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Electric arc in a plasma spray torch under modulated current

Plasma spraying of liquid feedstock is a relevant process for the deposition of finely structured ceramic coatings. The control of arc instabilities makes obtaining the desired material properties challenging; hence efforts are devoted to developing segmented direct current torches that intend to fix the length of the arc. An alternative method involves promoting the arc oscillations and associating a pulsed liquid injection with the torch. This paper presents a study of the influence of the amplitude modulation of a direct current on the arc dynamics confined in a plasma spray torch. The dependence of the electrical features of the torch on the current amplitude modulation is presented. Plasma speed and temperature are also measured. Time-resolved imaging of the arc attachment permits us to correlate its motion, particularly the arc reattachment on the anode surface, with arc current and voltage. Measurements are compared with a simplified model providing the time-dependence of the plasma properties. It is shown that the current modulation favors the arc restrike by modulating the arc radius, but also that the electric field strength of the arc column should be modified during current variations.