Plasma Torch - Thermal Plasma DC Torches

Thermal Plasma DC Torches

Thermal plasmas are generated in plasma torches by direct current (DC), alternating current (AC), radio-frequency (RF) and other discharges. DC torches are the most commonly used and researched, because when compared to AC: “there is less flicker generation and noise, a more stable operation, better control, a minimum of two electrodes, lower electrode consumption, slightly lower refractory wear and lower power consumption”. (For the sake of brevity and a preference for the aforementioned benefits, only DC torches will be detailed hereafter.)

In a DC torch (as can be seen above), the electric arc is formed between the electrodes (which can be made of copper, tungsten, graphite, molybdenum, silver etc.), and the thermal plasma is formed from the continual input of carrier/working gas, projecting outward as a plasma jet/flame (as can be seen on the right). In DC torches, the carrier gas can be, for example, either oxygen, nitrogen, argon, helium, air, hydrogen; and although termed as such, it does not have to be a gas (thus, better termed a carrier fluid).

For example, a research plasma torch at the Institute of Plasma Physics (IPP) in Prague, Czech Republic, functions with an H2O vortex (as well as a small addition of argon to ignite the arc), and produces a high temperature/velocity plasma flame. In fact, early studies of arc stabilization employed a water-vortex. Overall, the electrode materials and carrier fluids have to be specifically matched to avoid excessive electrode corrosion or oxidation (and contamination of materials to be treated), while maintaining ample power and function.

Furthermore, the flow-rate of the carrier gas can be raised to promote a larger, more projecting plasma jet, provided that the arc current is sufficiently increased; and vice versa.