Plasma arc constriction
The mechanical and thermal confinement of the plasma jet through a cooled nozzle orifice, resulting in an increase in current density, temperature, and plasma velocity. This maximizes cutting performance.
In plasma welding, the arc is constricted by a cooled nozzle. This makes the beam more tightly focused and much hotter than a free arc. This focusing provides higher energy density and a more stable, faster plasma jet. The trick in practice is to adjust the nozzle size and gas flow to match the material. If the constriction is too strong, the heat is too concentrated and you risk burn-through or cracks. If it's too loose, you lose the advantage of the process and the weld becomes unclean. So you need to develop a feel for how the nozzle must be matched to your workpiece and the desired weld.