射频和千赫兹驱动的毫米间隙放电的仿真研究

大气压短间隙放电是是产生冷等离子体的一种有效手段，常见的交流驱动电源方式有射频电源和kHz交流电源，而这两种不同频率电源所导致的气隙放电特性对比尚鲜有研究。本文以1 mm间隙的针-板电极这一极不均匀电场结构作为放电气隙，将之等效为球坐标系下的一维结构，建立基于迁移-扩散近似下的多组分、局部能量近似的经典等离子体流体模型，仿真研究了13.56 MHz射频（RF）电源或50 kHz交流（LF）电源所驱动的1 mm氦气（混合0.1%氮气）间隙的放电过程，关注了在1 mW和1 W这两种不同的沉积能量下的放电特性。结果表明：RF放电在1 mW时表现为电晕放电模式，此时间隙中的带电粒子密度低，且主要集中在功率电极附近；当沉积功率升高至1 W时，间隙放电则呈现出明显的辉光放电特征，电极附近出现鞘层，且气隙中间存在准电中性的等离子体区域；LF放电的起始电压幅值要高于RF，且LF放电随电压升高会较为平顺地从电晕放电模式过渡到辉光放电模式，而不存在明显的转换过程。对两种频率的放电而言，电晕放电模式下，潘宁电离是主要的电离路径；而辉光放电模式下，直接的电子碰撞电离成为主导的电离通道。此外，在相同的沉积功率下，LF放电的最大电子密度、电子温度和正离子温度都要要高于RF放电，但时间均匀性较差，呈现出明显的脉冲放电特性。

Simulation of the Discharges in Millimetre Gap Driven by Radio-frequency and Kilohertz AC Voltages

Atmospheric pressure short-gap discharge was an effective means to generate cold plasma. Common AC driving power sources included RF power supply and kHz AC power supply, but the comparison of air gap discharge characteristics caused by these two different frequency power sources was rarely studied. In this paper, the non-uniform field structure of an 1mm needle-plate gap was taken as the discharge structure, which was approximated as a one-dimensional model in the spherical coordinate system. A classical plasma fluid model based on multicomponent and local energy approximation under the drift-diffusion approximation was established. The discharge process of 1 mm helium (mixed with 0.1% nitrogen) gap driven by a 13.56 MHz radio frequency (RF) or 50 kHz AC (LF) power supply was simulated. The discharge characteristics at 1 mW and 1 W deposition energies were studied. The results showed that the RF discharge mode was corona discharge at 1mW, and the charged particles in the gap had a low density and mainly concentrated near the power electrode. When the deposition power increased to 1W, the gap discharge showed obvious glow-discharge characteristics, the sheath appeared near the electrode, and there was a quasi-electrically neutral plasma region in the middle of the gap. The igniting voltage amplitude of LF discharge was higher than that of RF, and LF discharge would transition from corona discharge mode to glow discharge mode smoothly without any obvious conversion process when increasing the voltage. For two kinds of discharge frequency, Penning ionization was the main ionization path in corona discharge mode. While the direct electron impact ionization became the dominant ionization channel in the glow discharge mode. In addition, under the same deposition power, the maximum electron density, electron temperature and positive ion temperature of LF discharge were higher than those of RF discharge, but the temporal uniformity was poorer, showing obvious pulse discharge characteristics.