Plasma Arc Welding:

The plasma-arc heat source might be considered as a development of the inert gas tungsten-arc. There are two kinds, the non-transferred and the transferred or constricted arc. If the inert-gas tungsten-arc torch is provided along a separately insulated water- cooled nozzle that produced a chamber round the electrode and the arc is struck from the electrode to this chamber, the arc plasma might be expelled from the nozzle in the form of a flame. It is the non-transferred arc which, with a powder feed into the plasma, is utilized for metal spraying.

While the arc is struck instead from the electrode to the workpiece the arc is contracted as it passes through the orifice in the nozzle - it is the transferred plasma arc and it is utilized for cutting because of the high energy density and velocity of the plasma. To begin an arc of this kind it is essential to have a pilot arc from the electrode to the nozzle.