Spectroscopic investigation of arc spot ignition on cold cathodes

Summary form only given. To investigate the ignition of arc spots on cold cathodes under defined conditions a special experimental set-up was developed. An arc ignited between two 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 as well by emission spectroscopy and high speed photography as by electrical measurements. 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 diffuse glow discharge which may pass into a constricted arc spot and an immediate formation of a constricted arc spot. Characteristic spectra of the filling gas and the commutation electrode could be related to the different modes.