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中国物理学会期刊

稠密等离子体焦点粒子-流体混合模拟研究

Particle-fluid hybrid simulation of dense plasma focus

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  • 稠密等离子体焦点作为一种高效的高能量密度等离子体装置,在聚变能源、辐射源开发及材料科学等领域具有重要应用潜力。然而,稠密等离子体焦点中复杂的多尺度物理过程及其耦合作用,使得传统单一数值模型难以实现对其全物理过程的高精度模拟。本文建立了粒子—流体的混合模型来研究稠密等离子体焦点的动力学过程。在混合模拟中,采用粒子方法模拟动力学离子,采用流体方法模拟准中性的无质量电子,同时采用麦克斯韦方程和时域有限差分方法求解电磁场。针对混合模拟的时间步推进问题,采用预估-修正方法实现了混合模拟的自洽显式迭代。通过与现有模拟及实验结果的对比,验证了混合模拟程序的准确性。最后基于验证后的混合模型,进行了DPF全物理过程研究,并针对DPF离子加速机制进行了初步探索。研究表明在箍缩阶段磁流体力学不稳定性会导致箍缩等离子体在局部产生极高的电磁场,电磁场加速产生高能离子,最终束靶互作用产生中子。

     

    The Dense Plasma Focus (DPF) is a high-energy-density plasma device capable of producing extreme plasma conditions, which makes it promising for applications in fusion energy, radiation sources, and materials science. However, the dynamical process of DPF involve multi scale processes, ranging from macroscopic magnetohydrodynamic (MHD) instabilities to microscopic kinetic ion behavior, which are not fully captured by traditional single approach simulations. In this study, a self consistent electromagnetic hybrid simulation framework is developed to investigate the complete physical process of DPF, including the run down, run in, pinch formation, and generation of high energy ion beams. In the proposed model, ions are treated kinetically using the particle in cell (PIC) method, while electrons are described as a quasi neutral massless fluid. The full set of Maxwell’s equations is solved by the finite difference time domain (FDTD) method without invoking the Darwin approximation, allowing electromagnetic waves to propagate self consistently in both plasma and vacuum regions. A predictor corrector iteration scheme is implemented to ensure stable and accurate time advancement in the hybrid algorithm. The model is first validated against simulation results from the LLNL laboratory obtained with the LSP code, showing good agreement in pinch timing despite slight differences in plasma geometry. Further validation with the UNU device confirms the accuracy of the hybrid model in simulating axial acceleration and circuit coupling. Using the verified model, the full DPF process is simulated. It is revealed that during the pinch phase, m=0 magnetohydrodynamic instabilities lead to the formation of localized plasma necks, where magnetic fields exceed 200 T and axial electric fields reach ~1011V/m. These extreme fields accelerate ions to energies up to 1.5 MeV, and a broad spectrum of ~100 keV ion beams is also observed. The results indicate that beam target interactions driven by these accelerated ions constitute the primary mechanism for neutron production in DPF. This work demonstrates that the hybrid PIC fluid FDTD approach provides an efficient and physically comprehensive tool for modeling multi scale plasma dynamics in DPF and similar high energy density devices.

     

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