Transitions between thermionic and vapour arc modes

A theoretical comparison between the thermionic and vapour arcs is presented which illustrates the essential similarity of the two arcs modes. Using the carbon arc in nitrogen as an example, the arc spot parameters are derived for both arc modes and are in good agreement with experiment. An extension of these models provides a quantitative explanation for the observed transitions from one arc mode to the other. The transition from the thermionic arc to the vapour arc is initiated by excessive excited particle loss from the arc spot, whereas the reverse transition to the thermionic arc is caused by nitrogen gas quenching of excited carbon atoms. A value for the cross section of this quenching process is derived.