Determination of ionization coefficient of atmospheric helium in DBD

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.