Analysis of arc-cathode system from in-situ diagnostics of tungsten electrode during operation

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An in-situ measurement of electrode work functions in gas tungsten arcs during operation has been carried out in order to analyze complex phenomena in arc-cathode system. This technique is based on the photoelectric effect occurring at the surface of a tungsten cathode with the use of a pulse laser system consisting of a Nd-YAG laser and a dye laser. Three types of tungsten electrode, namely, pure W, W-2% ThO_2 and W-2% La2O_3 were used in this project. The gas tungsten arcs were operated in argon at atmospheric pressure at a current of 200 A. The effective work functions of pure W, W-2% ThO_2 and W-2% La_2O_3 electrodes during operation were found to be, respectively, 4.6, 2.8 and 3.0 eV from in-situ measurements. These results were very close to the work functions of W, ThO_2 and La_2O_3 obtained from the literature.Furthermore, four types of tungsten electrode, namely, pure W, W-0.5% La2O_3, W-1% La2O_3 and W-2% La2O_3 were additionally used. The effective work function clearly decreased with increase of La2O_3 content of tungsten electrode, which showed the same tendency with the tip temperature.These experimental results of effective work function of cathode were given to a unified arc-electrodes model to take into account the close interaction between arc plasma and thermionic cathode. Calculations were made for predictions of current attachment at the thermionic cathode with different electron emitter materials at a current of 200 A in argon gas tungsten arc with a water-cooled copper anode. Theoretical results were compared with experimental results in current attachment area, shape of self-stabilized arc and surface temperature of cathode. It was concluded that the current attachment at thermionic cathode was dependent on work function, melting point and Richardson constant of emitter materials.