On the Electrode Damage and Current Densities of Carbon Arcs

Arc damage on carbon electrodes has been examined as a function of the charge passing through each arc in the range from 10-6 to 10-1 coloumb. Two forms of arcing damage were observed: that due to the so-called vapor arc, which has a very high current density (~6 × 107 A/cm2) and low erosion rate, and that due to the so-called thermionic arc with a lower current density (~2 × 103 A/cm2) and higher erosion rate. Unlike damage observed on metallic electrodes raised rims were not formed around the carbon craters. The vapor mode damage is compared with two known models of cathode erosion. Differences between the vapor and thermionic modes and the transition between them are discussed. A whitish-gray deposit observed on the arced electrodes appears to be a form of graphite.