## 3 Experiment  
### Experiment Setup  
Based on the correlations with the plasma characteristics [21], the geometry parameters for the waveguiding structure were set to improve the power transfer efficiency from the wave generator to the plasma region. In the quasi-coaxial line TEM waveguiding structure, microwave power was delivered through a TE/TEM coupler which was bent at 90\({}^{\circ}\), as shown in Figure 2a. To maximize the power transfer ratio when converting from a rectangular TE\({}_{10}\) to a quasi-coaxial line TEM waveguiding structure, a rod-shaped antenna was placed in the field concentration region. The rod-shaped antenna was made of copper as an inner conductor. The diameter of the inner conductor was 9 mm, and the axial length was 600 mm. The microwave propagated mainly in the space between the inner rod and the quartz tube. The space between the inner conductor and the quartz tube was filled with atmospheric pressure for the device cooling by the flow of compressed air [10]. Inside the plasma chamber, quartz window was placed to separate the field generator and plasma excitation region which is filled with argon gas at low pressure. The quartz window \(\epsilon_{r}=4\) was designed with an inner diameter of 32 mm, a thickness of 2 mm, and a total length of 550 mm in the axial direction. The compressive strength of the quartz window was 117,000 kg/cm\({}^{2}\), the Young modulus was 73,900 kg/mm\({}^{2}\), and the tensile strength was 510 kg/cm\({}^{2}\), which could sustain the mechanical load without deformation in the corresponding axial length. The plasma was ignited by the excitation of a discharge from the electromagnetic waves that penetrate or exist along the quartz window surface. The propagation wave mode was converted from the rectangular TE\({}_{10}\) wave mode to the circular TE\({}_{11}\) wave mode as shown in Figure 2b. In the circular TE\({}_{11}\) waveguiding structure, microwave power was transferred to the plasma through the continuous cylindrical slot antenna along the wave propagation direction. The circular waveguide was made of copper material with an inner diameter of 30 mm and a thickness of 1 mm. In the cylindrical slotted waveguiding structure, the cut surface of the circular TE\({}_{11}\) waveguide was cut to a radial thickness of 10 mm along with an axial length of 500 mm from the entrance of the plasma chamber to the end of the continuous axis. The quartz window and plasma generation process were set to be the same as in Figure 2a. When the inner diameter of the circular waveguide was 30 mm and was filled with free space material characteristics, the cut-off frequency in the circular waveguide was approximately 5.9 GHz. For the circular waveguide of 30 mm diameter, it was necessary to lower the cut-off fre  
Figure 2: Schematic diagram of microwave plasma source. (**a**) Quasi-coaxial TEM linear MWP and (**b**) circular TE\({}_{11}\) waveguiding structure. quency below 2.45 GHz for the wave propagation in the applied frequency. Accordingly, a rod type (diameter = 30 mm, \(\epsilon_{r}=9\)) alumina was inserted inside the circular waveguiding structure, and the cut-off frequency was reduced to a third of 5.9 GHz. To compare the source characteristics, experiments were conducted under 500 mTorr Ar plasma condition where 0.6 kW microwave power was applied. The experimental system used for this study is shown in Figure 3. Continuous wave (CW) power at 2.45 GHz was transmitted from a magnetron (ASTEX MKS FI20608-3 kW Mag Head, ASTEX Smart Match FI20606-3 kW) to the waveguide mode converter by a rectangular TE\({}_{10}\) wave mode through the WR340 waveguide. The power supply (MKS Smart Power generator FI20160-1-3 kW) was connected to a three-phase line and outputted 300-3000 W. As shown in Figure 3, the microwave transmission line from the generator to the plasma includes a WR340 3-Stub tuner, dummy load (FI20164), and circulator (AX3161) as the product of National Electronics.