Plasma-arc welding and TIG short time welding have in common that the welding- or main-arc burns between the tungsten electrode and the work piece. But in the TIG-process, the arc is burning free and unchanneled, whereas in the plasma-arc system, the arc is necked by an additional water-cooled plasma-nozzle. A plasma gas – almost always 100 % argon – flows between the tungsten electrode and the plasma nozzle. The necking effect of the plasma-arc results from the cooling effect of the nozzle (thermal pinch effect), amplifying the necking effect of the electromagnetic pinch effect.

During plasma-arc welding, the additional pilot-arc burns between the tungsten electrode and the plasma nozzle. Mostly, the current intensity of the pilot-arc lies between 3 and 15 amps. The plasma gas pushes the pilot-arc slightly out of the plasma nozzle, so that the arc can be seen here as a bright spot of light. The pilot-arc ionizes the arc gap which has as a result the extremely high ignition reliability that is typical for the plasma-arc welding system.

The weld pool is protected by the shielding gas, flowing between the outer shielding gas nozzle and the plasma nozzle. As shielding gas pure argon or argon-rich gas-mixtures with hydrogen or helium are possible.

Compared to conventional arc-welding systems, the necked plasma-arc has a markedly higher energy density and lower arc scattering. While the diameter of the TIG-arc grows markedly bigger from the tip of the electrode toward the work piece, the diameter of the plasma-arc grows only slightly.