High-speed motion analysis of surface melting phenomena of high-current vacuum arcs

In order to explore the physical mechanisms limiting the breaking capability of contact materials in vacuum interrupters, experiments have been performed to investigate the plasma- electrode interaction of high-current vacuum arcs in detail, in particular the surface heating effects leading to contact melting, erosion, and droplet formation. The arcs are drawn in a synthetic test circuit between industrially designed Cu-Cr contacts having butt and AMF geometries and strokes of up to 10 mm. Visible-light high-speed video recording is used to observe the dynamic structures of the molten electrode surfaces heated by the arc at rms currents of up to 23 kA. The individual video frames show locally stable arc configurations even at the interrupting limit of the contacts, melting the electrode surface homogeneously in a centimeter-wide area. They document the presence of agitated liquid contact material with protrusions of millimeter-size growing rapidly. Spatial details of the metal fluid flowing off the cathode and anode surface regions are found contributing to the total contact erosion as was reported. The dynamics of the observed surface structures are discussed in detail. The results stimulate the development of a theoretical model describing the formation mechanism of these structures in terms of hydrodynamic instabilities.