## 3 Results and Discussion
### Optical Emission Spectroscopy Diagnostics and the Charge Dynamics  
The optical emission spectra were measured at each power in Figure 2. Figure 9 shows the emission spectra at 138 W, corresponding to the first point in Figure 2, Figure 10 and Table 1 show the spectral lines and parameters of the argon we studied. The variation of atomic argon lines at 706, 738, 751, 794 nm with power is shown in Figure 11.  
Figure 8: The voltage and current waveforms with emission intensities curves from Figures 5–7 (836 W).  
Figure 9: Emission spectra of plasma at 138 W.  
Figure 11. Variation of intensities of each spectral line in Table 1 with power.  
Variation of intensities of each spectral line in Table 1 with power are shown in Figure 11. The first 8 points of each line (138-624 W) represent discharge in \(\alpha\) mode, and the last 4 points (686-836 W) represent discharge in \(\gamma\) mode. It can be seen that the  
\begin{table}
\begin{tabular}{c c c c} \hline  & \(\mathbf{\lambda/nm}\) & **Configurations** & \(\mathbf{A_{kk}/10^{7}s^{-1}}\) & \(\mathbf{E_{i}-E_{k}}\) (cm\({}^{-1}\)) \\ \hline ArI & 706 & \(3s^{2}3p^{5}(2^{\mathrm{P}_{0}}{}_{3/2})4s\)- & 0.38 & 93,143.7600–107,289.7001 \\ ArI & 738 & \(3s^{2}3p^{5}(2^{\mathrm{P}_{0}}{}_{3/2})4s\)- & 0.847 & 93,750.5978–107,289.7001 \\ ArI & 751 & \(3s^{2}3p^{5}(2^{\mathrm{P}_{0}}{}_{3/2})4s\)- & 4.02 & 93,750–107,054.2720 \\ ArI & 794 & \(3s^{2}3p^{5}(2^{\mathrm{P}_{0}}{}_{1/2})4s\)- & 1.86 & 94,553.6652–107,131.7086 \\ \hline \end{tabular}
\end{table}
Table 1: Parameters of Ar spectrum lines studied in the experiment.  
Figure 10: Species calibration of emission spectra in Figure 9.  
intensities of each curve increase with power, and after the mode transformation, the intensities of each spectral line increase quicker with power obviously. Among them, turning points (624-686 W) could be found from curves of 751 nm and 794 nm. This is considered to be caused by the apparent increase in electron density after the mode shift, which makes the concentration of various active species increase more rapidly with power.  
In every half cycle, electrons and argon ions were accelerated at the same time. Electrons were accelerated from cathode to anode and the avalanche happened, which leads to the first peak. In \(\gamma\) mode, positively charged argon ions were accelerated from anode to cathode. When the argon ions achieved a certain speed to break the sheath and bombarded the surface of the medium directly, a large number of secondary electrons yielded [6; 7; 8; 10; 11], which leads to the second peak on cathode surface.  
Additionally, we can see, in the every half cycle, the luminescence caused by the secondary electrons on the cathode surface is obviously later than the volume ionization, and the time interval between the peaks is 13 \(\pm\) 1 ns, which is thought to be caused by two reasons. The first is that argon ions are much heavier than electrons. The other reason is that the ions move farther than the electrons. In Figure 5, the value of the middle peak-valley and the third peak increased significantly, and the position of the peak-valley shift to the right. This is due to the large number of secondary electrons.  
Comparing Figures 5 and 7, it can be seen that the waveforms are essentially the same with interval of exactly half cycle, so the discharge characteristics on the surface of the two dielectrics are consistent. Although in Figure 6, there are always two peaks in the waveforms, the interval of which is about 36.7 \(\pm\) 1 ns, very close to half cycle of the current which is 36.87 ns.