激光能量沉积光路追踪法及其并行化

【作者】 吕信；

【导师】 莫则尧；

【作者基本信息】 中国工程物理研究院 ， 计算数学， 2009， 硕士

【摘要】 在激光驱动惯性约束聚变的数值模拟中,通常使用光路追踪法来计算激光能量沉积。它是把一束激光划分为大量的光线,然后根据每条光线通过物理求解域的路径和状态来计算能量沉积。本文讨论采用光路追踪法计算激光能量沉积的方法、实施过程中需要解决的实际问题。首先介绍了光路追踪法的基本思想,给出了一般的光路方程、激光能量沉积公式、激光脉冲强度的时间和空间分布模型,分别讨论了二维直角坐标和二维柱坐标下的激光能量沉积计算。其次,对已有光路追踪模块做了一些改进,如将计算节点电子数密度的插值方法改为双线性插值；增加了直线型的真空光路模型,给出了与双曲型模型的不同应用特点；为计算激光驱动产生X光激光的应用增加了线聚焦的计算模型。在此基础上,我们编制了一个新的三维光路追踪程序,可以计算任意方向的光线通过网格的轨迹,能够更准确地模拟真实的激光光束在柱型腔靶中的三维散射,提高了数值精度和计算效率,并设计了数据输入输出、光线绘制的可视化接口以便于分析模拟结果。编制的程序模块已经用于研究工作,在应用中取得了良好的效果。为了适应大规模、高置信度的数值模拟需要,本文还讨论了光路追踪法的并行策略,给出了一种分组流水线的光路追踪并行算法,并予以实施,结果表明它能够充分提高大量光线在区域分解网格下的并行性,使得大规模并行计算时的效率比未分组流水线方法提高百分之三十以上。更多还原

【Abstract】 In order to simulate the energy deposition in inertial confinement fusion droved by laser, a geometric optics approximation called the ray-tracing algorithm was used in general without solving Maxwell’s equations. In the algorithm, laser beams are discretized into a lot of rays, which are initialized with certain energy. Each ray is followed until it is absorbed or leaves the physical domain; energy of ray is transferred to the mesh by inverse bremsstrahlung. The practical problems applying the ray-tracing algorithm are discussed in this paper.Firstly, the ray-tracing algorithm is presented, the key problems that need to be solved is analyzed, equations for ray trajectories, energy deposition and the power of laser pulse are described. Numerical calculation in 2-D Cartisien and 2-D cylindrical geometry is discussed.Secondly, some improvement is realized in the module of laser deposition, for instance, the method to calculate density of electron number is changed to bilinear interpolation. A new model of ray trajectory in vacuum is added, which is consistent with trajectory in plasma. On the same time, for simulation of X-ray laser generated by laser, focus-line model is added as well.With all work mentioned above, we have developed a new laser deposition module, which separate the complexities of the geometrical-optical ray-tracing problem from the basic physics modules. A 3D ray trajectory through a cell with an arbitrary direction is more accurately calculated and numerical precision is improved. When the interface about date I/O is designed, ray trajectories are rendered in 2-D and 3-D perspective with graphic tools that the result of numerical simulations is conveniently analyzed. The module has been used to the research and works well.Because of the requirements of large scale and highly efficient simulation, the parallelization of ray-tracing algorithm is described. Ray-tracing computation combined with the existing decomposition of the domain is effectively dealt with by the group-pipeline strategy. The code was designed with the message passing programming environment, can significantly improve the parallel performance. The numerical results show that computing efficiency of large scale increased above 30 percent against the ungroup-pipeline strategy.更多还原