Optical investigation of the interaction of a dense plasma with cold cathodes in air

The magnetic blowing of an arc against a third so-called commutation electrode positioned perpendicularly to the discharge axis allowed a reproducible interaction of the plasma with this electrode to be achieved. Cathodic arc spot ignition was viewed on commutation electrodes made of Cu, Au, and Ni. High-speed photography revealed that arc spot ignition is initiated either by a luminous layer correlated with a precurrent which is followed by the development of a plasma channel, or by the formation of a plasma ball. The different structures are attributed to distributions of small sites on the electrode surface which differ in electron emissivity as a consequence of various ion current focusing. Scanning electron microscopy shows that the footprint of the channel or ball is marked with a burning spot with crater marks. The craters differ greatly in shape and size which depend strongly on certain surface properties.