It is well known, that the arc column has the shape of the truncated cone, the larger base of which is on the product regardless of polarity. However, the reason for the expansion of the arc column at the surface of the product has not yet been established. Therefore, we need to determine the processes that contribute to the expansion of the column at the surface of the product.
Let us first consider the schemes shown in Fig. 1, in case is the «product» is the non-melting plate.
For this, we consider all possible forms of the arc column. Let the diameter of the arc column near the plate surface be less than at the end of the electrode (see Fig. 1a). In this case, the highly heated gas flows reflected from the plate surface are directed approximately along the surface of the plate. These flows cover the arc column in its lower part, heat the gas surrounding the arc column, and first of all, the gas at the lower part of the arc column, which is facilitated not only by the proximity of the column and the reflected flow, but also by the fact that in this place the neutral gas is enclosed, as it were, between the post and the reflected flow, forming the acute angle between each other, which contributes to the even greater heating of the gas to a high temperature and, as a consequence, to the expansion of the column in the lower part. In Fig. 1, the a-direction of the column expansion is indicated by arrows. Consequently, when there is the smaller base of a conical arc on the non-melting plate, the column will tend to the cylindrical shape. Now, we consider whether the cylindrical form of the arc is stable in this case (see Fig. 1b).
For the scheme shown in Fig. 1b, one can draw the same arguments and come to the conclusion that the cylindrical form of the column arc is also unstable, since it tends to take the form of the cone with the large base near the surface of the plate. In Fig. 1c shows the arc column in the form of the truncated cone with the large base near the surface of the plate. In this case, the shielding gas approach to the lower cone base is freer than in the previous cases, which contributes to its more intensive cooling. In addition, the distance between the outer surface of the column in its lower part and the reflected gas flow is greater than in the previous cases, which also reduces the intensity of gas heating in this section. Consequently, as the lower part of the column arc expands (see Figs. 1a, b, c), the intensity of the gas heating near the lower part of the column arc decreases and the conditions for its cooling improve. This means that this process is damped with the certain taper of the arc and the direction of the reflected flow.