Low erosion rate of zirconium cathode in a plasma torch

Summary Form only given, as follows. Work on zirconium cathodes, which were able to operate as long as 38 h in air at very low erosion rates, is reported. To decrease the erosion rate, it is necessary to keep the temperature of the zirconium cathode surface as low as possible by optimizing the cooling. This depends critically on the method of holding the cathode insert. In the present study, copper clamps were used to secure the zirconium inserts. The depth of the erosion of the ceramic cavity formed was measured as a function of time while the arc was cycled (40 times during 38 h). The depth of erosion at 94 A in air can be described as a function of time, with low dispersion of data, by the equation delta =0.236 tau /sup 0.68/, with tau in hours and delta in millimeters. This shows that the erosion rate decreases with time. The erosion depth is 3 mm after 44 h and the calculated specific erosion is 4.4*10/sup -9/ g/ degrees C. This indicates that holding the zirconium inserts by copper clamps improves the conductivity as well as the reproducibility of the thermal contact between the lateral surfaces of the electrode and its support.<>