等离子体是一种被加热到至少部分电离状态的气体，因此能够传导电流。当电弧通过分隔两个电极的气柱形成时，气柱中的一些气体分子会电离。电离区或等离子体流由电弧气体中带正电的原子、数量基本相等的电子和中性原子或分子组成。等离子体流的电导率随电离度的不同而变化，但对于氩等离子体，电导率约为铜的0.05%。有关等离子体物理的更全面的讨论可以在文献中找到。 在任何电弧发生期间都存在等离子体。在自然界中，被闪电电离的空气构成等离子体。同样的电离现象也发生在焊接电弧、碳弧灯和电弧炉中。然而，近年来，“等离子弧”一词已与采用收缩电弧的工艺相关联。当电弧从电极传递到工件时，迫使电弧通过一个小喷嘴或开口，从而产生电弧收缩。

A plasma is a gas which has been heated to a condition where it is at least partially ionized and is, therefore, capable of conducting electric current. When an arc is established through the gaseous column separating two electrodes, some of the gas molecules in the column become ionized. The ionized zone, or plasma stream, consists of positively charged atoms of the arc gas, an essentially equal number of electrons, and neutral atoms or molecules. The electrical conductivity of a plasma stream varies with the degree of ionization but for argon plasma it is about 0.05% that of copper. More comprehensive discussions of plasma physics may be found in the literature. A plasma exists during any arc occurrence. In nature, the air that is ionized by a lightning bolt constitutes a plasma. The same ionization phenomenon occurs in welding arcs, carbon arc lights, and arc furnaces. In recent years, however, the expression "plasma arc" has become associated with those processes employing a constricted arc. Arc constriction is brought about by forcing the arc to pass through a small nozzle or opening as it passes from the electrode to the workpiece.