Optical investigation of arc spot formation on cold cathodes in noble gas atmosphere

An arc ignited by a high-voltage breakdown in pure argon at atmospheric pressure is blown magnetically against a third so-called commutation electrode (CE) positioned perpendicularly to the arc axis. Cathodic arc spot formation is initiated on this electrode by application of a potential difference between the bulk plasma and the commutation electrode. The first stage of this current transfer is investigated electrically and optically. Comparing high-speed photographs with the starting phase of the commutation current, it turns out that an abrupt current increase is correlated to a plasma ball in front of the cathode, and a precurrent (less than 1 A) is correlated to a luminous layer on the cathode surface and the subsequent formation of a bright channel between the cathode surface and the bulk plasma