Temperature measurement and metallurgical study of cathodes in DC arcs

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.