A Generalized Criterion of Transition to the Diffuse Column Vacuum Arc

The high-current diffuse column arc in vacuum consists of a bright central column of plasma, with some number of individual cathode spots burning on the remainder of the contact surface. The arc voltage imposed by the main column alters the plasma expansion and current flow of single jets burning alongside the column. Stable existence of the displaced single jets drives the transition into the high-current diffuse mode. The new model assumes that the displaced single jets are confined along the magnetic field, which is a combination of a self-magnetic field and an axial magnetic field. Azimuthal self-magnetic field leads to increase in the effective length of the jet path between two contacts. The longer effective length leads to higher critical voltage below which single jets can stably exist without a source of additional plasma. This produces conditions favorable to single-jet stability. A generalized criterion for single-jet existence is presented.

I. Introduction

The diffuse column arc is a mode of a high-current vacuum arc in which a visibly brighter column is surrounded by a region of a much lower current density that extends to the periphery of the contact gap [1]–[4]. The peripheral region consists of individual cathodic jets (50–100-A current in each [5]) moving over the cathode surface. In order to understand this phenomenon, a model predicting the stability conditions for a single cathode-spot jet burning to the side of a high-current plasma column was developed [6], [7]. This model is based on the fact that the arc voltage is set by the high-current column, which has higher voltage drop than a 100-A vacuum arc [8], [9].