Mechanisms of the formation of an electric arc of a helical shape in an external axial magnetic field

A numerical simulation of an open direct-current arc in an external uniform axial magnetic field is performed. A circuit analog of fluctuations of the temperature of electrons is used for the numerical implementation of the open electric arc column of a helical shape. It randomly generates asymmetry in the temperature distribution of electrons and, consequently, other plasma characteristics. It is shown that the formation of the arc column of a helical shape is due to the asymmetric influence of the Ampere force, which shifts the cross sections of the arc relative to the central discharge axis. Furthermore, the cross sections of the arc rotate around the central axis at different speeds. The rotational speed at the initial part from the side of the cathode decreases downstream and increases upon approaching the anode. Such a character of the convective heat transfer causes the formation of a helical arc. If the direction of the external axial magnetic field coincides with the direction of the electric current, then the spatial orientation of the helical arc as a whole is righthanded. Otherwise, there is a left-handed structure of the arc column. The directions of rotation of the arc sections at the cathode and anode sides are opposite each other. A helical shape of the open arc is not constant in time and is periodically destroyed. Two modes of destruction were revealed: first, as a result of bridging the spiral of the helix and, second, as a result of the transformation of several spirals into one. Apparently, an open arc cannot maintain a stable and constant helical shape in an external axial magnetic field.