# Dual-Frequency Microwave Plasma Source Based on Microwave Coaxial Transmission Line
## 3 Experiment Results  
Based on the simulation results, the experiment was carried out in accordance with processes listed in Section 2. Firstly, the 700 W 2450 MHz microwave was inputted into the reactor from Port 1 at a pressure of 80 Pa. The plasma can be excited with the assistance of an automatic impedance matchbox. In the center of the plasma reactor, the electron density and temperature measured by the Langmuir probe were \(n_{e}=1.56\times 10^{16}\) m\({}^{-3}\) and \(Te=2.46\) (eV). For microwave-excited plasma, electrons oscillate among the heavy immobile ions at the plasma electron frequency. At this time, if another microwave with the frequency below the plasma electron frequency is inputted from another port, the inertia of the electrons is sufficiently low for them to respond to the electric field in the incident electromagnetic wave, and the electron can, therefore, absorb energy from it. If the frequency of the incident microwave is higher than the electron plasma frequency, the inertia of an electron would be too high to enable it to respond fully to the incident microwave. Therefore, the interaction of the microwave with individual electrons is relatively insignificant. Thus, different interaction modes of microwaves with electrons can be achieved by controlling the frequencies of the second incident microwaves [27]. The electron plasma frequency is a function of the electron number density and is given by [27]. \[\omega_{\rm pe}=2\pi v_{pe}=\sqrt{\frac{n_{e}e^{2}}{\varepsilon_{0}m_{e}}}\ \ \ ({\rm rad/s}) \tag{1}\]  
where \(v_{pe}\) is the electron number density in Hz. The \(n_{e}\) is the electron density, the \(\varepsilon_{0}\) is the permittivity of free space, and the \(m_{e}\) is the mass of an electron. According to Equation (1), for \(n_{e}=1.56\times 10^{16}\) m\({}^{-3}\), the electron plasma frequency is \(v_{e}=1123\) MH, which is located between 2450 MHz and 915 MHz. Then, if the input microwave from Port 2 is 2450 or 915 MHz, the plasma would show different responding behaviors. To compare with the influence of the 915 MHz microwave, a 2450 MHz solid-state source was used instead of the 915 MHz solid-state source and the low-pass filter for comparative experiments. In the experiment, 915 and 2450 MHz microwaves were inputted from Port 2 to investigate the plasma characteristics changes, respectively. Figure 3  
Figure 2: COMSOL simulation of the electron density and temperature distributions at a fixed pressure of 80 Pa under Port 1 2450 MHz microwave power: (**a**) electron density (**b**) electron temperature. shows the effect of microwave inputted from Port 2 on the electron density and electron temperature measured by the Langmuir probe. Figure 3a shows that, when the second microwave was inputted from Port 2, the plasma electron temperature gradually increased as the 915 MHz microwave power increased and did not change sufficiently as the 2450 MHz microwave power increased. In contrast, Figure 3b shows that, when the second microwave was inputted from Port 2, the plasma electron density did not change sufficiently as the 915 MHz microwave power increased, but it gradually increased as the 2450 MHz microwave power increased. This means that the second incident microwaves with frequencies below the plasma electron frequency could increase the electron temperature and affect the electron density indistinctly, and the second incident microwaves with frequencies above the plasma electron frequency could increase the electron density and affect the electron temperature indistinctly. To investigate the physical mechanism of these phenomena, the OES results at different conditions were analyzed, and the OES data were all obtained at 10-ms integral times.