Effect of Fluctuations of DC Current on Properties of Plasma Jet Generated in Plasma Spraying Torch with Gerdien Arc M. Hrabovsky, V. Kopecky, O. Chumak, T. Kavka, A. Maslani, V. Sember, M. Konrad and P. Ctibor Institute of Plasma Physics, Za Slovankou 3, 182 00, Praha 8, Czech Republic hrabovsky@ipp.cas.cz Abstract. Two different power supplies used for dc arc plasma spraying unit with water stabilized plasma torch were compared - classical thyristor controlled unit with the frequency of the current ripple 300 Hz and the rectifier with the high frequency converter and frequency of the current modulation 30 kHz. For the both types of power supply the arc characteristics were measured, time averaged plasma jet properties were determined from spectroscopic measurements and plasma jet fluctuations were analysed on the basis of diagnostics with an array of high frequency photodiodes. The properties of coatings sprayed with the two power supplies were compared. While time averaged profiles of plasma jet were the same for the both power supplies, the fluctuations of plasma jet differed substantially. Also the dynamic arc voltage characteristics of the arc were different and could be described by modified Cassie equation. The differences of properties of TiO 2 coatings deposited with the use of the two power supplies were not significant. Key words: Plasma torch, dc arc, plasma jet, fluctuations, plasma spraying 1. Introduction Plasma jets used for plasma spraying technology are commonly generated in DC arc torches. The arc power and thus the heat carried by the plasma jet is not stable and fluctuates as a consequence of various mechanisms. The main cause of fluctuations of arc power are fluctuations of arc voltage connected with instabilities of arc column or with electrode phenomena. Another source of fluctuations is connected with a ripple of rectified current which is affected by power supply characteristics. These fluctuations can influence the plasma jet properties as well as the spraying process. The effect of fluctuations of plasma jet properties in direct current plasma spraying on particle parameters and coating properties has been studied in [1, 2] for fluctuations caused by High Temperature Material Processes 13 (2009) 229-240 229