Experiments and modeling of an instability of an atmospheric pressure arc

An instability of a free-burning atmospheric pressure $≈200$ A carbon arc is investigated experimentally and modeled analytically. The presence of the instability is found to depend critically on cathode dimensions. In particular, for cylindrical cathodes, the instability occurs only for a narrow range of cathode diameters. Cathode spot motion is proposed as the mechanism of the instability. A simple fluid model combining the effect of the cathode spot motion and the inertia of the cathode jet successfully describes the shape of the arc column during low amplitude instability. The amplitude of cathode spot motion required by the model is in agreement with measurements. The average jet velocity required is approximately equal to that found from applied oscillating transverse magnetic field experiments. The primary reasons for cathode spot motion are most likely cathode vaporization and interaction of arc current with the current distribution in the cathode. Cathode surface temperature distribution is likely to be the reason for cathode geometry dependence.