AN EXPERIMENTAL INVESTIGATION OF FACTORS AFFECTING ARC-CATHODE EROSION

A specially designed thermal plasma reactor system for the investigati on of arc-cathode erosion has been set up. By using an OMA-spectromete r system, emission spectroscopic measurements of electron temperature and electron number density in the cathode region have been performed, together with single-colour and two-colour pyrometry of cathode tempe rature distributions. Observation of cathode spot behaviour has been c arried out simultaneously by employing a telemicroscope and a high-spe ed vision system. Cathodes have been examined by SEM and EDX after arc ing. For pure tungsten cathodes, the initial cathode geometry has almo st no effect on the cathode spot's behaviour due to the molten state o f the cathode spot. The major erosion mechanism is the ejection of liq uid droplets from the cathode spot. However, the initial cathode geome try has a certain influence on the cathode's erosion for 2% thoriated tungsten cathodes. A highly non-uniform erosion pattern will occur if the cathode is overcooled, probably due to ion bombardment in the low- temperature regions of the arc-attachment spot.