiency, by increasing the backing pressure to 80 bars, the conversion efficiency of 1.9 107 neutrons/J of incident laser energy is achieved as the temperature is 217 K, which is almost an order of magnitude higher than the results before. Meanwhile, the fusion neutron yields have been analytically estimated according to the deuteron density, the energy spectrum of deuterons and the geometry of the plasma channel measured in the experiments, which are in agreement with those from experiments. 2.An experimental platform on relativestic intense laser-solid interaction for ion acceleration has been built up to investigate the effects of the laser prepulse on accelerated proton energy and divergence in target normal sheath acceleration. First, ultrahigh contrast (1011) laser pulses at a peak intensity of ~1018 W/cm2 are used to irradiate thin foils with different thickness, generating megaelectronvolt (MeV) proton beams with low divergence (100) at the optimum target thickness of 0.8μm. The two-dimensional (2D) particle-in-cell (PIC) simulations are carried out to investigate the effects of the foil thickness, the foil density and the density gradient on ion acceleration, revealing that the proton beam parameters of cutoff energy and divergence strongly depend on the density gradient at the back side of the foil. Second, experiments using ultrahigh contrast (1011) laser pulses with the higher peak intensity of 2 1019 W/cm2 to irradiate Al foils with different thickness are performed to study the effert of nanosecond-scale prepulse on proton acceleration, generating high-energy proton beams with the cutoff energy of 7 MeV at the optimum target thickness of 2.5μm. The results from 2D PIC simulations indicate that the ns-time-scale prepulse can bend a thin target, resulting in the reduction of maximum proton energy at the target normal direction. 3.Experimental investigations on ion acceleration by collisionless shock waves from diamondlike carbon foils irradiated by 800 nm, linearly polarized femtosecond laser pulses of peak intensity of 1.7-3.5 1019 W/cm2 at oblique incidence. By changing the laser energy, diamondlike carbon foils are heated by the prepulse of the high contrast laser pulse and expand to form plasmas of near-critical density before the arrival of the main pulse. A pronounced peak centered around 7.5 MeV in the carbon C6+ energy spectrum is observed as the prepulse-produced plasma is overdense. 2D PIC simulations reveal that carbon ions are reflected and accelerated by a collisionless shock wave in a slightly overdense plasma excited by forward moving hot electrons generated by the main pulse. This is the first experimental demonstration of shock acceleration driven by 800 nm femtosecond laser pulses. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15903]  |
| 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 | 张辉. 基于相对论强激光的聚变中子产生及离子加速研究[D]. 中国科学院上海光学精密机械研究所. 2015. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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