大角度碰撞对高密度等离子体能量弛豫过程影响的数值模拟研究

Large angle collisions are often ignored in previous studies. Nowadays, plasma densities up to several hundreds of grams per cubic centimeter had been created in laboratory the way of laser fusion. In such dense plasmas, the influence of large angle collisions can not be ignored any more. In this proposal, we mainly investigate theoretically and numerically the effect of large angle collisions on the energy relaxation process in dense plasmas. Based on the principle of binary collision, a computational model which can reasonably describe the probabilities of large angle collisions and small angle collisions is to be established, and a physical criteria distinguishing these two types of collisions will be formulated. The resulting model will be adopted to modify the collision module in our particle-in-cell (PIC) code by updating the Monte Carlo algorithm, with the aim to realize the sampling and calculation of the full range of deflection angle and to establish a PIC code that can consider the effect of large angle collisions self-consistently. Simulations on plasma relaxation process and particle slowing down process will be performed. A statistical method for different characteristic physical quantities will be developed to obtain the effect of large angle collisions from simulation results. A simplified model will be developed based on the theory of plasma collisions, and the influence of large angle collisions on the key physical quantities (relaxation time, the rate of energy change, the stopping power, etc.) is to be drawn out. The physical understanding and knowledge in this proposal is expected to be beneficial for improving the plasma model in laser fusion and broadening the basic plasma theory.

大角度碰撞在以往研究中通常不被考虑。人类利用激光聚变方式目前已产生了每立方厘米数百克的等离子体，在如此高密度条件下，大角度碰撞效应将不可忽略。本项目围绕大角度碰撞对高密度等离子体能量弛豫过程的影响开展理论分析和数值模拟研究。基于两体碰撞原理，提出合理描述大角度碰撞和小角度碰撞发生概率的可计算物理模型，并应用于现有粒子模拟（PIC）程序碰撞模块的改造，实现蒙特卡洛方法对全部偏转角的抽样计算，形成包含大角度碰撞过程的PIC程序；通过对高密度等离子体中弛豫过程和粒子慢化过程开展模拟研究，建立表征不同特征物理量的统计方法，从模拟结果中提炼出大角度碰撞效应的影响，结合理论分析，建立大角度碰撞对弛豫时间、能量变化率、阻止本领等关键物理量的影响规律，取得创新性成果。本项目的研究成果对改进激光聚变研究的物理建模、拓展等离子体物理理论具有重要的应用价值和科学意义。

高密度等离子体中大角度碰撞的研究，对于进一步完善激光聚变研究中的物理建模、拓展等离子体物理理论具有重要的应用价值和科学意义。获资助以来，本项目在高能量密度物理、惯性约束聚变物理等领域的高密度等离子体大角度碰撞及物理影响开展了研究。在激光惯性约束聚变物理方面，主要的研究成果有：完成等离子体粒子大角度碰撞的可计算建模和程序模块研制、大角度碰撞对高密度等离子体弛豫过程的影响规律、α粒子在高密度等离子体中的慢化/能量沉积规律、直接驱动内爆获得高密度等离子体的实验规律等。在高能量密度物理方面，主要的研究成果有：揭示了相对论电子束在高密度等离子体中传输的新演化行为、两束强激光在等离子体中作用过程的能量转移物理、提出优化质子加速的密度调制靶方案、提出产生百GeV金离子的物理方案、提出阿秒脉冲的精细表征方法等。通过对上述物理问题的研究，本项目建立了大角度碰撞过程的可计算物理模型并完成了相应的粒子模拟程序模块，获得了大角度碰撞效应对高密度等离子体弛豫过程、高能粒子慢化过程的影响规律、直接驱动内爆过程物理规律、揭示了相对论电子束传输过程的新物理现象和机制、强激光加速离子的优化物理方案、固体表面高次谐波的精细时空表征方法等。通过本项目建立的物理理论、程序模块、物理规律，有望对深入理解及拓宽基础等离子体物理、激光惯性约束聚变等领域起到一定的参考价值。