Modeling the effects of an axial magnetic field on the vacuum arc

A quantitative two-dimensional model of a vacuum arc in an applied magnetic field is presented. For the particular case, a uniform axial magnetic field B/sub Z/, it provides a unified condition for the two regimes of arc voltage versus B/sub Z/, i.e. ? the steeply falling voltage branch (low B/sub Z/), and the slowly rising voltage branch (high B/sub Z/, which characterizes the diffuse arc mode). The transition to the high-B/sub Z/ regime is complete when the average mixing distance of the individual expanding cathode-spot plasma jets, measured from the cathode plane, is equal to the contact gap length. The model also predicts the observed smooth transition and minimum arc voltage between the two branches, and the independence of the are voltage on the arc current in the high-B/sub Z/ regime. Calculated results are found to be in good agreement with reported experimental data from several independent investigations over a wide range of gap, B/sub Z/ and arc current I/sub ARC/.

I. Introduction

It IS KNOWN that a vacuum arc remains in a diffuse mode up to a high current if a uniform axial magnetic field (AMF) is imposed, and that the arc current–voltage characteristics are changed by varying the AMF [1]–[4]. For above a few thousand amperes (depending on the gap and contact diameter), there is a range of in which the arc voltage drops as increases from zero. This happens because the common-channel plasma constriction due to the self-magnetic field is reduced by the increasing imposed , thus causing the energy flux concentration near the anode to decrease [5], [6]. After reaching a minimum at a particular value of , the arc voltage is observed to increase slowly with further increasing [2] [3] [4].