Showing papers by "Joachim Heberlein published in 1994"

Analysis of the arc-cathode interaction of free-burning arcs

Abstract: A theoretical model has been formulated describing the influence of the arc condition and the cathode material and geometry on arc cathode erosion. To arrive at a self-consistent description for the entire arc cathode attachment region, a realistic one-dimensional sheath model has been used. This sheath model is supplemented by an integral energy balance of the ionization zone between the sheath and the arc, and by a differential energy balance of the cathode. For the case of a tungsten cathode in an argon arc, it has been shown that the ion current density is almost 50% of the total current density at low arc currents, while it decreases to about 18% of the total current density and the thermionic electron current density increases to about 82% of the total current density at high currents. It has also been found that heat conduction within the cathode and radiation from the cathode surface control energy transport from the cathode spot at low currents, and that dissipation by thermionic electron release dominates at high currents.

Theoretical study of factors influencing arc erosion of cathode

Abstract: A theoretical model has been formulated describing the influence of the arc condition and the cathode material and geometry on arc cathode erosion. To arrive at a self-consistent description for the entire arc cathode attachment region, a realistic, one-dimensional sheath model has been used. This sheath model is supplemented by an integral energy balance of the ionization zone between the sheath and the arc, and by a differential energy balance of the cathode. For the case of a tungsten cathode, it has been shown that arc constriction has a strong effect on the current density at the cathode spot but little influence on the cathode temperature. It has also been found that heat conduction within the cathode and radiation from the cathode surface control the energy transport from the cathode spot at low currents, and dissipation by thermionic electron release dominates at high currents. The conclusions are that for high-current applications erosion can be minimized by using a cathode material with a low vapor pressure, and that for these conditions the thermal design of the cathode plays a secondary role. In contrast, for low currents the erosion will be determined by the thermal characteristics of the cathode material. >

Diagnostics of a thermal plasma jet by optical emission spectroscopy and enthalpy probe measurements

Abstract: An accurate determination of electron density, temperature, and velocity distributions is of primary interest for the characterization of steady-state thermal plasma spray jets. Our diagnostic capabilities based on optical emission spectroscopy include measurements of absolute emission coefficients and Stark broadening. In addition, enthalpy probe diagnostics has also been used for temperature and velocity measurements. Observation of large discrepancies between temperatures derived from absolute emission coefficients, Stark broadening, and from enthalpy probe measurements indicate that severe deviations from LTE (local thermal equilibrium) exist in various regimes of plasma spray jets. Nonequilibrum characterization of such turbulent thermal plasma jets suggests that diffusion of high-energy electrons into the fringes of plasma jets and deviations from chemical equilibrium due to high velocities in the core of plasma jets and entrainment of cold gas, are the main reasons for these discrepancies. The establishment of a reliable data base, taking these nonequilibrium effects into account, is a prerequisite for meaningful modeling of real plasma jets.