The characteristic feature of a dielectric barrier discharge (DBD) is the dielectric barrier placed between the electrodes. In the present work, the influence of the dielectric barrier to the properties of a DBD in air was investigated. Spectroscopic characterization of the DBD and electrical measurements were carried out. It was shown that the efficiency of a DBD can be considerably improved by optimizing the dielectric barrier. The dielectric material should possess an appropriate relative permittivity and thickness. For thin dielectric barriers, a high secondary emission coefficient becomes important. Additionally, the use of only one dielectric barrier is advantageous.

For surface treatments with plasma under atmospheric pressure, it is important to obtain a homogeneous discharge, ideally a glow discharge. Normally, a discharge under atmospheric pressure is filamentary. The aim of this work is to examine the influence of various parameters on the plasma properties and therefore on its homogeneity. For this purpose, the plasma was characterized by optical emission spectroscopy and electrical measurements. It was found that a smaller gap leads to more electronic excited species and to an increased plasma power. Furthermore, the reduction of the gap distance results in a lower rotational temperature and, above all, to an improved homogeneity of the plasma. The changes in the plasma properties with variation of the external parameters, such as gas flow or applied voltage, are much slighter with a smaller gap. Therefore, a plasma with a smaller gas gap can be run over a wider region without significant changes in its properties.

In this paper, a non-thermal atmospheric pressure plasma jet at high streaming velocity operating with ambient air is highlighted. In the present technological approach, the employment of air poses a significant challenge. The high oxygen concentration in air results in a reduced concentration of reactive species in combination with a short species lifetime. The plasma jet assembly presented here contains a special dielectric barrier with a high secondary emission coefficient. In this way, the electron density and in turn the density of reactive species is increased. In addition, the plasma jet assembly is equipped with a short electrode. This leads to a higher voltage across the discharge gap and in turn to an increased density of reactive plasma species. The plasma jet is formed within and emitted by a small conical nozzle. A high-speed gas flow with gas velocity of 340 m/s was achieved at the end of the nozzle. In the jet the concentration of toxic and unwanted neutral plasma species like O3 or NOx is significantly reduced because of the shorter residence time within the plasma. The range of short-lived active plasma species is in turn considerably enhanced. The jet efficiency and action range measured through the oxidation of a test surface were determined by measuring the increase of surface tension of a polypropylene substrate via contact angle measurements after plasma treatment. Numerical modeling of the plasma plume indicates that oxygen atoms are in fact the main active species in the plasma plume.

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