Investigations of the ignition of arc spots on cold cathodes in a noble gas atmosphere

To investigate the ignition of arc spots on cold cathodes under defined conditions, a special experimental setup was developed. An arc ignited between horn electrodes in a pure argon gas atmosphere is blown magnetically against a third so called commutation electrode, which is negatively biased against the arc plasma. The ignition of arc spots on this cathode was investigated by electrical measurements and high-speed photography. The arc traces of short current pulses were examined by in situ optical microscopy of the cathode surface. Two different modes of arc-spot ignition were observed: an initiation by a diffuse glow discharge, which may pass into a constricted arc spot, and an immediate formation of a constricted arc spot. The two modes of arc-spot ignition at atmospheric pressure were attributed to different surface structures, which are characterized by broad or narrow distributions of local ion current density enhancement factors. Ion current density enhancement may raise the field strength and temperature on the tips of microprotrusions so far that they emit electrons. A sufficiently high density of small emission sites produces locally such a high average current density that a plasma channel and an arc spot on the cathode surface arc formed. With lower pressure, the influence of the surface structure is reduced and pushed back by Townsend-γ emission