The behavior of vacuum arcs between spiral contacts with a fixed-polarity arc shield

A framing camera was used to study drawn vacuum arcs between spiral-type contacts. This contact design imposes a transverse (radially directed) magnetic field to drive the arc column along the edge of the gap. The fixed electrode was electrically connected to a cylindrical arc shield. With the fixed electrode and shield as cathode, the manner of ignition of cathode spots on the shield depended on the peak arc current I_p. For I_p≳25 kA, the arc entered an intense columnar mode, and cathode spots were produced on the shield when the arc column expanded radially outward from the gap for brief intervals during its running motion. For I_p=12.6 kA to ~25 kA, cathode spots ignited on the shield when cathode spots moved over the adjacent edges of the fixed contact and large spikes occurred on the arc voltage. With the fixed electrode and shield as anode, there was less interaction of the arc column with the shield. In this polarity, there was no formation of an anodic arc root on the shield at least up to I_p ≈38 kA, and a different mode of high-current arcing was observed