Voltage/current characteristics at the metal shield surrounding high-current vacuum arcs

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Abstract
Electron and ion currents collected at the metal shield and floating shield potential are measured to study the behavior of high-current (up to 40 kA) vacuum arcs. An arc with a half cycle of 60 Hz is burned between 20-mm-diam electrodes spaced 4 mm apart bounded by a 200-mm-i.d. metal shield in a demountable vacuum chamber. The electron current is measured by connecting the metal shield to the anode and the ion current is collected by connecting the metal shield to the cathode. The floating shield potential is differentially measured relative to the cathode by two resistive potential dividers. It was found that the electron current increases, while the ion current peaks and the shield potential decreases with an increasing arc current. The direction of the ion-current flow is reversed during the discharge of an arc current at a level of more than 20 kA. Correspondingly, the shield potential becomes negative relative to the cathode. This newly found phenomena is attributed to the radial electric field generated from the flow of negatively charged carriers to the shield that originates from the interaction between the anodic jets of vapor and plasma and the cathodic plasma. A correlation between this finding and the anode-spot activity causing severe erosion of the anode is also shown.

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