Anode Voltage Drop and Anode Spot Formation in dc Vacuum Arcs

Abstract

Factors influencing the formation of anode spots in vacuum arcs have been investigated. For anode diameters of 1.25 and 5 cm, copper vapor arcs have been drawn to electrode spacings of 2.5 cm, and spot formation threshold currents of 400 and 2100 A have been determined for the small‐ and large‐diameter anodes, respectively. Comparison of the arc voltage vs current characteristics, together with probe observations of potential distribution, reveals that an anode voltage drop is established prior to spot formation. The magnitude of this drop increases with both electrode spacing and arc current, and decreases with increase of anode area. By analogy with phenomena in mercury arcs, anode voltage drop formation is associated with vapor starvation. Photographs reveal that, even in the presence of an anode spot, high‐voltage arcs can exist with a multiplicity of cathode spots and a diffuse arc column. Delays in the onset of the spot phenomena indicate that a temperature rise of the anode surface to the melting temperature precipitates the vapor release necessary for spot formation.