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首页> 外文期刊>Journal of Computational Physics >A PIC-MCC code for simulation of streamer propagation in air
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A PIC-MCC code for simulation of streamer propagation in air

机译：用于模拟拖缆在空中传播的PIC-MCC代码

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A particle code has been developed to study the distribution and acceleration of electrons in electric discharges in air. The code can follow the evolution of a discharge from the initial stage of a single free electron in a background electric field to the formation of an electron avalanche and its transition into a streamer. The code is in 2D axi-symmetric coordinates, allowing quasi 3D simulations during the initial stages of streamer formation. This is important for realistic simulations of problems where space charge fields are essential such as in streamer formation. The charged particles are followed in a Cartesian mesh and the electric field is updated with Poisson's equation from the charged particle densities. Collisional processes between electrons and air molecules are simulated with a Monte Carlo technique, according to cross section probabilities. The code also includes photoionisation processes of air molecules by photons emitted by excited constituents. The paper describes the code and presents some results of streamer development at 70 km altitude in the mesosphere where electrical discharges (sprites) are generated above severe thunderstorms and at similar to 10 km relevant for lightning and thundercloud electrification. The code is used to study acceleration of thermal seed electrons in streamers and to understand the conditions under which electrons may reach energies in the runaway regime. This is the first study in air, with a particle model with realistic spatial dependencies of the electrostatic field. It is shown that at 1 atm pressure the electric field must exceed similar to 7.5 times the breakdown field to observe runaway electrons in a constant electric field. This value is close to the field where the electric force on an electron equals the maximum frictional force on an electron - found at similar to 100 eV. It is also found that this value is reached in a negative streamer tip at 10 km altitude when the background electric field equals similar to 3 times the breakdown field. At higher altitudes, the background electric field must be relatively larger to create a similar field in a streamer tip because of increased influence of photoionisation. It is shown that the role of photoionization increases with altitude and the effect is to decrease the space charge fields and increase the streamer propagation velocity. Finally, effects of electrons in the runaway regime on negative streamer dynamics are presented. It is shown the energetic electrons create enhanced ionization in front of negative streamers. The simulations suggest that the thermal runaway mechanism may operate at lower altitudes and be associated with lightning and thundercloud electrification while the mechanism is unlikely to be important in sprite generation at higher altitudes in the mesosphere. (c) 2008 Elsevier Inc. All rights reserved.
机译：已经开发了粒子代码来研究空气中电子在放电中的分布和加速。该代码可以跟踪放电从背景电场中单个自由电子的初始阶段到电子雪崩的形成以及向电子流转变的过程。该代码位于2D轴对称坐标中，允许在拖缆形成的初始阶段进行准3D仿真。这对于在空间电荷场必不可少的问题（如拖缆形成）中进行逼真的模拟非常重要。带电粒子在笛卡尔网格中跟踪，并根据带电粒子密度用泊松方程更新电场。根据横截面概率，使用蒙特卡洛技术模拟了电子与空气分子之间的碰撞过程。该代码还包括受激发成分发射的光子对空气分子的光电离过程。本文介绍了该规范，并提出了在中层大气中70 km处彩带发展的一些结果，在这些区域中，雷暴上方产生了放电（雪碧），与闪电和雷云电气化有关的距离约为10 km。该代码用于研究拖缆中热种子电子的加速度，并了解电子在失控状态下达到能量的条件。这是首次在空气中进行的研究，其粒子模型具有静电场的实际空间依赖性。结果表明，在1个大气压下，电场必须超过击穿电场的7.5倍，才能在恒定电场中观察到失控的电子。该值接近于电场，该电场上的电子力等于电子上的最大摩擦力-近似于100 eV。还发现，当背景电场等于击穿场的3倍时，在10 km高度的负拖缆末端达到了该值。在更高的海拔高度，由于光电离的影响增加，背景电场必须相对较大才能在拖缆尖端中产生相似的电场。结果表明，光电离的作用随着高度的增加而增加，其作用是减小空间电荷场并增加拖缆的传播速度。最后，介绍了失控状态下电子对负拖缆动力学的影响。结果表明，高能电子在负离子流的前面产生增强的电离。模拟表明，热失控机制可能在较低的高度运行，并与闪电和雷云带电有关，而该机制在中层较高高度的子画面生成中不太重要。 （c）2008 Elsevier Inc.保留所有权利。

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