Transition to the diffuse mode for high-current drawn arcs in vacuum with an axial magnetic field

The opening of electrical contacts while passing current generates a drawn arc. In vacuum, the arc begins as a bridge of molten metal connecting the contacts, which then ruptures to form a bridge column arc. For butt contacts without an AMF at currents up to 16 kA peak, the bridge evolves into a transition mode before switching to a diffuse or diffuse column mode. Previous work observing the development of high-current drawn arcs in vacuum with an axial magnetic field (AMF) measured the time period required for the bridge column to evolve into the diffuse mode. Arc visualization experiments on Cu-Cr contacts with an AMF have now determined that the transition to the fully diffuse mode has a more complicated structure. The opening sequence begins with the rupturing of the molten metal bridge, forming the bridge column. This column evolves into the transition mode, and then into the fully diffuse mode. The transition mode consists of a region of concentrated cathode spots, similar to the transition mode for butt contacts. Over a period of a few milliseconds, individual cathode spots begin to appear outside the concentrated region until a diffuse arc forms. Increasing the axial field strength reduces the lifetime of the transition mode. This reduction correlates with changes in the arc voltage. In addition, the transition mode appearance also agrees with previous studies of contact melting in an AMF.