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.