Metal atoms are explosively accelerated from the cathode spots into the switching gap. Near the cathode surface a positive space charge region is formed. In a distance of a few micrometres from the cathode surface, a metal
vapour plasma is formed; its density distribution is very non-homogenous and reduces strongly in the direction towards the anode. The number of emission centres is very much dependent on the current amplitude and the contact material. The average current per cathode spot is in the range of some tens to 100 A depending on the cathode material . In case of low current arcs, the anode is passive and acts only as a sink of electrons and there are many cathode spots on the cathode surface, which are moving very fast all over the contact surface. The cathode spots are the only powerful light source in the gap between the contacts. There is no confined or well-defined arc column, and there is no visible foot point of the arc on the anode side. A characteristic weak diffuse light is emitted from the arc between the electrodes, and this type of arc is thus usually called diffuse mode vacuum arc.
In this short movie we can see diffuse mode and arc rotating in vacuum interrupter
