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.

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.

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.

Partial discharge (PD) is an important factor leading to insulation degradation of electrical equipment. Understanding the PD mechanism is of great significance for maintaining the safety of power systems. Compared with the previously reported PD models, this work develops a well-validated plasma fluid model considering more practical particles and reactions in order to explain the PD behaviour from the physicochemical point of view. The simulation results reveal that, under the power frequency voltage (50 Hz), the PD in a small dry air cavity operates at the weak Townsend mode. Besides, since the electric field nearby the perpendicular wall is significantly distorted by the void geometry, the local intense discharge presents a radial migration. Such a migration route of the local intense discharge can enlarge the discharge area per radial distance and is believed to be capable of inducing the oscillation of the current pulses. Furthermore, the main reaction pathways of ions production/destruction are elaborately analyzed, as well as the contribution of different ions to the surface charge accumulation. Finally, the generation law of the main oxidative product of the dry air discharge-O ₃ is obtained. It is revealed that the O ₃ concentration elevates with a fixed increment during each voltage half-cycle in the steady-state, and such an increment basically increases linearly as the voltage amplitude rises. The corresponding discussions in this work would be helpful both to achieve a better understanding of plasma dynamics in PD and to predict the PD intensity or position in the practical application scenarios.

We report a systematic numerical study regarding the control of discharge pulse number in an atmospheric helium dielectric-barrier discharge with multiple current pulses per half cycle. The results indicated that by replacing the original sinusoidal applied voltage with a peak-levelled one, the manipulation of current pulse number in each discharge half cycle could be achieved, and the initial moment of flat-top stage t ₀ , which reflects the duration of peak-levelled applied voltage, was set to be the controlling parameter. With the increase of flat-top duration, the current pulse number per half cycle decreased monotonously from the original value to zero. In this process, the current amplitude elevated, the peak phase delayed, and the energy efficiency in terms of electron production enhanced. Further analyses revealed that the above alterations in discharge performance were not caused by changes in seed electron level, but by the variations in discharge current components and surface charge level. Finally, based on the criteria of pulse number alteration, a preliminary analytical method was proposed to switch the original multiple-current-pulse discharge into a single-current-pulse one. The results obtained in this work are expected to help achieve the mode manipulation and stabilization of the discharge in industrial application scenarios.