Characteristics of helium DC plasma jets at atmospheric pressure with multiple cathodes

A novel DC plasma torch with multiple cathodes is developed for generating laminar, transitional and turbulent plasma jets. The jet's characteristics, including jet appearance, voltage fluctuation, thermal efficiency, specific enthalpy, and distributions of temperature, pressure, and velocity, are experimentally investigated. The results show that as the gas flow rate increases, the plasma jet transforms first from the laminar state to the transitional state and second to the turbulent state. Compared with the transitional/turbulent jet, the laminar jet possesses not only a better stability and a longer hightemperature zone but also a higher average/core temperature and a higher specific enthalpy at the nozzle's outlet. With the change of jet states from the laminar to the turbulent flow, the core pressure and velocity at the nozzle's outlet increase,while the decaying rates of temperature/pressure/velocity along the jet's axial direction increase sharply. Furthermore, applications of laminar, transitional and turbulent jets for zirconia spray coating are described. The test results indicate that the long laminar jet is favorable for the deposition of a high-quality coating because the powder particles injected into the laminar jet may have better heating and lower kinetic energy.     

Production of a large area diffuse arc plasma with multiple cathode

An arc channel at atmospheric pressure tends to shrink generally. In this paper, a non-transferred DC arc plasma device with multiple cathode is introduced to produce a large area arc plasma at atmospheric pressure. This device is comprised of a 42-mm diameter tubular chamber, multiple cathode which is radially inserted into the chamber, and a tungsten anode with a nozzle in its center. In argon/helium atmosphere, a large area and circumferential homogenous diffuse arc plasma, which fills the entire cross section surrounded by the cathode tips, is observed. Results show that the uniformity and stability of diffuse arc plasma are strongly related to the plasma forming gas. Based on these experimental results, an explanation to the arc diffusion is suggested. Moreover, the electron excitation temperature and electron density measured in diffuse helium plasma are much lower than those of constricted arc column, which indicates the diffuse helium plasma probably deviates from the local thermodynamic equilibrium state. Unlike the common non-transferred arc plasma devices, this device can provide a condition for axial-fed feedstock particles. The plasma device is attempted to spheroidize alumina powders by using the central axis to send the powder. Results show that the powder produced is usually a typical hollow sphere.