## 3 Results and Discussions
The average discharge duration slightly increased, up to \(\sim\)2.95 \(\pm\) 0.35 ms, when the V\({}_{\rm G}\) was set to 90 V. It should be noted that the increase in the voltage from 50 V to 250 V increased the arc discharge duration almost linearly, up to 5.21 \(\pm\) 0.61 ms (Figure 6). It was determined that the arc velocity gradually decreased as the arc glided along the electrodes, mainly due to the reduction of the driving force as the gas flow velocity is lower at a longer distance [38]. Thus, it could be concluded that the duration of the discharge was highly  
Figure 3: Photo images showing the arc evolution over time at 50 V, 90 V, and 130 V voltages. The optical emission spectrum of the air plasma produced using a gliding arc discharge at V\({}_{\rm G}\) = 250 V is given in Figure 7. The emission spectrum offers us information about the type of excited species existing in the air plasma. It was observed that the main particles in the air plasma were exited nitrogen molecules. The highest intensity peaks observed at 316 nm, 337 nm, 353 nm, 358 nm, and 379 nm are attributed to the (0, 1), (0, 0), (2, 1), (1, 0), and (2, 0) N\({}_{2}\) vibrational modes, respectively [6,11,22]. The appearance of peaks in the range of the 280-450 nm wavelengths is related to the various vibrational mode transitions of the second positive system of N\({}_{2}\) in the air plasma [6,15,22,39,40,41]. It was established that the highest intensity emission peaks in the range of 280-430 nm are attributed to the second positive system of N\({}_{2}\) (C\({}^{3}\Pi_{\rm u}\)\(\rightarrow\)\({}^{3}\Pi_{\rm g}\)) and the first negative system of N\({}_{2}\)\({}^{+}\) (B\({}^{2}\Sigma^{+}_{\rm u}\)\(\rightarrow\)\({}^{3}\Sigma^{+}_{\rm g}\)), which are usually obtained in air or nitrogen plasmas [6,11,15,21,22,31]. The low-intensity emission peaks at 391 nm, 419 nm, and 470 nm are attributed to the first negative band of the molecular nitrogen ion (N\({}_{2}\)\({}^{+}\)) (Figure 7). The N\({}_{2}\) molecules react with the plasma electrons through direct impact; part of the electrons' energy is transferred to the molecule and the excitation or ionization of N\({}_{2}\) proceeds. It was determined that the formation of N\({}_{2}\)\({}^{+}\) in air plasma can be attributed to the direct electron-impact ioniza  
Figure 6: Variation of gliding arc discharge duration versus output voltage. Figure 7: Optical emission spectrum of air plasma when the output voltage was 250 V.  
tion of the nitrogen molecules from their ground state (\(\mathrm{X^{1}\Sigma_{g}}^{+}\))\(\mathrm{V}_{\mathrm{z}=0}\) to \(\mathrm{N_{2}}^{+}\) excited states (\(\mathrm{B^{2}\Sigma_{u}}^{+}\))\(\mathrm{V}_{\mathrm{z}=0}\) via the following reaction:  
\[\mathrm{N_{2}}\ (\mathrm{X^{1}\Sigma^{+}}_{g})+\mathrm{e}\mathrm{\rightarrow} \mathrm{N_{2}}^{+}\ (\mathrm{B^{2}\Sigma^{+}}_{u})+\mathrm{e}+\mathrm{e} \tag{1}\]  
Another way to produce a molecular nitrogen ion (\(\mathrm{N_{2}}^{+}\)) is a two-step ionization process via the following reactions:  
\[\mathrm{N_{2}}\ (\mathrm{X^{1}\Sigma^{+}}_{g})+\mathrm{e}\mathrm{\rightarrow} \mathrm{N_{2}}^{+}\ (\mathrm{B^{2}\Sigma^{+}}_{g})+\mathrm{e}+\mathrm{e} \tag{2}\]  
Figure 2:0  
Firstly, due to electron collisions with the nitrogen molecule, ionization takes place and the \(\mathrm{N_{2}}\) molecule moves from its ground state to a molecular nitrogen ion (\(\mathrm{N_{2}}^{+}\)) ground state (Equation (2)). In the second step, the transition from the \(\mathrm{N_{2}}^{+}\) ground state to the \(\mathrm{N_{2}}^{+}\) excited state takes place due to the excitation induced by the electron impact (Equation (3)).