Determination of ionization coefficient of atmospheric helium in DBD

Z. Liang, H. Luo, B. Lv, X. Wang, Z. Guan, L. Wang
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Abstract

Dielectric barrier discharge of helium at atmospheric pressure was investigated. Two plane-parallel electrodes, each covered by a 1-mm thick quartz plate, are 50 mm in diameter and the gas gap is 5 mm in length. Powered by an alternative voltage with a frequency of 33 kHz, a homogenous discharge was produced and characterized by one current pulse per half cycle of the applied voltage. The development of the discharge during one current pulse was recorded by taking a series of side-view photographs of 20 ns exposure time using an ICCD camera. It was important to find that a weakly luminous layer close to the anode was observed even at the time far ahead of the current pulse, which was considered as the result of a weak Townsend discharge. The distribution of light intensity in the gap was obtained by processing the photograph taken at the time of this weak Townsend discharge. The curve of this light distribution shows a shape quite similar to that of the total electron number in an electron avalanche as a function of the distance through which the avalanche passes. This suggested us that the curve could be used to determine ionization coefficient alpha in the Townsend discharge. The method is based on the proportionality of light intensity to the electron density in a discharge gap of a uniformly distributed electric field. By fitting a theoretically derived formula with the measured curve of light distribution, alpha was determined. It was found that the value of alpha is quite high even at relatively low reduced field. For instance, alpha =31 cm-1 for E/p = 3.6 V. cm-1 . Torr-1. The reason for this higher value of alpha may lie in the contribution of Penning ionization of helium metastables with impurities, especially with nitrogen molecules.
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DBD中大气氦电离系数的测定
研究了常压下氦的介质阻挡放电。两个平面平行的电极,每一个被1毫米厚的石英板覆盖,直径为50毫米,气隙长度为5毫米。由频率为33 kHz的交替电压供电,产生均匀放电,其特征是每半周期施加电压一个电流脉冲。利用ICCD相机拍摄了一系列曝光时间为20ns的侧面照片,记录了一个电流脉冲放电的发展情况。重要的是发现,即使在电流脉冲之前很远的时候,也可以观察到靠近阳极的弱发光层,这被认为是弱汤森放电的结果。通过处理弱汤森德放电时拍摄的照片,得到了缝隙中的光强分布。这种光分布曲线的形状与电子雪崩中总电子数作为雪崩通过距离的函数的形状非常相似。这表明该曲线可用于确定汤森德放电中的电离系数。该方法基于均匀分布电场放电间隙中光强与电子密度的比例关系。通过理论推导公式与实测光分布曲线拟合,确定了alpha值。结果表明,即使在较低的还原场下,α的值也相当高。例如,E/p = 3.6 V. cm-1时alpha =31 cm-1。Torr-1。α值较高的原因可能是氦亚稳态与杂质,特别是氮分子的Penning电离的贡献。
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