电极表面等离子体形成与扩散的数值建模及其在粒子模拟中的实现

In magnetically insulated transmission line (MITL), high power diodes and other devices in pulse power system, there would be plasma formation and expansion on electrode surfaces under strong field, thereby resulting in impedance collapse, gap closure and other adverse phenomena. It becomes a bottleneck to further raise the performance of pulsed power system. So it is an urgent problem to be solved that establishes a suitable model for plasma formation and expansion and implements it in particle-in-cell (PIC) codes. This project proposes a semi-empirical model of it which bases on actual surface physic processes. Because several most important mechanism of plasma formation are considered, and experimental data are based on, the model is more self-consistent than the traditional phenomenological model. Then using both of fluid-particle boundary algorithm and particle-in-cell-Monte Carlo collision (PIC-MCC) algorithm, we implement the model in the three dimensional conformal PIC codes. In the first algorithm, fluid equations are used to calculate the plasma boundary, and on the plasma boundary, a dynamic shift boundary is set up to simulate emission. With experimental validation of the models and the codes on MITL, two kinds of high power diode would be simulated. This project contributes to promote the understanding of the physical processes of metal electrode surface under strong field and solve issues related to device gap closure.

在脉冲功率系统的磁绝缘传输线和高功率二极管等器件中，金属电极表面在强场作用下会产生等离子体并向对面电极扩散，从而造成阻抗降低、间隙闭合等不利现象，成为进一步提高脉冲功率系统性能水平的瓶颈问题。建立并实现一个适用于粒子模拟的电极表面等离子体形成和扩散模型成为一个亟需解决的问题。本项目提出一种基于表面实际物理过程的半经验建模方法，通过对几种重要的等离子体产生机制进行理论分析和综合，结合实验数据建立起比传统唯象方法更自洽的等离子体形成和扩散模型。然后分别采用流体-粒子边界算法和粒子-蒙特卡罗碰撞算法在三维共形粒子模拟中加以实现，其中第一种方法中，利用流体力学方法计算得到等离子体的边界，再结合动态移动边界来实现边界上的发射的模拟。利用磁绝缘传输线对模型和软件进行实验验证，并对两种高功率二极管进行模拟研究。本项目对推动强场作用下金属电极表面物理过程的认识、解决相关器件间隙闭合等工程问题具有重要意义。

在脉冲功率系统的磁绝缘传输线和高功率二极管等器件中，金属电极表面在强场作用下会产生等离子体并向对面电极扩散，从而造成阻抗降低、间隙闭合等不利现象，成为进一步提高脉冲功率系统性能水平的瓶颈问题。本项目通过对几种重要的等离子体产生机制进行理论分析和综合，建立基于表面实际物理过程的阴极电子发射模型、中性气体扩散模型、蒙特卡罗碰撞模型等来产生等离子体；结合粒子模拟和拉格朗日-欧拉流体模拟方法，建立了动力学-流体混合粒子模拟模型，实现动力学粒子和流体粒子的互作用和相互转化，并进行了编程实现和测试验证，能够高效实现高密度等离子体与高能粒子的模拟；基于BERTHA电路模型和Mur吸收边界模型实现磁绝缘传输线的路场耦合模拟；对磁绝缘传输线和二极管中存在电极等离子体的情况进行了模拟，得到了等离子体对电压、电流、和阻抗的影响，模拟得到的等离子体扩展速度与实验结果基本一致。本项目对推动强场作用下金属电极表面物理过程的认识、解决相关器件间隙闭合等工程问题具有重要意义。