Distributed Arc sources

Abstract

This chapter discusses classification of vacuum arcs by the mean temperature of the source electrode. The electrode heating, necessary to generate distributed arcs can be achieved by the discharge itself in the case of a distributed cathodic arc or by means of an auxiliary power source. Generally, obtaining a vacuum arc on a hot evaporated anode is more straightforward than on a hot evaporated cathode; however, the degree of ionization for cathodic arcs, whether discrete or distributed, is mentioned to be typically higher. A distributed cathodic arc occurs when the average cathode temperature is larger than some critical value. The arc vapor is composed of cathode material since anode erosion is negligible. The basic difference between this vacuum arc and cold cathode arcs is the lower average cathode current density. Because of this lower current density, distributed arcs do not exhibit the high frequency oscillations in discharge voltage and chaotic cathode spot motion characteristic of cold cathode discrete arcs. Among these phenomena, evaporation and electron emission from the cathode surface are important for the existence of the arc. The working medium is produced as a result of evaporation. The current continuity at the cathode surface is maintained by the emission. The temperature necessary for the formation of a distributed cathodic arc can be achieved by preheating the cathode to the required temperature by means of an external source or by configuring the source so that the arc itself heats the electrode.