are generated in plasma torches by direct current, alternating current, radio-frequency 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".
Thermal plasma DC torches, non-transferred arc, based on hot cathode
In a DC torch, the electric arc is formed between the electrodes, and the thermal plasma is formed from the continual input of carrier/working gas, projecting outward as a plasma jet/flame. In DC torches, the carrier gas can be, for example, either oxygen, nitrogen, argon, helium, air, or hydrogen; and although termed such, it does not have to be a gas. For example, a research plasma torch at the Institute of Plasma Physics in Prague, Czech Republic, functions with an H2O vortex, 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, 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. The plasma flame of a real plasma torch is a few inches long at most; it is to be distinguished from fictional long-range plasma weapons.
Transferred vs. non-transferred
There are two types of DC torches: non-transferred and transferred. In non-transferred DC torches, the electrodes are inside the body/housing of the torch itself. Whereas in a transferred torch one electrode is outside, allowing the arc to form outside of the torch over a larger distance. A benefit of transferred DC torches is that the plasma arc is formed outside the water-cooled body, preventing heat loss—as is the case with non-transferred torches, where their electrical-to-thermal efficiency can be as low as 50%, but the hot water can itself be utilized. Furthermore, transferred DC torches can be used in a twin-torch setup, where one torch is cathodic and the other anodic, which has the earlier benefit of a regular transferred single-torch system, but allows their use with non-conductive materials, as there is no need for it to form the other electrode. However, these types of setups are rare as most common non-conductive materials do not require the precise cutting ability of a plasma torch. In addition, the discharge generated by this particular plasma sourceconfiguration is characterized by a complex shape and fluid dynamics that requires a 3D description in order to be predicted, making performance unsteady. The electrodes of non-transferred torches are larger, because they suffer more wear by the plasma arc. The quality of plasma produced is a function of density, temperature and torch power. With regards to the efficiency of the torch itself—this can vary among manufacturers and torch technology; though for example, Leal-Quirós reports that for Westinghouse Plasma Corp. torches “a thermal efficiency of 90% is easily possible; the efficiency represents the percentage of arc power that exits the torch and enters the process”.
