Spatially and temporally resolved measurements of argon metastable atoms in the effluent of a cold atmospheric pressure plasma jet

Although atmospheric pressure plasma jets (APPJs) have been used for several years in a wide field of applications, there is still a lack of knowledge about physical aspects, namely the phenomenon of spatio-temporally localized luminous effects in the effluent of APPJ, so-called 'plasma bullets'. This paper reports on investigations on the effluent of a low frequency (kilohertz-range) driven APPJ in argon atmosphere. To gain insight into the spatio-temporal structure of the effluent, laser absorption measurements, probing the optical depth of the 4s 3 P 2 –4p 3 D 3 transition of argon (811.531 nm), which is proportional to the column density of the metastable 4s 3 P 2 argon atoms, and investigations with an intensified video camera were performed. Simultaneous recordings of the discharge current show that plasma bullets are connected only to a certain current pulse in a series of four pulses of the discharge cycle. A good correlation between the propagation speeds of the plasma bullets (5–20 km s −1 ) and of the generation zone of metastable argon atoms in the effluent of the APPJ (8–25 km s −1 ) was found. Both the appearance of plasma bullets and spatio-temporal evolution of the generation zone of metastable atoms can be explained by the effect of a self-propagating ionization front.