Transition mode of the vacuum arc in an axial magnetic field: comparison of experimental results and theory

The transition mode for drawn vacuum arcs in an axial magnetic field (AMF) occurs over a few milliseconds during the evolution from the initial bridge column arc to the diffuse arc mode. This dynamic period of arc evolution is studied using a diffuse column arc model for the behavior of individual cathode spot jets burning in parallel with a high-current arc column. The model calculates a critical arc voltage, which is largely a function of the contact gap and only weakly dependent on the AMF strength. Experimental observations of the transition mode show that when the arc voltage exceeds the critical voltage, the arc is in one of two modes. The first mode consists of a central column with few or no cathode spots outside the column, and the second involves the appearance of some cathode spots outside the column. When the arc voltage drops below the critical voltage, the arc converts into a high-current diffuse mode for alternating currents ranging from 5-28 kA rms and axial magnetic field strengths from 2.5-7.5 mT/kA. Further modeling work shows that the fraction of current flowing through the column decreases with time during the transition mode.