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) occurred over a few milliseconds during the evolution from the initial bridge column arc to the diffuse arc mode. This dynamic period of arc evolution could be 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 calculated a critical arc voltage, above which the sign of the anode sheath for the individual spots changed from negative to positive with respect to the inter-contact plasma. In this case, the individual jet could continue burning only if the anode supplied B significant fraction of its near-anode plasma. This change could hinder the existence of individual cathode spot jets outside the column. Experimental observations of the transition mode showed that when the arc voltage exceeded the critical voltage, the arc was in one of two modes. The lint mode consisted of a central column with few or no cathode spots outside the column, and the second involved the appearance of some cathode spots outside the column. When the arc voltage dropped below the critical voltage, the arc converted into a high-current diffuse mode. The diffuse column model can be used to explain the observed transition mode behavior. Cathode spots could freely burn over the cathode surface when the arc voltage was less than the critical voltage because of the change in sign of the anode sheath. This could divert current out of the arc column, thereby lowering the magnetic pinch pressure on the column and causing it to collapse. The theoretical predictions of the critical voltage agreed well with the observed transition to a high-current diffuse mode for AC currents ranging from 5-28 kA rms and axial magnetic field strengths from 2.5-7.5 mT/kA.