# A Simple and Stable Atmospheric Pressure Electrodeless Water Vapor Microwave Plasma Torch
## 3 Results and Discussion  
Images of the plasma torch are shown in Figure 3a for when the water vapor is heated and evaporated, reaches the center of the waveguide and is excited. The figure shows the torch images under 300, 500, 700, and 900 W microwave power, respectively. It can be seen that the brightness and height of the torch increase with the increase in power. Figure 3b shows in detail the variation of the height and duration of the plasma torch with power. It can be seen here that the minimum excitation power of the torch is 300 W, and its torch height is 25 mm. With the increase in power, its height can reach 90 mm at 1000 W. It can  
Figure 1: Schematic of the water vapor microwave plasma setup. Figure 2: Water vapor plasma is excited by microwave discharge; the left is the actual image of the torch flame on the ceramic tube. also be seen from the figure that the duration of the flare is different under different powers. When the power is 300 W, the duration is only 2 min. With the increase in power by 800 W, it can be continuously and stably excited and no longer extinguished. The reason is that when the microwave power is low, the evaporation of water is too small to provide enough free radicals to excite the plasma torch continuously and stably. The process of exciting water vapor plasma in the experiment is as follows: first, add a little alcohol to the wick to burn it well, use the igniter to ignite the wick, and turn on the microwave power supply. At this time, the ignited wick burns rapidly under the action of the microwave and carbonizes, as shown in Figure 4a. In the figure, 0 s indicates that the microwave has just been turned on, and 20 s indicates that the wick is carbonizing. Then the carbonized wick begins to discharge, which makes the ceramic tube rapidly heated and absorbs the water in the container. Finally, the water at the top of the ceramic tube begins to evaporate and ionize, and the discharge position begins to transfer to the ceramic tube. After the water vapor plasma torch is formed on the top of the ceramic tube, the top of the ceramic is quickly heated to a molten state. The water vapor at the lower part continuously comes to the top of the ceramic tube. It is excited by microwaves, resulting in periodic expansion and contraction of the ceramic, as shown in Figure 4b. The above description can be confirmed by the temperature measurement of the infrared camera. Figure 4c shows the infrared image of the torch and the change of its maximum temperature with time. In the absence of microwaves, the maximum temperature of the flame is concentrated in the flame center, and the maximum temperature of the flame is 360 \({}^{\circ}\)C. As the microwave begins to act on the flame, the flame is enhanced, and its high-temperature increases to 420 \({}^{\circ}\)C. Then the wick began to discharge in a small range, the core temperature of the device was concentrated at the discharge of the wick, and the temperature changed significantly. At about 60 s, the discharge position is finally transferred to the ceramic tube. At this time, the maximum temperature in the device reaches the maximum temperature of 2000 \({}^{\circ}\)C that can be tested by the infrared camera, and a continuous plasma torch begins to appear. After each hour of operation, the weight of the ceramic tube is measured after drying. It is found that the weight of the whole ceramic remains about 18.5 g, with little change, as shown in Figure 4d, which shows that the water can be ionized on the top of the ceramic to form a plasma torch without losing the quality of the ceramic. Figure 3: **(a)** Water vapor plasma torch image captured by a high-definition camera with different power (multimedia view: Video S1). **(b)** The torch height and duration of water vapor plasma vary with power. After the top of the ceramic tube is discharged, the image of the ceramic tube before and after discharge is compared and analyzed. It can be found that the molten part on the top becomes very transparent.