基于频域全息法的激光尾场测量技术研究

【作者】 董军；

【导师】 彭翰生； 魏晓峰；

【作者基本信息】 中国工程物理研究院 ， 光学工程， 2009， 博士

【摘要】 超短超强脉冲激光在稀薄等离子体中激发的等离子体振荡波(也称为尾场)用来加速电子,是目前激光驱动等离子体加速器的重要工作机制之一,因其具有超高加速梯度和较小的加速结构而具有广阔的应用前景,特别是激光尾场加速器(LWFA)在实验上已经被证明具有产生超高电场梯度(超过100GeV/m)的能力,因此成为当前国际学术界十分关注的前沿课题。激光尾场加速器产生相对论电子束的光束质量取决于等离子体振荡波的时空结构,能够直接观测这种结构对了解和控制激光尾场加速器的电子束输出是至关重要的。本论文的研究内容是:利用频域全息探测方法(Frequency-domain holography,FDH),对以接近光速传播的等离子体波实现单束瞬态时空测量开展理论性研究。主要工作包括以下几个方面:1.简述激光—尾场电子加速器的需求背景、意义以及发展现状。重点介绍了基于频域干涉探测法(Frequency-domain interferometry,FDI)和频域全息探测法的激光尾场测量技术国内外进展,特别是针对近些年国际上有代表性的尾场测量技术方案及其技术指标进行了较为详细的调研分析。2.概括性地介绍了激光尾场的激励原理、加速电子的工作机制和四种技术方案。针对尾场的时空测量,特别介绍了超短超强脉冲激光与等离子体相互作用共振激发等离子体波的条件,尾场的时空特征及其对探测脉冲的相位调制,为测量实验参数的选择给出理论依据。3.由于频域全息探测方法的理论基础是两束啁啾脉冲之间的频谱干涉(SpectralInterferometry,SI),因此对傅里叶变换频谱干涉技术(Fourier Transformation SpectralInterferometry,FTSI)的工作原理以及确定两束相干宽带脉冲之间相对延迟时间的方法进行了理论、模拟和实验上的研究。重点探讨了线性啁啾脉冲的频谱干涉技术的原理和特性。4.阐述频域全息探测技术的基本原理,对实现频域全息探测技术的两种方法:基于直接映射模式的频—时相移转换和基于傅里叶变换模式的频—时相移转换法分别进行了系统的理论分析,并对两种相移转换方法进行了比较。指出基于傅里叶变换模式的频—时相移转换法更适合激光尾场的瞬态时空测量。5.提出基于皮秒啁啾脉冲的频域全息测量法对激光尾场的瞬态时空演变进行实时测量的方案。基于小信号下啁啾高斯脉冲的宽带二次谐波的频谱特性,在理论上和实验验证两个方面展开共轴光路下参考脉冲.探测脉冲产生机制的研究。6.基于中国工程物理研究院激光聚变研究中心的SILEX-Ⅰ超短激光装置平台,以800nm,30fs,30TW脉冲作为泵浦光,400nm,1ps啁啾脉冲作为参考-探测光脉冲对,展开尾场测量的实验设计。并根据频域全息方法激光尾场探量技术的理论分析,对各光路中所需的光学实验参数进行理论计算。为具体实验实施打下理论基础。在低密度等离子体中激发尾场来加速电子,是一个非常复杂的过程,将受多种不稳定性的制约。本文通过对基于频域全息方法探测尾场瞬态时空变化的理论研究,对实现尾场的实时测量和控制,以获得高质量的高能电子具有非常重要的意义。更多还原

【Abstract】 Using ultra-short ultra-intense laser pulse to excite the electron plasma wave(wakefield、Langmuir waves or electron density "bubbles") is one of main mechanism for laser-driven plasma accelerators to accelerate electrons in the low-density plasma.This accelerator with ultrahigh accelerating gradients and compact size is promising for the next-generation particle accelerators,and is one of the most attractive frontiers.In particular,it has been experimentally demonstrated that laser wake-field acceleration(LWFA) has great potential to produce ultrahigh field gradients excited by intense ultrashort laser pulses(＞100GeV/m). However,the beam quality of relativity electron beams produced by LWFA depends on the space-time structures of the wake-field,and it is important for controlling electron beams from LWFA that these structures are directly observed.Based on frequency domain holography(FDH),the single-shot real-time measurement for temporal and spatial evolution of the wake-field propagating near light speed,is investigated theoretically in this dissertation, including the following research topics:1.The development,application and general status of laser wake-field accelerator are summarized.Domestic and oversea development for measuring techniques of laser-wakefield, using Frequency domain interferometry and Frequency domain holography,are emphatically introduced,respectively.Especially,representative technical schemes and qualifications are given a presentation in detail.2.Excitation principle of laser-wakefield,mechanism of electron acceleration and four schemes for laser driven plasma accelerators are briefly depicted.The resonance conditions between the ultra-short ultra-intense pulse laser and plasma,temporal and spatial characteristics of wakefields and phase perturbation of the probe pulse imposed by wakefields are specially discussed for the appropriately choice of experiment parameters.3.Because supporting technology of the frequency-domain holography is spectral interference(SI) between two chirped pulses,FDH is an extension of Frequency domain interferometry(FDI).Fourier-transform spectral interferometry(FTSI) and the measurement of delay time between two pulses are analyzed theoretically,numerically and experimentally, respectively.Thereinto,the characteristics of spectral interferometry between two linear chirped pulses are mostly studied.4.Two methods for frequency-domain holography,direct mapping mode and full Fourier transform mode,are theoretically investigated,respectively.With the direct mapping mode, the spectral phase shifts are transformed to time-domain by time-frequency relation,while the spectral phase shifts are transformed to time-domain by Fourier transform with the full Fourier transform mode.Comparison of the two modes is given,and results show that more accuracy can be obtained through the full Fourier transform mode for the measurement of the wake-field.5.The measurement scheme for transient time-space evolution of the wakefield induced by the ultra-short ultra-intense laser pulse is presented,based on FDH using picosecond chirped pulses.The coaxial generation of the reference and probe pulses is investigated theoretically and experimentally,based on spectral characteristics of the generated pulse in the process of broadband second-harmonic generation.6.According to the theoretical analysis of FDH,the optical layout and experimental parameters of FDH are designed based on the SILEX-I,a Ti:sapphire laser system at CAEP, where the wake-field is excited in a He gas jet pumped by a pulse of～30TW/800nm/30fs,and probed by two chirped pulses of～1ps/400nm.更多还原