Analysis of arc spot ignition on cold lamp electrodes

The ignition of arc discharges in lamps does not proceed straightforwardly in many cases. The discharge remains in the state of a glow discharge since the formation of an arc spot at the cold cathode does not take place. To investigate the arc spot ignition on cold cathodes under defined conditions, a special experimental setup was developed. An arc ignited between horn electrodes in a clean 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. It could be shown that the arc spot ignition is mainly determined by surface structures. These structures are able to produce local field enhancement factors in the order of 100 which raise the local electron emission more than any known depression of the work function. The results can be explained by a model in which a sufficiently high density of small emission sites produces by field emission locally such a high average current density that a plasma channel and an arc spot on the cathode surface is formed.