Dynamic Behavior of Plasma in a Cascade Plasma Torch with External Magnetic Field

The effect of operating conditions on the arc voltage fluctuations and dynamic behavior of plasma were investigated for a cascade plasma torch with an external magnetic field. The key factors that affect the fluctuations of the arc voltage were studied, which is useful for generating stable plasma jets. A ring-shaped permanent magnet was used to produce an external magnetic field having a flux density of approximately 0.27 T at the center of the magnet. The operating current, gas flow rate, and anode-cathode distance were varied in the range of 40-160 A, 10-40 L/min, and 0-10 mm, respectively. The width of the ring-shaped anode was set to 1 mm or 3 mm. The end-on images of the plasma were captured by high-speed photography, and the associated arc voltage was measured simultaneously. When an anode of 1 mm width was used, a periodic sine-like voltage waveform with low amplitude was observed at a low gas flow rate. In that condition, an arc root rotated along the anode wall continuously with no observable breakdown process. The rotational frequencies of the arc exceeded 1 kHz and were strongly dependent on the square root of the operating current.