Plasma constriction effect
The contraction of the arc column by intrinsic magnetic fields (Lorentz force) and external cooling gases, significantly increasing energy density in the center.
In plasma welding, the arc is constricted by a nozzle. This increases the energy density, giving you a narrower, hotter arc than in normal TIG welding. Plasma is an ionized gas that brings heat to the material in a focused way. If you don't keep the nozzle clean or the gas flow isn't right, the arc becomes unstable and the weld irregular. Then it doesn't burn in cleanly, and you risk lack of fusion or undercut. So make sure the nozzle isn't dirty and the gas flow stays constant. With a steady hand and even travel speed, you get a clean, narrow weld. This process is ideal for thin sheets because you put little heat into the part.