Driven Motion and Instability of an Atmospheric Pressure Arc
Max Karasik
Date of PPPL Report: December 1999
A dissertation presented to the Faculty of Princeton University in candidacy for the degree of Doctor of Philosophy in the Department of Astrophysical Sciences.
Atmospheric pressure arcs are used extensively in applications such as welding and metallurgy. However, comparatively little is known of the physics of such arcs in external magnetic fields and the mechanisms of the instabilities present. In order to address questions of equilibrium and stability of such arcs, an experimental arc furnace is constructed and operated in air with graphite cathode and steel anode at currents 100-250 A. The arc is diagnosed with a gated intensified camera and a collimated photodiode array, as well as fast voltage and current probes.
Experiments are carried out on the response of the arc to applied transverse DC and AC (up to approximately 1 kHz) magnetic fields. The arc is found to deflect parabolically for DC field and assumes a growing sinusoidal structure for AC field. A simple analytic two-parameter fluid model of the arc dynamics is derived, in which the inertia of the magnetically pumped cathode jet balances the applied J x B force. Time variation of the applied field allows evaluation of the parameters individually. A fit of the model to the experimental data gives a value for the average jet speed an order of magnitude below Maecker's estimate of the maximum jet speed.
A spontaneous instability of the same arc is investigated experimentally and modeled analytically. The presence of the instability is found to depend critically on cathode dimensions. For cylindrical cathodes, 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 arc shape 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 inferred from the transverse magnetic field experiments. Reasons for spot motion and for cathode geometry dependence are discussed.
An exploratory study of the instability of the arc in applied axial magnetic field is also described. Applicability of the results of the thesis to an industrial steelmaking furnace is considered.