## 3 Results and Discussion
### Discharge Characteristics  
The ignition (electric-breakdown or discharge-firing) test is presented in Figures 3 and 4, while the electrode arrangements of the planar DBDs are shown in Figure 1 with the alumina dielectric plate. The gap distance is an essential parameter for the ignition along the discharge path between the metallic electrodes on the surface of the dielectric layer. Figures 5 and 6 with the various dielectric materials listed in Table 1 show the discharge mode and the current-voltage (I-V) characteristics, respectively, while the DBD structure of Figure 2 is applicable for the actual plasma pads [35; 36]. For the DB discharge between the face-to-face electrodes in Figure 1a, Figure 3 shows the spatial-discharge photographs in (a) and the Paschen curve in (b) according to the spatial gap distance \(d\) between the upper electrode and the dielectric-layer surface. The dielectric layer is the alumina ceramic with the \(T\) = 0.5 mm in thickness and the copper electrodes are \(W\) = 10 mm in width and \(L\) = 30 mm in length. The upper electrode was applied to a high voltage and the lower electrode was grounded. For the discharge ignition with the proposed-inverter output voltage in the range of 0.5-3.0 kV, the \(d\) of this experiment is from 0.1 to 0.5 mm. The firing voltage \(V_{f}\) = 0.99-1.76 kV corresponding to the \(d\) = 0.1-0.5 mm and the \(Pd\) = 7.7-38.0 (Torr cm) for the \(P\) = 760 Torr. The firing voltage \(V_{f}\) and the current were measured three times and averaged within an error range of less than 5%, and the firing voltage was plotted in Figure 3b. The data of the gap distance, firing voltage, and current are (\(d\), \(V_{f}\), \(I\)) = (0.1 mm, 0.99 kV, 5.5 mA), (0.2 mm, 1.16 kV, 4.7 mA), (0.3 mm, 1.35 kV, 4.5 mA), (0.4 mm, 1.53 kV, 4.4 mA), and (0.5 mm, 1.76 kV, 4.2 mA). As the \(d\) was increased, the \(V_{f}\) increased for the Paschen breakdown while the current decreased, even for the increasing of the brightness intensity of the visual plasma. Considering the decreasing current, the amount of the displacement current that can be passed through the dielectrics and the real current (plasma current other than the capacitive type) were determined. It was expected the displacement current would be high for the high capacitance of the short gap, and therefore the total current increased to a high value for the short gap, while the total current for the long gap is low, even though it was expected the plasma current would be high with the high \(V_{f}\) for the long gap. From the Paschen data of Figure 3b, it can be interpolated that the \(V_{f}\) is  
Figure 2: Schematic diagram of the planar-discharge experiments performed in the atmospheric environment with the various dielectric materials listed in Table 1.  
-21 kV for the \(d\) = 1 cm, together with the reported operating voltage of the same order as that of the earlier atmospheric DBD experiments where the \(Pd\) is \(\sim\)760 Torr cm [33; 34]. In the coplanar discharge of Figure 1b, which is the typical structure of the microdischarge in the plasma display panel (PDP), the ignition was not observed with the alumina plate of the \(T\) = 0.5 mm in this experiment. With the proposed inverter in the output-voltage range of 3 kV, the insulation breakdown occurred between the two electrodes of the short gap \(d\) < 1 mm, and the ignition on the dielectric-layer surface did not occur for the \(d\) > 1 mm. As a result, the coplanar discharge cannot be ensured unless the dielectric layer is quite thin. In the case of the microdischarge PDP with the dielectric-layer thickness of less than 100 \(\upmu\)m, the reported ignition voltage is approximately 300 V for the low-pressure values of less than a few tens of Torr. Therefore, the DBD atmospheric discharge is not suitable for the coplanar discharge. The ignition tests with the planar discharge of Figure 1c are shown in Figure 4. Two electrodes are both sides of the dielectric layer (alumina ceramic) with the \(T\) = 0.5 mm.