基于AMAR算法的大气压脉冲放电等离子体快速数值模拟研究

Atmospheric plasmas generated by pulsed discharges have received a growing attention due to the immense application potential, especially in the area of plasma medicine. For the very fast variation of the applied voltage even in nanosecond time scale many new discharge physics have been observed by experimental devices in atmospheric plasmas driven by pulsed voltage, which could not be described or interpreted by the well accepted Townsend and streamer discharge mechanisms. In this study a proposed spatial-temporal hybrid model will be constructed which combines adaptive mesh refinement with automatic selection of kinetic or continuum solvers in different parts of the computational domains during the pulsed-on phase. The particle model is used in the high field region with the fluid model applied in the area where electron density is high and electrical field is low. And the continuum simulation will be always applied when the pulsed voltage is off. Therefore this hybrid model is computationally efficient. By means of this model, the kinetic behavior of atmospheric pulsed plasmas can also be obtained; even the long-time dynamics of the pulsed atmospheric plasmas can be possible simulated with the consideration of reactivity and stability of atmospheric plasmas. The breakdown mechanism will be investigated with this hybrid model in nanosecond pulsed discharges, to unveil the new physics which cannot be described by the previous theories, especially to discuss the roles of the high energy electrons or the runaway electrons during the breakdown process. The experimental results have shown that more reactive species with higher densities can be generated, which is believed one of the advantages of atmospheric pulsed discharges. The production and optimization of the reactive oxygen species (ROS), including atomic oxygen, excited atomic oxygen and ozone, in atmospheric pulsed discharges with helium-oxygen mixture or air as the working gas will also be discussed, and the influence of discharge parameters on the generation of ROS will be studied based on the simulation results. The principles of optimization of the discharge parameters could be given under a given power or power density condition. The hybrid model can be easily expanded to two-dimensional form to study the stability of pulsed atmospheric discharges with the parallel algorithms. It is well accepted that the homogenous large-volume atmospheric plasmas can be achieved even in air with uneven gas distance in atmospheric pulsed discharges. Using the 2D hybrid computational model the underpinning physics of stability will be shown and the optimization of pulsed voltages to keep stability will be discussed. Based on the results of this computational study, the breakdown mechanism in nanosecond pulsed discharges will be interpreted deeply, and the ways to generate the uniform atmospheric plasmas with stability and reactivity driven by pulsed voltage will be revealed.

大气压脉冲放电是近年来广受关注的放电类型，不但在材料表面处理、等离子体医学等领域具有广泛的应用前景，而且由于外加激励电压变化迅速而产生了一系列新的放电物理现象。本项目基于AMAR算法思想，拟构建一种时空混合模型对大气压脉冲放电等离子体进行全区域、长时间、高维度的快速数值模拟研究。在脉冲电压作用期间，该模型在待模拟区域内依据放电等离子体的物理特性自动选择粒子模型或流体模型，而在脉冲电压结束后则采用优化后的流体模型。利用该混合模型研究纳秒脉冲放电的击穿过程，并与传统的放电理论进行比较；探讨脉冲放电中基态氧原子等多种活性成分产生与消失的过程，并分析电压波形对活性成分密度的影响；讨论大气压脉冲放电具有高稳定性的内在机理，分析电压波形、放电间隙等因素对稳定性的影响。通过本项目的研究，揭示纳秒脉冲放电的击穿机制，总结出脉冲电压波形的优化原则，探讨如何获得兼具稳定性与化学活性的大气压脉冲放电等离子体。

大气压脉冲放电是近年来广受关注的等离子体产生方式，其可以产生大体积均匀的大气压非平衡等离子体，并且在合适的放电条件下可以产生种类繁多、数量丰富的活性粒子。然而，由于脉冲放电，特别是纳秒级的脉冲放电放电剧烈，等离子体的产生与演化过程比较负责，实验诊断面临着较多的困难，同时由于脉冲电源的限制，也很难对脉冲放电进行较为系统的实验研究，这样，数值模拟将在脉冲放电的研究中起着重要作用。但是，虽然流体模拟具有较高的计算效率，且可以研究多种活性粒子的产生于演化，然而脉冲放电的特性决定了在很多情况下必须使用粒子模拟来反映脉冲等离子体的动理学特性。在本项目中，我们综合使用流体模拟与粒子模拟算法系统的研究了脉冲放电、脉冲调制射频（高频）放电的放电特性、等离子体演化以及活性粒子的产生与分布。在脉冲放电中，通过理论分析给出了击穿电压近乎线性的依赖于脉冲电压的上升率，以及放电参数对放电特性的影响，并且结合计算数据验证了该结果；并借助于粒子模拟给出了放电演化过程中的电子能量分布函数，并讨论了放电参数对电子能量分布函数的影响，并结合流体模拟与粒子模拟研究了微等粒子体中放电结构的相互转化，研究表明二次电子提供的高能电子对于微等粒子体具有非常重要的影响。脉冲等离子体中产生大量的几十种活性粒子，涉及到数量众多的化学反应，流体模拟可以比较高效的胜任此种类型的模拟，但是需要精确的估计电子能量。模拟结果表明，从理论上来说，可以通过精确调控脉冲放电参数，来调控活性粒子的生成，使其满足不同的应用需要。特别是在本项目的支持下，我们系统的研究了同短脉冲放电极为类似的高频脉冲调制放电现象，加深了对大气压非连续性放电的影响，并进而综合流体模拟与粒子模拟讨论了占空比与调制频率对首电流脉冲现象的影响。本项目的研究充分体现了流体模拟与粒子模拟的结合，对大气压脉冲放电的稳定性与化学活性进行了深入系统的研究，将加深人们对大气压非连续性放电的理解。