Driven Motion and Instability of an Atmospheric Pressure Arc Page: 3 of 177
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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 ~ 1 kHz) magnetic fields. The arc is found to deflect paraboli-
cally 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 in-
ertia 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 ap-
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Karasik, Max. Driven Motion and Instability of an Atmospheric Pressure Arc, report, December 1, 1999; Princeton, New Jersey. (digital.library.unt.edu/ark:/67531/metadc626691/m1/3/: accessed February 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.