## 4 Results and Discussion  
The experiments were conducted as follows: Secondary air, which can be gradually increased to 1000 Nm\({}^{3}\)/h using an electronically controlled fan, was delivered into the burner. During the measurements, the temperature of the secondary air drawn from the atmosphere, which was used as the working gas, was 30 \({}^{\circ}\)C. As shown in Figure 1, the pipe diameter through which the secondary air enters is 150 mm. The secondary air velocities, which were gradually increased, were measured at the same port on the pipe. After repeated measurements, the secondary air flow rates obtained from eight separate measurements are shown in Table 1.  
To form the plasma flame, the secondary air was directed into the burner through a 150 mm diameter pipe using an automatically controlled fan. Then, the magnetron power was set to 18.75 kW, which is 25% of its total power of 75 kW, and maintained at this level, with the frequency set to 915 MHz. A spark is created when a metal rod with a copper tip, moved by a double-acting pneumatic valve operated with compressed air, strikes the metal \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Parameters** & **M1** & **M2** & **M3** & **M4** & **M5** & **M6** & **M7** & **M8** \\ \hline static pressure [Pstatic, Pa] & 181 & 193 & 205 & 222 & 225 & 230 & 234 & 251 \\ temperature [T, \({}^{\circ}\)C] & 30 & 30 & 30 & 30 & 30 & 30 & 30 \\ density [\(\rho\), kg/m\({}^{3}\)] & 1.08 & 1.08 & 1.08 & 1.08 & 1.08 & 1.08 & 1.08 & 1.08 \\ \({}^{*}\) differential pressure [\(\Delta\)Pdifferential, Pa] & 7.44 & 15.13 & 24.90 & 30.56 & 36.74 & 51.68 & 68.60 & 88.20 \\ velocity [V, m/s] & 3.93 & 5.52 & 7.08 & 7.84 & 8.59 & 10.20 & 11.77 & 13.32 \\ secondary air flow [Q, Nm\({}^{3}\)/h] & 250 & 352 & 451 & 500 & 548 & 650 & 751 & 848 \\ \hline \hline \end{tabular} \({}^{*}\) The differential pressure values represent the average values measured at each port.  
\end{table} 
Table 1: Secondary air measurements. rod made of tungsten in the center of the coaxial line. Microwave energy transforms the secondary air, serving as the working gas, into plasma near the open end of the coaxial line through the spark generated. After the formation of plasma, the magnetron power is increased to 37.5 kW, which is 50% of the total power. The ignition process was then repeated with a gradual increase in the airflow rate. When the secondary air flow rate reached 250 Nm\({}^{3}\)/h, the first plasma flame was obtained. About 3 min after the initial plasma formation was observed, an overcurrent fault signal appeared on the control system, causing the magnetron to deactivate. When the air flow rate was increased to 350 and 450 Nm\({}^{3}\)/h, the control system detected an overcurrent fault, similar to what occurred at an airflow rate of 250 Nm\({}^{3}\)/h, resulting in the automatic shutdown of the magnetron in both attempts. When the air flow rate is increased to 500 Nm\({}^{3}\)/h and ignition is performed, plasma forms, and no overcurrent fault signal is observed during ignitions at various air supply rates ranging from 500 Nm\({}^{3}\)/h to 1000 Nm\({}^{3}\)/h. During the ignition tests, the three-stub tuner was adjusted each time to minimize the reflected power. The evaluation of the results determined that the airflow rate of the secondary air directed to the burner as a working gas should be at least 500 Nm\({}^{3}\)/h. A high-resolution camera was temporarily mounted 1 m away from the burner outlet, as shown in Figure 5a, to observe the transformation of secondary air entering the burner through direct flow into plasma. When the air flow rate was increased to 500 Nm\({}^{3}\)/h and ignition is performed, plasma forms as shown in Figure 5b. The temperature of the plasma flame formed at different air flow rates via an electronically controlled fan was measured using a thermocouple mounted at the burner outlet. Initially, the secondary air flow rate was set to 500 Nm\({}^{3}\)/h, and plasma was generated through the ignition process.