Properties of arc cathode spots

Arc cathode spots operating by formation of surface-microplasmas are reviewed for discharges in vacuum and in gases. New experimental studies using advanced optical methods (image converter cameras and laser absorption techniques) reveal that the spot is composed of fragments with a size below 10 mu m. In vacuum these are highly mobile, for copper on a time scale < 50 ns. The motion and the rapid sequences of extinctions and re-ignitions determine the behavior of the whole spot. Cu-spots in vacuum exhibit displacements in times < 100 ns. A tentative explanation of the spot movement is given on the base of the fragment dynamics. In gases the fragments are more separated than in vacuum. The ignition of fragments is discussed in the framework of recent studies on the glow-to-arc transition in high pressure discharges. Finally, new mechanisms for enhanced field electron emission are mentioned.