## 3 Results and Discussion
### Discharge Characteristics
A high voltage was applied to the lower electrode covered with the insulation pad, and the ground was applied to the upper electrode. All the electrodes are 10-mm wide and 30-mm long, except the upper electrode of the 5-mm width, as shown in Figure 4a. All the data of ignition voltage \(V_{f}\) and current were measured three times and averaged as shown in Figure 4a-d and as plotted in Figure 4e within an error range of less than 5%. The negative value of gap distance d means the overlapped electrode. The upper electrode is overlapped as \(d\) = \(-\)5 mm over the center of the lower electrode in Figure 4a, where the \(V_{f}\) = 1.0 kV and the current is \(I\) = 5.3 mA for both the plasma-generated upper-electrode sides. In Figure 4b with the overlapped electrodes of \(d\) = \(-\)2 mm at the edge, the \(V_{f}\) = 1.1 kV and the current is \(I\) = 2.3 mA. In Figure 4c with the \(d\) = 0 between the two electrodes, the ignition voltage is \(V_{f}\) = 1.2 kV and the current is \(I\) = 1.4 mA. In Figure 4d with the \(d\) = 2 mm, the \(V_{f}\) = 2.2 kV and the current is \(I\) = 1.8 mA. With the overlapped electrodes of (a) and (b), the \(V_{f}\) is low while the current is high, since the displacement current is high for the high capacitance between the overlapped electrodes. Comparing the \(d\) = 0 in (c) with the \(d\) = 2 mm in (d), the \(V_{f}\) for the \(d\) = 2 mm is approximately two times higher than that for the \(d\) = 0. Further, for the wide-gap distance of \(d\) = 2 mm in (d), the discharge is unstable and the plasma is nonuniform on the discharge surface. With the wider \(d\) of > 3 mm, the ignition will not occur in the range of 3 kV with respect to the proposed inverter power system. Therefore, the \(d\) = 0 is proper in terms of the planar DBD. Thus, the structure of Figure 1c-iii was chosen for the following experiment, shown in Figure 2, for the bio-medicine application. Figure 3: Cross-sectional pictures of the spatial discharge at the ignition voltage (\(V_{f}\)) according to the gap distance (\(d\)) with the dielectric barrier discharge (DBD) structure shown in Figure 1a in (**a**) and the Paschen curve with the firing voltage (\(V_{f}\)) versus the \(d\) in (**b**). In Figures 3-5, the shape of the discharge mode represents the multichannels of the streamer discharge that typically appear in the DBD [33; 34]. Figure 5 shows the photographs of the discharges on the surface of the glass sample of Table 1 from the experiment shown in Figure 2. The plasma generation occurred on the dielectric surface over the high-voltage electrode with the 5-mm diameter. As shown in Figure 5a, the plasma was generated at the boundary line between the high-voltage and ground electrodes. As the current and voltage were increased, the plasma was observed throughout the overall boundary of both the electrodes, as shown in Figure 5b,c. The discharge was gradually enlarged to the center of the surface of the high-voltage electrode, as shown in Figure 5d,e. Figure 4: Pictures of the planar discharge on the dielectric surface at the ignition voltage (\(V_{f}\)) in the dielectric barrier discharge (DBD) structure of Figure 1c with the both-side electrodes: overlapped 5 mm at the center in (**a**), overlapped 2 mm at the edge in (**b**), no gap distance (\(d\)) = 0 in (**c**), and \(d\) = 2 mm in (**d**). And the graph of firing voltage and current versus the gap distance \(d\) in (**e**) where the negative value of gap distance d means the overlapped electrodes. Figure 5: Photographs of the discharge shapes in the dielectric barrier discharge (DBD) structure of Figure 2 in (**a**–**e**) with the increasing current and voltage on the glass-plate surface. Figure 6 presents the typical I-V characteristics, the so-called _filamentary mode_[33; 34; 42; 43], in terms of the typical DBD operation with the atmospheric pressure.