The overall contribution of this thesis consists in the establishment of electric probes as a valuable diagnostic tool in thermal arcs. This requires the development of special experimental techniques with respect to low pressure plasma environments, but results in the ability to measure, independently of the thermodynamic state of the plasma, quantities such as electron temperatures and current densities. These are of primary importance for the description of the interface between a thermal arc and a bounding surface. Part I reports and discusses the experimental study of plasma surface interactions in two standard arc configurations. In Section I.A., the anode boundary layer of a free-burning arc is investigated. This configuration is of direct interest to TIG welding applications, but the results are of value in the broader area of electrode design for arc plasma generators. Quantitative values for the electron temperatures and peak current densities are presented. A consistent picture of the transport mechanisms dominating the anode boundary layer is proposed which affords predictive capabilities. In Section I.B., the near-wall region of a wall-stabilized arc is investigated by means of a double floating probe. A theoretical relationship is derived for the probe current-voltage characteristics under nonuniform plasma conditions. Electron temperature profiles are obtained near the wall, where strong deviations from equilibrium occur and impair spectrometric methods most seriously. In Part II, some modeling issues are tackled. Section II.A points out and corrects some inconsistencies in the formulation of the energy equation for a LTE plasma. In Section II.B, a collisional-radiative model for the excitation/ionization kinetics is coupled to the conservation equations allowing for deviations from LTE in a fully developed arc column. These sections provide an essential connection between the interpretation of the experimental results presented in Part I and the available experimental and computational information regarding thermal arcs. Finally, in Section II.C, some work regarding the heat transfer to fine particles immersed in a plasma will be presented. Probe theory and probe results are here combined to support a model which consistently accounts for deviations from equilibrium and from continuum conditions.
- Pub Date:
- January 1990