基于多光束参量调控的组束匀滑技术研究

In the high-power laser facilities used for inertial confinement fusion (ICF), the physical experiments require well defined laser focal spots. To precisely control the distribution of laser focal spots in spatial domain, various phase plates are developed and widely used. While, phase plates break the beam into a series of beamlets, and the interference between these beamlets will lead to speckles, causing instabilites and filamentation in the laser fusion experiments. Therefore, beam smoothing technologies in time and polarization domain are required to be adopted to suppress the laser-plasma instabilities, and attain symmetry compression of the target capsules. For the moment, several laser beam smoothing technologies have been studied for the single beam circumstance,in time, spatial and polarization domain, such as smoothing by spectral dispersion (SSD), continuous phase plates (CPP) and polarization smoothing (PS). While, for the overlapping focusing properties of multiple beams, systematic research is still scarce. Considering megajoule laser drivers with hundreds of beams, combined beam focusing will be needed to adapt to beam orientation in the target area. Therefore, characteristics of combined beam smoothing are required to be analyzed to support the design of laser drivers. Here, combined beam smoothing using novel parameter manipulation methods in time, spatial and polarization domain will be studied to improve the target illumination uniformity. The research will be focused mainly on the followings: influences of the direction of polarization and angular spectral dispersion on target illumination uniformity of combined laser beams; optimization and realization ways of new polarization smoothing methods. The subject will provide theoretical and experimental support for the development of combined laser beam technology of high-power laser facilities.

惯性约束聚变物理实验要求高功率激光装置精确控制靶面光强分布，因而广泛采用了相位板技术以进行焦斑空域整形。由于光束内部子束间的相干性，相位板会在靶面产生散斑结构，从而在打靶时产生各种不利的非线性过程和成丝，影响靶丸对称压缩，因而还需要采用时域匀滑和偏振匀滑等措施。目前已对单束光的时域、空域和偏振匀滑特性进行了研究，但对多束光的组束匀滑特性国内外尚未开展系统研究。而在多达数百束的兆焦耳级激光驱动器中，为满足靶场光束排布需采用组束聚焦，因此亟需对组束匀滑特性进行研究以支撑驱动器设计。本项目将对组束打靶过程中的时域、空域和偏振等多个参量开展创新性调控技术研究，从而掌握组束匀滑的参量优化设计和实现方法。重点要突破的难点包括：多光束组束打靶时子束偏振态、光谱角色散方向等参量对靶面辐照均匀性的影响；新型偏振调控方法的优化设计和实现方法。本项目将为高功率激光装置组束匀滑技术的发展提供理论和技术支撑。

在多达数百束的高功率激光驱动器中，为满足靶场光束排布的要求需采用组束聚焦，因此亟需对组束聚焦的光场特性和束匀滑优化方法进行研究以支撑驱动器设计。结合高功率固体激光装置光束传输与控制特点，提出了一系列多参量调控组束匀滑解决方案。自主开发了包含时域、空域、偏振匀滑计算能力的组束匀滑模拟分析软件。提出破坏束组内各子束近场间的相关性是提升组束匀滑性能的重要原则。对于组束匀滑中的CPP设计，模拟结果表明，在同一集束中，相同面形的CPP因不同束间位相大体相同而不能得到最优结果，可将同一集束中各束激光CPP设计为完全独立面形。对于集束匀滑中的时域匀滑，采用“多色+多频”SSD模式时，组束匀滑获得的焦斑均匀性优于“多色+相同调制频率”SSD的焦斑均匀性，优于“单色+单频”SSD。其中，多色（指多种中心波长）是为了增加带宽，缩短相干时间；多频（指多种调制频率）可使集束中两束光的频谱结构不同；各子束的最小频差应大于SSD带宽；在同一集束中，各束激光SSD扫动方向不同也有利于提升靶面束匀滑效果。在组束匀滑光束偏振调控方面，对偏振控制板阵列及柱矢光束聚焦特性进行了理论模拟，研究了偏振调控对靶面光强分布均匀性的改善能力。设计并加工了偏振控制板阵列，通过调节偏振控制板阵列单元排列方式，可获得多种偏振态分布；采用偏振控制板阵列后远场偏振态分布相比不采用时偏振态极大丰富。为提升组束匀滑和各子束光偏振调控的灵活性，研究了基于液晶空间光调制器的偏振匀滑技术。实验研究了加与不加阵列液晶器件的激光焦斑分布，通过使用液晶偏振器件，焦斑上幅度调制较剧烈的区域得到了抑制，使用液晶偏振器件的焦斑与不使用时相比均匀性更佳，rms减少至78%，FOPAI曲线超过平均光强三倍部分的强度接近减半。该量化结果证明了液晶偏振器件的高效束匀滑功能，从原理上证明了其可作为高功率激光装置的偏振匀滑器件在线使用。本课题研究结果为高功率激光装置组束匀滑方案的优化设计提供了重要的理论和技术支撑。