# Dual-Frequency Microwave Plasma Source Based on Microwave Coaxial Transmission Line
## 2 Experiment Design  
The schematic diagram for the presented dual-frequency microwave plasma source based on the microwave coaxial transmission line is shown in Figure 1.  
The plasma reactor is a coaxial line, whose outer diameter is 150 mm and the inner diameter is 8 mm. A quartz glass tube surrounds the inner conductor. Between the outer conductor and the quartz glass tube, there is a vacuum region in which the plasma is excited. There are two microwave ports at the two ends of the coaxial transmission line. One is used to input the 2450 MHz microwave, and another is used to input the 915 MHz microwave. A magnetron microwave power generator (ASTEx AX2110) is used to generate the continuous-wave 2450 MHz microwave. An isolation circulator with a water load in an isolated way is connected to the magnetron microwave power generator. In this way, the microwave feedback will be entirely absorbed by the water load. An ASTeX SmartMatch is used to match the impedance so that the maximum amount of power can be coupled with the plasma. The 2450 MHz microwave is transmitted from the generator to the plasma reactor by the waveguide. A waveguide-to-coaxial adapter is designed and installed to guide the microwave from the waveguide into the coaxial transmission line plasma reactor through Port 1 [15]. A custom-made solid-state microwave power source (Wattsine Electronic Technology Co., Ltd. Chengdu China) is used to generate a continuous-wave 915 MHz microwave. The 915 MHz microwave is transmitted by N-type coaxial cables. The incident and reflected powers are monitored by a bi-directional coaxial coupler and Keysight power meter (Narda N1914A). A coaxial-to-coaxial adapter is designed and installed to guide the microwave from the N-type coaxial cable into the coaxial transmission line plasma reactor through Port 2. A coaxial low-pass filter is installed into N-type coaxial cables to avoid the solid-state microwave power source affected by 2450 MHz. There are several KF flanges on the outer conductor, which are used to connect a vacuum pump, gas feed mass flow meter, and diagnostic devices. A Langmuir probe and an optical emission spectrometry (OES) are used to diagnose and investigate the plasma characteristics from two KF flanges in the center of the plasma reactor. The experimental processes are as follows: First, pump the pressure in the reactor below 10 Pa by a vacuum pump; second, adjust the argon flow rate at 15-20 sccm to stabilize the discharge pressure at 80 Pa by the mass flow meter; then, turn on the microwave source  
Figure 1: Schematic of the coaxial line plasma driven by the dual microwave sources. at Port 1 to excite plasma discharge; finally, turn on the microwave source at Port 2 to adjust plasma characteristics and measure the plasma parameters. As shown in Figure 1, the coaxial transmission line is a non-resonant structure. Thus, ultra-wide band electromagnetic waves could be inputted into the reactor. However, it would only be discharged at the area near the input Port 1, and not at the whole reactor. To produce a dual-frequency operation, the plasma must be affected by microwaves from both ports. Therefore, the plasma excited by 2450 MHz from Port 1 should be distributed in the whole reactor. To obtain the plasma distribution in the reactor, COMSOL Multiphysics is used to simulate the plasma distribution at different microwave powers from Port 1. In this investigation, the argon plasma considered in the simulated model contains only electrons (_e_), positively charged argon ions ( _Ar\({}^{+}\)_ ), metastable-state argon (_Ars_), and argon (_Ar_) atoms. For these species, the main transportation equations are provided in Table 1[20]. Stepwise ionization (Reaction 5 in Table 1) plays an important role in sustaining low-pressure argon discharges. Excited argon atoms are consumed via superelastic collisions with electrons, quenching with neutral argon atoms, ionization, or Penning ionization where two metastable argon atoms react to form a neutral argon atom, an argon ion, and an electron.