In this paper, a 2D fluid model is employed to investigate the radial evolution of the discharge structures in a helium dielectric barrier discharge with 100 ppm nitrogen impurity. By elevating the applied voltage amplitude (V am ), the discharge exhibits some distinctive radial evolution features that comprise three aspects: (1) the lateral migration of the peripheral filament, i.e. outward filament migration nearby the electrode edge; (2) the reduced intervals between two successive filaments; (3) the growth in the number of filaments. It is revealed that the radial position of the peripheral filament is basically consistent with that of the initial local intense discharge, whose location is closely related to the surface charge distribution during the initial breakdown. An increase in V am reduces the duration of the current pulse, and hence, the displacement of space charges is restrained. When more charges are restrained in the gap rather than being attached to the dielectric surfaces, the surface charge distribution becomes more uniform, which contributes to the lateral migration of the peripheral filament. Meanwhile, the lateral uniformity of Penning ionization rate is improved with V am increasing, and the seed electron level in the intervals becomes comparable to that in the filamentary channels, leading to a more uniform radial seed electron profile that attenuates the electric field distortion. As a result, the intervals between two adjacent filaments are shortened. With the lateral migration and reduced intervals as V am increases, we observe the growth in the number of filaments, and the improvement of the radial discharge uniformity.
A two-dimensional model is employed to investigate the evolution of radial discharge columns (or filamentary channels) and the potential mechanism in an atmospheric argon dielectric barrier discharge (DBD). As the applied voltage amplitude increases, the number of discharge columns first increases and then deceases, and finally, the discharge evolves into the diffuse mode. With a lower voltage amplitude range, the more uniform distribution of surface charge density makes the original discharge column move outwards, providing a wider inner space to increase the filament number. A similar filamentation process is also observed in atmospheric helium. However, when the voltage amplitude is further increased, considering the lower ionization threshold of argon, even the relatively small amount of residual electrons diffusing from filaments to adjacent regions can serve as seed electrons to activate the former inhibition positions, which makes the filament number further increase. Moreover, influenced by the stronger radial electric field between the central column and its neighborhoods, more electrons located at the column near the middle position will drift toward the center. As a result, once charged particles move over the inhibition region with voltage amplitude rising further, the two discrete discharge columns will merge, causing the decrease in the filament number. Finally, it is revealed in our simulations that when the voltage amplitude exceeds one certain level, seed electrons of the preionization stage get harder to gather and all discharge columns vanish. These results may help to provide a new perspective on the evolution of radial filamentary channels in an atmospheric argon DBD.
In this paper, a two-dimensional axisymmetric fluid model was established to investigate the influence of nitrogen impurity content on the discharge pattern and the relevant discharge characteristics in an atmosphere pressure helium dielectric barrier discharge (DBD). The results indicated that when the nitrogen content was increased from 1 to 100 ppm, the discharge pattern evolved from a concentric-ring pattern into a uniform pattern, and then returned to the concentricring pattern. In this process, the discharge mode at the current peak moment transformed from glow mode into Townsend mode, and then returned to glow mode. Further analyses revealed that with the increase of impurity level, the rate of Penning ionization at the pre-ionization stage increased at first and decreased afterwards, resulting in a similar evolution pattern of seed electron level. This evolution trend was believed to be resulted from the competition between the N 2 partial pressure and the consumption rate of metastable species. Moreover, the discharge uniformity was found positively correlated with the spatial uniformity of seed electron density as well as the seed electron level. The reason for this correlation was explained by the reduction of radial electric field strength and the promotion of seed electron uniformity as pre-ionization level increases. The results obtained in this work may help better understand the pattern formation mechanism of atmospheric helium DBD under the variation of N 2 impurity level, thereby providing a possible means of regulating the discharge performance in practical application scenarios.
We report a systematic numerical study concerning the effect of traces of nitrogen admixtures on the discharge pattern dynamics in an atmospheric helium dielectric barrier discharge. By gradually increasing the N 2 concentration from 1 to 1000 ppm, the discharge mode transformed from the stationary pattern, via quasi-uniform structure, stationary pattern, into the complementary pattern. Analyses revealed that Penning ionization exerted a significant influence on the electron distribution, surface charge deposition and electric field profile over the gap and thus induced the transitions in pattern structure. With the increase of N 2 content, the average rate of Penning ionization within the afterglow stage presented a non-monotonic variation trend, which conformed to previous results, and seed electron level of the main discharge varied in a similar way. When the discharge was ignited with a large amount of seed electrons, the electron distribution prior to the breakdown phase tended to homogenize and the level of radial electric field during the pre-ionization process was restrained, contributing to the homogenization of the discharge. Furthermore, the formation of complementary discharge pattern was believed to be resulted from the excessive reduction of average electron density between two adjacent main discharges at above 500 ppm N 2 level. The reduced electron concentration made the volume ionization more susceptible to the surface charge distribution and led to the disruption of discharge channel succession between positive and negative half cycles.
Air is a typical and arguably unavoidable impurity in atmospheric pressure dielectric barrier discharges (DBDs). The introduction of air may bring rich plasma chemical effects on DBDs and lead to a significant change of discharge characteristics. Here we implement a two-dimensional fluid model to study the spatial discharge behavior in a helium–dry-air DBD under the air impurity level (N air) of 10–200 ppm. The simulation results reveal that under low impurity content (less than 30 ppm), the gas gap cannot be ignited due to the feeble Penning ionization during the breakdown. However, with an elevation in the impurity level, the progressively enhanced Penning ionization makes the DBD experience three different spatial modes, namely uniform, columnar, and complementary quasi-uniform modes. Of particular note is that the improvement of discharge uniformity observed after the second mode transition is not directly controlled by seed electron level—a previously reported qualitative indicator of the discharge uniformity concluded by helium DBDs with only nitrogen traces. And the main contributor to this phenomenon is the complementary spatial structure appearing in successive two discharges induced by the further reinforced Penning ionization with extra oxygen doped. The result suggests the necessity of considering oxygen in helium–air DBDs when the impurity effect of air is concerned.
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