Vacuum interrupter arcs show two different modes of either diffuse or constricted
arcs. The arc voltage of the diffuse arc is relatively low. In contrary, the constricted
arc is characterized by a single bulk plasma column similar to an arc observed in
gases carrying a large current.
The boundary of the two arc modes depends on the current level, the contact gap,
the shape, and material of the contact.
Figure shows a schematic of the arc behavior during current interruption in a
vacuum for a half cycle. The arc is initiated in the diffuse mode with a small gap
after contact separation. With an increase of the current, the vacuum arc may be
shifted to the constricted.The constricted arc is a single thick arc carrying a large
current mode with a high plasma density . It has high atmospheric pressure due to
excessive metal vapor with an arc voltage higher than that of the diffuse arc.
When the current decreases, the constricted arc transforms to a diffuse arc
composed of many arc columns with cone-shaped plasma directed from the small
cathode spot to the anode
