Dynamic behavior of anode arc jets in a cascade plasma torch with an external magnetic field

The effect of using a cascade electrode and an external magnetic field on the current-voltage characteristics and the dynamic behavior of anode arc jets was investigated for a non-transferred DC plasma torch in argon. A ring-shaped permanent magnet was used to generate an external magnetic field of 0.27 T on the center axis of the plasma torch. The gas flow rate, the operating current, and the anode-cathode distance were varied in the range 20-60 lpm, 40-100 A and 0-10 mm, respectively. The end-on images of the anode arc jets were captured by high-speed photography and the associated arc voltage waveforms were discussed. The time-averaged arc voltage showed a positive trend with the increase of the anode-cathode distance. Higher arc voltage tends to be obtained by applying the external magnetic field for each operating condition. High-speed photography showed that voltage steep drops were caused by successive arc root jumps on the anode. The voltage fluctuations were significantly reduced under a certain condition that the attachment of the anode arc root keeps to rotate in circumferential direction.