## 5 Results and Discussion  
Normally, the generation of optical emission is associated with the Ar* excitation state species, which means electron heating occurs. The electron heating mechanism is based on the electric field profiles, and in this study, the wave field was fed by a wave launcher from the end of the tube. In the linear MWP, which applied power to one side port, electron density attenuation appeared along the linear axis due to the difference in the power absorption along the wave propagation direction, which could result in a specific electron density distribution. Moreover, if the reflection of the waves was made at the opposite boundary, the formation of a standing wave occurred. A standing wave formed in the antenna structure and made inhomogeneous line strengths of an electric field, which caused a spatial difference in the plasma power absorption along the axial direction. Figure 7 shows that the microwave power is applied from the left side in the front view and the right side in the side view. For TE\({}_{11}\) wave mode, as shown in Figure 7a, the brightness of the emitted light is higher at the beginning of wave propagation, and the faint pattern becomes clearer towards the end. From Figure 7b, we see that from the right-side view the optical emission pattern can be observed more clearly. In the quasi-coaxial line TEM wave mode, the same optical emission pattern and the decrease in brightness along the axial directions are observed as in the TE\({}_{11}\) mode. On the other hand, emitted light pattern in the TEM structure was different in the intensity of the emitted light brightness along the axial line and the wavelength of the emitted light wave pattern. In the TEM structure, emitted light brightness concentrated at the beginning of the axial line was relatively lesser and was evenly distributed, as shown in Figure 7c. The wavelength of the emitted light pattern was longer than that of the TE\({}_{11}\) mode. The half wavelength observed on the optical emission pattern was measured to be 23 mm in TE\({}_{11}\) and 47 mm in TEM. Figures 8-10 show the simulation results of the spatial profiles of the electric field, electron density, and electron temperature, respectively, for each source along the axial and radial directions. Figure 8b shows that the coordinate direction is based on the center of the waveguide, and the X-axis is directed perpendicular (radial direction, \(\theta=0^{\circ}\)) from the waveguide to the plasma column; the Y-axis (radial direction, \(\theta=90^{\circ}\)) is the direction of the height, and the Z-axis is the wave propagation direction (axial direction) in the waveguiding structure. Figure 7: Images of MWP plasma optical emission patterns of Ar circular TE\({}_{11}\) waveguiding structure. (**a**) Front view, (**b**) right side view, quasi-coaxial TEM, (**c**) front view, and (**d**) right side view. In Figure 8, the E-field profile (norm \(|E|\) (E-field amplitude, \(\sqrt{E_{x}^{2}+E_{y}^{2}+E_{z}^{2}}\))) is calculated in the 3-D domain. In Figure 8, at TEM wave mode, the direction of the radially emitted wave electric field is perpendicular to the tangential plane of the quartz window  
Figure 8: Electric field norm \(|E|\) (E-field amplitude, \(\sqrt{E_{x}^{2}+E_{y}^{2}+E_{z}^{2}}\)) simulation results. (**a**) TEM mode of quasi-coaxial linear MWP and (**b**) circular TE\({}_{11}\) mode waveguiding structure MWP. Figure 10: Electron temperature at 500 [mTorr] MWP. (**a**) TEM mode of quasi-coaxial linear MWP (**b**) circular TE\({}_{11}\) mode waveguiding structure MWP. Figure 9: Electron density profile at 500 mTorr MWP. (**a**) TEM mode of quasi-coaxial linear MWP and (**b**) circular TE\({}_{11}\) mode waveguiding structure MWP. surface in the XY plane. In the radial direction, the electric field strength was maximum at the copper antenna surface (inner conductor) and decreased as it was emitted in all directions of plasma-dielectric contact window surfaces [10].