Z箍缩微观动力学过程及相关物理问题的粒子模拟研究

Although a large amount of studies have been carried out on the experiments and numerical simulations of Z-pinches so far, the researchers are still not satisfied with the obtained knowledge and understanding about Z-pinch physics yet. There is the trend that the convergence ratio of load radius often becomes too large in the simulation of Z-pinch by radiation magneto-hydrodynamics (RMHD) code if the electrical conductivity of plasma is not modified artificially. The measured X-ray energy radiated from Z-pinch might be much greater than the “kinetic” or “theoretical” magnetic energy coupled to the pinching plasma during the implosion. The high energy particles and hard X-rays, the energies of which might be higher than applied voltage between the cathode and anode, had been probed in axial direction in Z-pinch experiments. These simulated and experimental results could not be well explained under the current RMHD model. That means that there would be something limited in the RMHD model. Therefore, we propose this proposal to develop the Z-pinch explicit/implicit particle-in-cell (PIC) simulation codes, based on the theory frame of interaction of particles and electromagnetic wave, to simulate the practical Z-pinch processes. By means of the developed codes, we plan to study the evolving diagram of particles and electromagnetic fields, from which the implosion trajectory of particles, trailing mass and current, and particle energy probability distributions can be obtained. The electrical conductivity, thermal conductivity, and the transferred energies are expected to calculated in detail. By means of the implicit PIC code, the impact of outer plasma shell driven by Lorentz force with the inner plasma shell will be simulated to evaluate the production of high energy particles. The results of Z-pinch simulations through PIC and RMHD will be compared. The goals of this proposal are to develop the model used in RMHD, explain the experimental phenomena required further understanding, and to enhance the understanding of the dynamical hohlraum physics. They will finally help to the realization of the inertial confinement fusion driven by Z-pinch.

虽然Z箍缩进行了大量的实验和模拟研究，但对它的物理理解还不能令人满意。如果不对电导率进行唯象调整，则辐射磁流体力学程序对它的模拟总倾向于压得过紧，甚至“压死”。测量的X射线能量明显地大于等离子体内爆动能，甚至大于耦合到等离子体上的“理论”电磁能量；在负载轴向测量到比极板电压还高的高能粒子束和硬X射线。这些Z箍缩模拟和测量结果都还得不到满意解释，表明Z箍缩辐射磁流体力学模型有局限性。因此，本项目提出发展和研制Z箍缩显/隐格式粒子模拟程序，在带电粒子和电磁波相互作用的理论框架下研究Z箍缩过程的粒子空间演化图像，给出电磁场演化、质心内爆轨迹、拖尾质量和电流、粒子能谱；粒子体系中电导和热传导过程及能量转换的微观机制；内外层等离子体壳高速碰撞时高能粒子的产生。比较粒子模拟与流体模拟结果，提出Z箍缩辐射磁流体力学模型的改进方案，解释尚未理解的物理现象，以促进对动态黑腔的物理理解和它的惯性约束聚变应用。

本项目主要是发展和研制Z箍缩显/隐格式粒子模拟程序，在带电粒子和电磁波相互作用的理论框架下研究Z箍缩的微观动力学过程及相关物理问题。采用二阶形状因子和双时标方法，提高了二维显格式粒子模拟程序的电磁场计算精度和运算效率；比较分析了低密度Z箍缩等离子体在粒子和流体描述下的径向受力情况，以及它们的模拟结果，发现它们的箍缩机制是不同的，在单流体MHD模拟中Z箍缩等离子体将表现出过强的压缩性，从而导致在流体模拟中常常需要人为地增大等离子体电阻率，增强磁场扩散，以降低其可压缩性，此外，在冲击波形成以前它们的箍缩图像也差别很大；在国内首先研制了矩方法隐格式一维Z箍缩粒子模拟程序，通过了测试，得到了Z箍缩微观动力学过程的合理图像；提出了新的高精度电荷守恒电流分配方法，大大地提高了等离子体粒子模拟中电磁场的模拟精度；独立地发展了类似于美国LSP程序的二维直接隐式粒子模拟程序，程序通过了初步测试，得到了符合物理预期的低密度等离子体Z箍缩模拟结果；发展了用于对粒子模拟结果进行统计处理，得到压强、黏性、电导率和热传导系数等输运系数的程序；改造了开源的激光等离子体相互作用程序MULTI2D，并用于模拟8MA电流水平的Z箍缩动态黑腔形成过程；对50MA点火级驱动电流的Z箍缩动态黑腔形成过程及其定标关系进行了模拟研究，得到了一些规律性认识。这些研究工作，为Z箍缩物理和应用研究提供了丰富的程序模拟工具和算法，提高了对Z箍缩微观物理机制和图像的物理理解，加深了对磁流体力学模拟方法局限性的认识，给出了Z箍缩动态黑腔形成过程和特征的准确理解。发表了9篇相关的学术论文，参加了5次国际会议，主办了第11届稠密Z-pinch国际会议，培养毕业了2名硕士研究生，培养在读博士研究生2名、硕士研究生2名，完成与2名博士后的合作研究。