Abstract
Introduction The need to improve plasma spraying processes, waste treatment and plasma synthesis has motivated us to investigate plasma in the anode attachment area of DC arc plasma torch. Studying the processes in this area helps to extend the lifetime of the anode, stabilize the plasma flow and better understand a movement of the anode attachment in the restrike mode.
For this mode, the anode attachment moves periodically downstream along the anode surface. The movement is the result of the imbalance between the drag force caused by the interaction of the incoming plasma flow over the arc attachment and the electromagnetic force caused by the double curvature of the arc [1].
However, the cause of the second curvature of the arc close to the anode surface and the arc reattachment process is still not well explained. In the majority of publications, the anode processes were observed only indirectly.
This study follows publication [2]. Methods For our experimental investigation of the plasma in the anode attachment area, we used the hybrid water-gas DC arc plasma torch with the external anode (Figure 1), the high- Figure 1: Sketch of the hybrid water-gas DC arc plasma torch speed monochromatic camera and synchronized cathode- anode voltage measurements (sample rate 80 MHz).
We directly observed and analysed the movement of the anode attachments and the plasma flow above them. Observations and Results The reattachment process is visible in two camera images in Figure 2.
The attachment inclines downstream because of the drag force. The electric current flows mainly perpendicular to the anode surface (through the shortest path); therefore, there is the second lower curvature of the arc.
As the attachment moves downstream, the electrical resistance between the positions X1 and X3 increases. The new current path and consequently a new attachment arises between X1 and X2 because this new path starts to have a smaller resistance than path X1-X4.
In time 10 µs, only the new attachment remained. Figure 2: Mechanism of arc reattachment process in restrike mode.
We calculated the averaged electrical conductivity σ of the arc plasma above the anode from the voltage between new and former attachment, their distance and the constant electric current flowing through the attachments to the anode. We also extended and refined our calculations of dwell frequencies, dwell times and attachment velocities in publication [2] and compared the results for new and worn anode.
Conclusion We present a new view of reattachment process, explanation of the second curvature of the arc (both consistent with our experiments) and a new way for calculation of the electrical conductivity of the plasma above the anode, during the restrike mode. For the first time, the process of punching small craters (during the dwelling) into the anode surface by the attachments was studied in such detail.
Acknowledgements The work was supported by the Grant Agency of the Czech Republic under the project GA15-19444S. References [1] Wutzke SA, 1967.
Conditions governing the symptom- matic behavior of an electric arc in a superimposed flow field. Ph.D. thesis, University of Minnesota [2] Ondac P, et al. 2016.
Investigation of the arc-anode at- tachment area by utilizing a high-speed camera. Plasma Physic and Technology J.: 3(1): 1-5