# Current Filaments in Asymmetric Surface Dielectric Barrier Discharge
## 1 Introduction  
Dielectric barrier discharges (DBD) have found many industrial applications as ozone generators, plasma discharge panels, excimer lamps and in different types of surface modifications [1; 2; 3; 4; 5]. DBD are characterized by the presence of an insulating material between the electrodes to prevent the occurrence of high electrical current arcs between them (arcing). They produce a non-thermal plasma at (or near) atmospheric pressure in the discharge gap region between electrodes (Figure 1a) [6; 7]. The DBD geometry results in a quite uniform plasma distributed between electrodes, providing a large flow of activated or reactive compounds for the treatment of materials [8; 9; 10]. This technique enables the application of plasma processes at atmospheric pressure without a damaging heat load [11; 12; 13; 14; 15]. DBD have been used, for instance, to increase the wettability of polymers, to grow silicon oxide thin films, to perform sterilization by UV irradiation, and for air purification [16; 17]. Variants of DBD are surface dielectric barrier discharges (SDBD) (Figure 1b), obtained by using a particular configuration of electrodes in direct contact with the dielectric material. To extend SDBD applications, asymmetrical electrode layouts (one isolatedand the other exposed to the air) were developed, in particular in the context of aerodynamics [18; 19; 20; 21]. In the latter, the main purpose was the production of an ionic wind, flowing along the wing surface [22; 23]. The resulting enhanced mixing of air flow and ionic wind was found to be beneficial for an active boundary layer control, pursuing drag reduction [24], lift enhancement [21], and stall prevention [25]. In this work, we perform extensive statistical analysis of SDBD, complementing the study on DBD performed in [26], using an asymmetric device with an exposed (grounded) metal electrode, glued onto the dielectric surface, and a lower electrode covered by an insulating material (see Figure 1b). The purpose of our work is the characterization and the analysis of the mechanisms taking place during the discharge formation in SDBD, and its robustness when operating under steady state conditions. This is a basic requirement that a working plasma device should fulfil in order to be used in large-scale industrial applications. The study of SDBD properties are also interesting from a fundamental physics point of view, since they have not been thoroughly investigated, while DBD have not been thoroughly investigated either [6; 26]. In the presence of high-voltage oscillating signals, SDBD display a complex pattern of electric discharges in the form of short bursts of activity, concentrated within each half-cycle of the wave signal period. They are often referred to as forward discharge or forward stroke (FS), and backward discharge or backward stroke (BS), depending on the sign of the applied voltage [27]. The two strokes are composed of a relatively large number of individual events, carrying electrical currents in the discharge area, for which the expression microdischarge was coined [6]. It is still not clear yet what the role played by FS and BS microdischarges is in the production of the induced airflow around the plasma [22; 28; 29]. At atmospheric pressure in air, SDBD operate in the so-called microdischarge regime, characterized by narrow current filaments with very short lifetimes (few tens of ns) [6; 30]. Microdischarge filaments, or streamers, extend from the exposed electrode towards the dielectric surface above the covered electrode, more or less perpendicularly to the electrodes [30]. This spatial region roughly corresponds to the region where the electric field strength is at its maximum. At a more fundamental level, it is not even clear if the discharge mechanism is the same during the two AC semi-cycles [30; 31]. The origin of the differences between the two strokes may be due to the different asymmetries that, as a matter of fact, can be present in typical SDBD setups.