Plasma arc is the torch makes gas to ionize to form an arc (also called compressed arc) between cathode and cold nozzle or between cathode and work piece.
When the arc passes through the hole of the water-cooled nozzle, it is cooled by the cold air flow and the hole wall of the water-cooled nozzle. The cross-sectional area of the arc column is reduced due to increase the density of charged particles and the intensity of electric field through compression effect of thermal, mechanical and self-magnetic. This compressed arc is called plasma arc which has high temperature, heat concentration, good thermal stability and controllability.