## 3 Results and Discussion
### ICCD Diagnostics of Plasma in One Cycle with Exposure Time of 2 ns  
In total, 36 photos were captured with a single exposure time of 2 ns within a current cycle of 73.7 ns, each photo was spaced 2 ns apart. As the existence of negative glow, we analyzed the average intensity of 8 lines of pixels (basically covering the negative glow region) at the top, the middle 8 lines of pixels at the middle and 8 lines of pixels at the bottom of plasma region (952 \(\times\) 44 pixels) (952 \(\times\) 44 pixels). Therefore, the relation curves represent the average emission intensity of different longitudinal positions varies with time in one cycle were obtained. The results are shown in Figures 5-7.  
Figure 4: Three-dimensional view of the emission intensity vary with gap position and power of 4.4 mm argon DBD.  
Figure 5: The curves of luminescence intensity on surface of the upper electrode in 4.4 mm argon DBD in one current cycle at different power.  
As we can see from Figures 5-7 the luminous intensity increases with the power at all times and all positions in one cycle, the 12 curves from bottom to top correspond to points 1-12 in Figure 2. At the power of the point 1 to point 8 (138-624 W) in Figure 2, the intensity of the three longitudinal positions of the plasma has two peaks in one cycle, which match the two peaks of the current. However, it can be seen from Figures 5-7 that the three peaks are different in time. The first peak of intensity appears first on the dielectric surface of the upper electrode (cathode), the following is the middle position, and finally, the dielectric surface of the lower electrode. Three peaks from top to bottom appeared at 17.7 \(\pm\) 1 ns, 20.1 \(\pm\) 1 ns and 20.5 \(\pm\) 1 ns, respectively. In the second half of the cycle, the second peak appears first on the dielectric surface of the lower electrode (cathode), followed by the middle position, and finally on the dielectric surface of the upper electrode. Three peaks from bottom to top were 54.5 \(\pm\) 1 ns, 56.9 \(\pm\) 1 ns and 57.3 \(\pm\) 1 ns, respectively, which means the generation of electron avalanche on anode surface is about 3 ns later than that on cathode surface. The time intervals reflect the process of electron avalanche from cathode to anode in every half cycle.  
It can be seen from the curves at the four maximal powers in Figures 5-7 (686-836 W) that a third peak appears obviously on the upper electrode surface in the first half cycle and the lower electrode surface in the second half cycle, and the intensity increases rapidly with  
Figure 6: The curves of luminescence intensity of middle part in 4.4 mm argon DBD in one current cycle at different power.  
Figure 7: The curves of luminescence intensity on surface of the nether electrode in 4.4 mm argon DBD in one current cycle at different power.  
power (686-836 W). In Figure 8, voltage and current waveforms are shown with emission intensity curves from Figures 5-7 (836 W), from which we can see the upper electrode in the first half is cathode, same as the lower electrode in the second half cycle, the new peak is identified as cathode luminescence caused by secondary electrons, which is an important characteristic of \(\gamma\) mode. Thus, it can be judged that the discharge has changed from \(\alpha\) mode (624 W, point 8 in Figure 2) to \(\gamma\) mode (686 W, point 9 in Figure 2) [6,13].