Arc voltage fluctuations are studied for two different plasma torches. One of them is home-made, the other is a Sultzer Metco plasma torch. Both work under similar operating conditions, but they show very different voltage waveforms and spectra, depending on their electrode configurations. The first torch shows a rather intermittent behaviour and works under the restrike mode with a rather low fluctuation amplitude and with non-reproducible spectral components. The second torch also shows characteristic features related to the restrike mode, but they are superimposed on more regular oscillations which are due to pressure variations in the cold gas chamber, located upstream the arc region. This part of the torch together with the nozzle channel appears to be a Helmholtz resonator, whose resonance frequency is theoretically evaluated as a function of the torch geometry and of the operating conditions. The theoretical predictions are in very good agreement with the measured frequency of the main peak in the voltage spectrum of the second torch. A discussion about the coupling between the pressure and the voltage is proposed to explain how the torch design could influence the Helmholtz resonance.

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