## 3 Results and Discussions  
The electric characteristics of the gliding arc discharge were obtained by determining the waveforms of the voltage and discharge arc current in the air plasma. The electrical characteristics were measured by changing the output voltage from 50 V to 250 V. The typical voltage and current waveforms of the GAD in air plasma are presented in Figure 2. The voltage waveform is a serrate-like shape, which is due to the periodic change in the length of the arc column over time. The arc current also increases slightly as the arc length increases (Figure 2a,c). It should be noted that, with the increase in the output voltage of voltage generator from 50 V to 250 V, the arc column becomes longer, and both the voltage and current waveform last longer [20; 38]. Similar behaviors of the voltage and current waveforms were obtained for different gliding arc discharges [13; 16; 20]. The voltage increases to its maximum value as the gliding arc column is elongated. The arc breakdown occurs when the distance between the electrodes becomes too long and the applied electric field is too low to sustain the discharge. The voltage drops very rapidly and the arc current rises instantly as the breakdown occurs (Figure 2). The growing arc requires more power or a higher voltage to sustain itself, as the air/gas breakdown voltage depends on the gap distance between the electrodes (_d_) and the pressure (_p_), according to Paschen's law. The _pd_ value is enhanced with the increase in the arc length, and at the critical arc length the voltage becomes insufficient to sustain the elongation of the discharge [9]. It should be noted that the duration of the discharge, the voltage, and current values depend on the shape of the electrodes as well as on the flow rate of the gases and the voltage values [13; 16; 20; 38]. However, the shape and period of the waveforms were slightly different even when the output voltage was kept constant (Figure 2b). With the increase in the voltage, the electric field is increased, and the arc propagates and reaches further distances (Figures 2 and 3) [38]. In order to determine the evolution of the arc during the discharge a high-speed camera was used. The images of the arc's evolution in air plasma at various V\({}_{\rm G}\) values are presented in Figures 3 and 4. These images were also used to determine the discharge duration and compared with the data obtained from the voltage and current waveforms. Figure 2: Waveforms of current (**a,c**) and voltage (**b,d**) for the GAD discharge with a V\({}_{\rm G}\) of 90 V (**a,b**) or 250 V (**c,d**). It can be seen that the arc is ignited at the minimum gap between the electrodes and that the carrier air pushes the arc forward. The voltage increases with the movement of the arcs through the electrodes and as they increase in length (Figures 2 and 4). When the arc reaches its maximum length, the arc breaks up and is immediately reignited at the point of the smallest gap, and then the arc glides further (Figure 4, see the first or last photo images). With the increase in the voltage, the plasma volume and length become greater (Figure 5). The visual increase in the plasma volume and area can be clearly seen from the photo images in Figure 5. The higher voltage can support the arc running across a higher resistance (larger gap). The increase in current values versus time with the expansion of arc could be due to fact that the voltage increases faster than the resistance between the electrodes. The average values of the life-time of the arc discharge, determined from the electric characteristics and the images taken by a high-speed camera, are presented in Figure 6. It was found that the average discharge duration was 1.71 \(\pm\) 0.29 ms when a voltage of 50 V was used.