Characterisation and origin of arc fluctuations in a F4 DC plasma torch used for thermal spraying

The fluctuating behaviour of a Sulzer Metco F4 DC plasma gun has been investigated. The time dependencies of the arc voltage and current and of the optical emission of the plasma jet have been measured. An optical fibre inside the gun allows direct measurement of optical fluctuations of the arc. Spectral and statistical analysis of the recorded signals are, presented for various working gas mixtures and for two types of gas injection. There is a dominant fluctuation frequency around 4 kHz which appears to be almost insensitive to the working parameters. It is suggested that the circuit impedance might dominate the arc physics with respect to the fluctuations at this particular frequency. On the other hand, an increase in the amplitude of the fluctuations and the increase of high frequency components in the power spectra are observed as the electrodes become worn. This might be used as a process control tool. An optical technique has been set up to estimate the jet velocity from the time of flight of the emission fluctuations.