温稠密碳等离子体的价态分布对离子结构和输运性质的影响

Effect of Charge State Distribution on Ionic Structure and Transport Properties in Warm Dense Carbon Plasmas

  • 摘要: 温稠密等离子体处于高温高密的极端物理条件下,原子易发生部分电离,形成多种价态离子共存的复杂体系。探究不同离子价态对体系微观结构及宏观输运性质的调控机制,是惯性约束聚变和天体物理研究的关键科学问题。本研究采用基于Yukawa势函数的多价态离子分子动力学(Multi-ion Molecular Dynamics, MIMD)模拟方法,结合Saha方程求解各价态离子丰度,系统研究多价态环境下温稠密碳等离子体的结构与输运特性。模拟结果表明:体系的局域有序度主要由温度和密度共同调控,而离子价态则决定了局域原子排布的具体特征;由于高价态离子间存在更强的库仑排斥作用,其径向分布函数显著降低。相应地,离子自扩散系数随价态升高呈单调下降趋势,随温度升高而增大,随体系密度增加而减小,呈现出明确的规律性。除此之外,本研究还讨论了考虑离子速度的动态屏蔽Yukawa势进行对比模拟,结果显示:动态屏蔽效应会增大体系的有效屏蔽长度并增强离子间库仑相互作用,导致速度关联函数衰减速率加快,最终表现为自扩散系数的降低。该效应在高温、低密度条件下更为显著;在同种元素体系中,低价态离子受动态屏蔽的调制作用更强。

     

    Abstract: Warm dense plasma is subject to extreme physical conditions of high temperature and high density, where atoms tend to be partially ionized, forming a complex system with multiple ionic charge states coexisting. Investigating the regulatory mechanism of different ionic charge states on the microscopic structure and macroscopic transport properties of the system is a key scientific issue in inertial confinement fusion and astrophysical research. In this work, a multi-ion molecular dynamics (MIMD) simulation method based on the Yukawa potential is adopted, combined with the Saha equation to calculate the abundance of ions in different charge states. The structure and transport properties of warm dense plasma under a multi-charge-state environment are systematically investigated. Simulation results show that the local order of the system is mainly regulated by both temperature and density, while the ionic charge state determines the specific characteristics of the local atomic arrangement. Due to the stronger Coulomb repulsion between high-charge-state ions, their radial distribution functions are significantly reduced. Correspondingly, the ionic self-diffusion coefficient decreases monotonically with increasing charge state, increases with rising temperature, and decreases with increasing system density, exhibiting clear regularity. In addition, a dynamically screened Yukawa potential considering ion velocity is introduced for comparative simulations. The results reveal that the dynamic screening effect increases the effective screening length of the system and enhances the interionic Coulomb interaction, accelerating the decay rate of the velocity correlation function and ultimately leading to a reduction in the self-diffusion coefficient. This effect is more pronounced under high-temperature and low-density conditions. In single-element systems, low-charge-state ions are more strongly modulated by dynamic screening.

     

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