Abstrakt
Direct current arc plasma torches with an external anode are widely used in various industrial applications, like plasma spraying or bio-waste gasification. The processes in their anode area influence a lifespan of the whole plasma torch, and the characteristics of their plasma jet.
These processes are still not fully understood. We focused on the measurement of a plasma electrical potential in the anode area of a hybrid water-argon DC arc plasma torch with an external anode.
For our measurements, we used electric Langmuir and emissive probes (1 mm spatial and 50 ns temporal resolution), a high-speed camera (300,000 fps, 0.29 µs exposure time) and a system for Schlieren imaging (with a knife-edge). The electric measurements and camera videos were synchronized.
The measurements were made under different values of the arc electric current or ambient pressure, during the restrike mode with a constricted anode arc attachment or during a supersonic mode with a diffuse anode arc attachment. Floating potential of the used probes, the electric field in the plasma jet, V-A characteristics of the Langmuir probes, electron and ion saturation currents in the probes, and plasma electron temperature were measured in the anode area.
Three methods were compared to obtain accurate values of the plasma electrical potential. Two methods were compared to obtain values of electric field.
In addition, we tested the electrical conductivity of turbulent vortices around the plasma jet in the anode area and measured potential drops across the anode arc attachment. The results provide first experimental data on plasma electrical potential inside the plasma jet in hot anode area of plasma torches with an external anode.
They help to better understand the corresponding anode processes and confirm an accuracy of a non-intrusive approach to the measurement of the plasma electric field in the anode area during the restrike mode.