Abstrakt
We investigate plasma in the anode attachment area of DC arc plasma torch. The processes in this area influence lifetime of the anode, stability of the plasma flow in a plasma jet and their studying helps better understanding of a movement of the anode attachment in the restrike mode 1 .We used the high-speed camera (max. 1,080,000 fps and min. exposure time 0.25 μs) and synchronized cathode-anode voltage measurements (sample rate 80 MHz).
The anode used was external and therefore it was possible to directly observe the attachment motion during the restrike mode. For this mode, the anode attachment moves periodically downstream along the anode surface.
It very often comes to a stop (dwells) at anode surface inhomo geneities, where anode material vaporizes more intensively than in the immediate vicinity. The average total dwell time then describes irregular wearing of the anode surface.
We found dependencies of attachment velocities, hydrodynamic wave velocities (above the attachment), dwell times (z. 10 μs), dwell frequencies (z. 10 kHz) and electrical resistivity of the plasma, on the electric current - both for new and worn anode and for two values of argon flow rate (12.5 and 22.5 slm). The cathode-anode electric current ranged from 300 to 600 A.
We were able to stop the movement of the attachment and examined forces acting on it. From the results, it can be seen how the anode surface is worn under various plasma torch parameters.
The results also bring better understanding of the anode attachment motion. Based on our experiments we offer an explanation of the restrike anode mode and of the second curvature of the anode attachment, close to the anode surface, proposed in earlier publications