Abstract: For perfecting plasma control technique, the mecha- nism of plasma cloud current in plasma arc welding is studied on the basis of physical theory of plasma. Analyzing the reason of producing the electric current with probe. If the probe is located in the plasma, there is a negative electromotive for plasma special property. The value has something to do with tempe-rature. Setting out from physical theory of plasma, establishing a simple model for plasma arc welding that taking plasma as a power source with resistance. The model satisfying interp-rets the regularity of the plasma cloud charge signal decreasing monotonously with the increase of the probe-center distance. At the same time , it gives the right location of probe to detect the plasma cloud .

Key words: Plasma arc welding, Plasma cloud, Simple model