Influence of an acoustic resonator on non-transferred plasma torch parameters

Summary form only given. For many years, strong efforts have been devoting to the understanding of arc fluctuations in non-transferred dc plasma torches. In case of one-cathode plasma torches using common plasma forming gases for spraying applications, it is well known that the amplitudes of the arc voltage is almost as high as the mean arc voltage. Consequently, the time evolution of the available specific enthalpy is strongly transient. For conventional plasma spraying, this effect can be smoothed due to response times of heat and momentum transfers to plasma treated materials. However, when spraying solutions of suspensions of submicron particles for the elaboration of finely structured coatings, the fragmentation processes of liquid phase or heat transfer processes are deeply affected by arc fluctuations.In previous papers, it was shown that the main arc voltage fluctuation is similar to a Helmholtz type oscillation described in acoustics[Coudert, J.F. and Rat, V.; 2008]. This causes the main arc motion with frequencies between 3 and 5 kHz on which are superimposed the arc restrike phenomena with characteristic times of about 10 μs. The main arc motion results in pressure variations in the cathode cavity which are suspected to modulate the gas flow rate[Rat, V. and Coudert, J.F.; 2010]. This assumption is backed by the positive effect of an acoustic resonator mounting on the plasma torch and connected with the cathode cavity. This resonator indeed causes non negligible arc voltage attenuation[Rat, V. and Coudert, J.F.; 2010]. This paper will present results concerning the influence of the acoustic resonator on the time dependence of the arc voltage and also pressure, simultaneously measured, for different plasma forming gases, cathode volumes, and also cathode position relatively to the anode wall.