Initiation of Arcing at a Plasma-Wall Contact

Conditions for the initiation of arcing in a plasma-wall contact were studied experimentally. Arcing could only be initiated by applying a bias voltage producing a net ion current to the wall. Results obtained show that crucial parameters for the initiation of arcing are the wall material, surface contaminations, the voltage across the plasma-wall sheath, and the total current to the wall. The current density was found to have no significant influence on arcing. For fixed surface conditions, arcing can be described by initiation characteristics. Also characteristics of the burning arc were determined 5 ¿s after initiation. The arc operates with voltages and currents lying between the initiation characteristic and the burning characteristic. This result can be interpreted in terms of a nonstationary mechanism of the cathode spot.