1D PIC-DSMC simulations of breakdown in microscale gaps
1D PIC-DSMC simulations of breakdown in microscale gaps
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DOI：
10.1063/1.4769601
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发表时间：
2012-11
期刊：
影响因子：
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通讯作者：
C. Moore;M. Hopkins;P. Crozier;J. Boerner;L. Musson;Russell Hooper;M. Bettencourt
中科院分区：
文献类型：
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作者：
C. Moore;M. Hopkins;P. Crozier;J. Boerner;L. Musson;Russell Hooper;M. Bettencourt
利用具有复杂边界条件和直接模拟蒙特卡罗(DSMC)粒子碰撞的显式静电粒子格子(PIC)程序，研究了STP中空气隔开的两个电极之间的一维直流击穿。模拟模型包括阴极的俄歇中和和冷场电子发射以及电子-中性的弹性、电离和激发相互作用。将模拟的不同电极间隙下的击穿电压与实验数据和Paschen曲线进行了比较。研究发现，冷场电子发射可以解释击穿电压与测量的小间隙的帕森曲线的偏差。大间隙中的击穿在多个离子传输时间尺度上进行，因为通过阴极处的离子的俄歇中和而产生的电子迅速流过间隙，产生新的离子，这些离子加速向阴极，并释放另一个电子脉冲。如果产生的电子脉冲比初始脉冲大，那么这个过程可以在间隙中建立一个重要的准中性等离子体，并且穿过间隙的电压降将主要发生在(薄)鞘上。如果阴极表面的电场足够大，足以产生较大的冷场发射通量，则击穿会加速，从而增加等离子体密度，减小德拜长度，从而减小鞘层尺寸，进一步增加阴极表面的电场和冷场发射通量。发现气压间隙的击穿对电子-中性相互作用的微分散射截面很敏感。与使用更精确的正向偏向散射分布相比，弹性碰撞的各向同性散射在中等间隙(几个平均自由程)处产生较低的击穿电压，而在较大间隙尺寸时产生较高的击穿电压。击穿电压对散射分布的依赖是由于后向散射的增加导致更大的跨越禁带的有效路径长度与电子能量分布函数的变化之间的竞争。
An explicit electrostatic particle-in-cell (PIC) code with complex boundary conditions and direct simulation Monte Carlo (DSMC) particle collisions is utilized to investigate one dimensional direct current breakdown between two electrodes separated by air at STP. The simulation model includes Auger neutralization and cold field electron emission from the cathode as well as electron-neutral elastic, ionization, and excitation interactions. The simulated breakdown voltages at various electrode gap sizes are compared to experimental data and the Paschen curve. It is found that cold field electron emission can explain the breakdown voltage deviation from the Paschen curve measured for small gaps. Breakdown in large gaps proceeds over multiple ion transit timescales as electrons created via Auger neutralization of ions at the cathode quickly stream across the gap, creating new ions which accelerate towards the cathode and release another "pulse" of electrons. If the resultant pulse of electrons is larger than the initial pulse, then this process can build up a significant quasi-neutral plasma in the gap and the voltage drop across the gap will occur primarily across the (thin) sheath. Breakdown is accelerated if the electric field at the cathode surface is large enough for significant cold field emission flux, which increases the plasma density and decreases the Debye length and thus the sheath size, further increasing the electric field and cold field emission flux from the cathode surface. Breakdown in air pressure gaps was found to be sensitive to the differential scattering cross section for electron-neutral interactions. Isotropic scattering of elastic collisions results in lower breakdown voltages at moderate gaps (several mean free paths) and higher breakdown voltages for large gap sizes compared to when more accurate forward-biased scattering distributions are used. The dependence of breakdown voltage on the scattering distribution is due to a competition between increased backscattering resulting in a larger effective path length across the gap versus changes in the electron energy distribution function.