## 3 Results and Discussion  
### Arc Evolution Behavior  
Figure 3 was taken with a digital camera with an exposure time of 10 milliseconds. Arc discharge images with and without an extension tube were taken from the front and side, respectively. It can be seen directly from the figure that the filamentary arc was very obvious without the extension tube, and the root of the contact between the arc and the discharge electrode was relatively fixed. The photo with the extension tube shows that the arc was more uniform and the rotation trend of the arc can be clearly seen from the photo taken from the front. At the same time, the side photographs show that the arc extended conically when the tube was lengthened, but without the extension tube, the plasma torch expanded at the exit, and the cone shape was not obvious. Figure 4 shows the evolution of the gliding arc from the side view when the No. 5 anode cone without the extension tube was used for discharge. Two tension fracture cycles of the gliding arc are shown in Figure 4. It can be seen from the figure that the gliding arc began to reorganize at 1 ms and 4.33 ms, at which the head of the arc broke and was reconnected upstream and then stretched forward. Due to the small gap between the electrodes, the outlet velocity of air flow was higher, which made the disturbance of air flow at the center of the axis larger, and the gliding arc fluctuated violently on both sides of the axis, as shown in Figure 5. At the same time, the gliding arc rotated circumferentially and stretched forward under the influence of the rotating airflow. After the arc was elongated, the gap resistance of the discharge increased due to the arc path lengthening, and the required maintenance voltage increased sharply. Before the arc is stretched to the point where the loading voltage cannot maintain the arc and breaks, there is a space position in the process of arc evolution, which minimizes the normal distance on both sides of the gliding arc. At this point, the arc will break down and conduct,  
Figure 2: Schematic diagram of discharge device with or without extension tube. and then undergo a tensile fracture process. In this paper, this phenomenon is called the regeneration process of the gliding arc short circuit. This phenomenon is very common when the gap between the electrodes is small, which was the case of the No. 5 and No. 6 electrodes in this experiment. The regeneration of the gliding arc short circuit can be effectively avoided by installing an extension tube at the grounding end. The two arc evolution situations of whether to install an extension tube or not were compared when using the No. 5 anode cone, as shown in Figure 6. In both cases, a complete gliding arc growth process was demonstrated. It can be seen that the new arcs developed from inside the extension tube to outside after the gliding arc prolongation and fracture with the addition of the extension tubes, and the probability of short-circuit regeneration was higher when there were no extension tubes. In this paper, the extension tube size was 100 mm. As the arc needs to break down and develop once again at the electrode gap after the gliding arc is extinguished, the arc undergoes a long stretch in the tube, which causes the single arc growth process to take a long time. Figure 6 shows that the arc stretching time was 2.57 ms when there was an extension tube, and 0.93 ms when there was no extension tube. The tangential velocity component will be generated by the air after the swirler. The closer the distance to the swirler, the more intense the air swirl. The discharge without the extension tube was greatly affected by the direction of the swirl air velocity; as shown in Figure 5, the arc fluctuated sharply. However, the extension tube had a rectification effect. Although the tangential velocity component still exists in the flow, the farther the flow is from the end of the electrode, the smaller the velocity component in the vertical axis direction, and the more stable the flow in the center of the extension tube.