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Abstract-The geometrical effects of an anode nozzle in a nontransferred plasma torch on air entrainment are examined by measurements of plasma composition using a quadruple mass spectrometry. In addition, the radial and axial distributions of plasma enthalpy, temperature, and velocity are measured by using an enthalpy probe method. Two types of anode nozzle geometry, i.e., cylindrical and stepped nozzles, are employed for the torch in this experiment. As a result of gas composition measurements, the new stepped nozzle turns out to produce a thermal plasma jet having lower air content in it compared with the conventional cylindrical nozzle. The plasma jet produced by the stepped nozzle exhibits higher enthalpy and temperature, especially around the core of the plasma flame, due to less intrusion of ambient air. Furthermore, the axial velocity distribution with a slowly changing variation is observed in the stepped nozzle case because of the plasma flow less disturbed by air entrainment. From these experimental results of thermal plasma characteristics and nozzle geometry effects on air entrainment, a high quality of coating products are expected in plasma spraying by using the stepped nozzle due to higher plasma enthalpy and temperature and lower velocity drop along the plasma jet.Index Terms-Air entrainment, enthalpy probe, nozzle geometry effects, plasma spraying, plasma torch, quadruple mass spectrometry.