Optical and Electrical Investigation of Transition From Anode Spot Type 1 to Anode Spot Type 2

This paper presents differences between anode spot type 1 and type 2 and the discharge conditions for a transition from anode spot type 1 to type 2 are investigated by means of optical and electrical measurements. The transition from the diffuse mode to the high-current anode modes (footpoint, anode spot type 1, and anode spot type 2) is studied with respect to spatial and temporal distributions of different ionization levels of Cu lines, voltage-current waveforms, transferred charge, and aspect ratio. The driving current pulse was applied by a high-current generator which corresponds to a pulsed dc of 10-ms duration. Spectral lines of Cu I, Cu II, and Cu III are analyzed for various anode modes using a combination of an imaging spectrograph and a high-speed camera. The results show that during the formation of the anode spot type 2, the intensity of all lines change noticeably. Significant differences are found, for example, in the spatial structure of Cu III lines in the anode spot modes. A jump in the arc voltage corresponding to the time of intensity changes appears at the transition from anode spot type 1 to type 2. The impact of transferred charge, instantaneous current, and aspect ratio on the formation of the anode spot type 2 is also investigated. The transition from anode spot type 1 to type 2 is dominated by the ratio of electrode diameter to gap length. The formation of an anode plume is always observed after extinction of anode spot type 2 and before current zero.