Water-cooled Electrostatic Probe Measurements on the Temperature Distributions of Electron and Heavy Particle in DC-RF Hybrid Plasma Jets

A water cooled electrostatic probe, suggested by J. Gray as an enthalpy probe electrically insulated except the area of probe tip, is used to measure the electron and heavy particle temperature distributions in argon DC-RF hybrid plasma jets generated at atmospheric and reduced pressures. The electron temperature is firstly obtained from a current-voltage characteristic curve of the water-cooled electrostatic probe under the conditions that the probe sheath is much smaller than probe dimensions and a mean free path of ion. And an ion saturation current is approximated based on the continuum plus free fall theory considering the effect of convection on the current-voltage characteristics for finding electron density. Secondly, the heavy particle temperature is determined from the plasma enthalpy found by the enthalpy probe measurement, and it is presented compared with the electron temperature for estimating a deviation from the LTE (locally thermodynamic equilibrium) state depending on the operating pressure. Finally, the results of spectroscopic measurements are compared with those obtained from the water-cooled electrostatic probe measurements in this work to prove the validity of the probe methods for measuring the electron temperature distribution, and the degree of deviation from the LTE state in DC-RF hybrid plasma jets is discussed according to the operating pressures.