Showing papers by "Joachim Heberlein published in 1996"

Line-by-line method of calculating emission coefficients for thermal plasmas consisting of monatomic species

Abstract: A theoretically rigorous method for obtaining radiation property data of thermal plasmas composed of monatomic, nonhydrogenic species is presented. An exact line-by-line technique is utilized in order to avoid assumptions that are commonly made in handling line radiation. Doppler, natural, Van der Waals, resonance, and quadratic Stark broadening mechanisms are included in determining the line half-widths. In addition to line radiation, free-bound continuum and free-free continuum for both ions and neutral particles are considered. The fundamental radiation property determined by the program is the spectral emission coefficient. From this property the spectral absorption coefficient can easily be determined. Properties that are more suitable for engineering analysis, such as the wavelength integrated emission coefficient and the net emission coefficient, are also calculated. Comparisons to experimentally determined wavelength integrated emission coefficients for argon plasmas and analytically determined net emission coefficients for argon/copper plasmas are presented.

Temperature measurement and metallurgical study of cathodes in DC arcs

Abstract: Arc-cathode erosion has been studied fur pure tungsten and 2% thoriated tungsten cathodes. Measurements of cathode temperature distributions with high spatial resolution have been carried out using an optical multichannel analyzer (OMA)/spectrometer system. The cathode spot behavior has been observed with a tele-microscope and a high speed vision system simultaneously with the temperature measurement. Cathodes have been examined by scanning electron microscope (SEM) and energy dispersive x-ray spectrometer (EDX) after arcing. The results show that the initial cathode shape has no effect on cathode erosion for tungsten cathodes because the major erosion mechanism of tungsten cathodes is the ejection of molten cathode material. For 2% thoriated tungsten cathodes, the cathode geometry has little effect on the cathode tip temperature but plays an important role in the erosion. The thorium is depleted from the cathode spot due to fast evaporation, which increases the work function of the cathode. Deposition of thorium and tungsten crystals at the cathode surface outside of the cathode spot occurs. The conclusions with respect to cathode design are that steep temperature gradients resulting from overcooling should be avoided, and that an optimization of the thermionic emitter needs to take into account both a low work function and an appropriate diffusion rate in the metal matrix.