Cathode spot types and spot motion of arcs in ultra high vacuum have been investigated with large area cathodes that consisted of two adjacent pieces of Mo and Cu. Arc currents were 20-60 A dc and 8-20 kApulse (duration about 1 ms). Two spot types occured with different velocities and surface erosion: Type 1 spots are typical for surfaces covered by oxides or thick adsorption layers, whereas clean surfaces show only type 2 spots.
During arc-conditioning both types exist simultaneously inn complex mutual dependence. Type 1 spots react weakly on the cathode material, while type 2 spots burn preferentially on Cu and a t the boundary line between Mo and Cu. The motion of type 1 spots is determined by the expanding spot plasma, whereas type 2 spots show a step-by step motion. determined by explosions in the arc craters. Generally 8 spontaneous formation of type 2 spots beneath hhe arc plasma takes place only with contaminated surfaces (probably by a transition from type I to type 2 spots). Thus a breakdown between plasma and cathode surface requires the presence of contaminations. The observed effects occiir in low current dc-arcs as well as in high cnrrent pulse arcs. They are discussed for different spot models.