Analysis of the spontaneous oscillations of the voltage of an arc burning on heterogeneous electrodes by current injection.

It has been shown that large amplitude voltage oscillations may be present in free burning arcs in special conditions, These conditions concern the ca thode nature, the ambient gas, the gap length, the are current. To investig ate the origin of these oscillations, we inject in an oscillating are a ste pwise current pulse, and record the induced are voltage variations Dependin g on the sign of the injected current Delta I vs, are current I and on the relative time position of the current injection vs. voltage level, we obser ve either stable are behaviour, with a voltage variation proportional to th e injected current : Delta U = u(i). Delta I / I , where u(i) is the voltag e drop in the plasma, or sensitive unstable levels where the voltage exhibi ts anomalous large amplitude variations. In this latter case, with Delta I > 0, the are voltage shifts towards the higher level for a short period, th en rapidly falls to the lower level. For the stable are case, we compute th e minimum are voltage and the are impedance.