High speed monitoring of the discharge regimes of a non-thermal atmospheric pressure plasma jet

Abstract

Summary form only given. The observation of striated microdischarges in a miniaturized RF capillary plasma jet operating at 27.12 MHz is reported for the first time. Although the occurrence of striated structures in gas discharges has been reported quite early and effects linked to these phenomena have been reviewed in the past, these microstructures present in RF plasma jets at atmospheric pressure have not yet been monitored. This is based on the characteristic time windows of only a few ns. The high speed monitoring can inspire a better understanding of the micro-scaled spatial and temporal development of the discharge. As these plasma sources are currently gaining more attention e.g. as emerging surface modification technique, this can contribute eventually to optimized plasma tools for such technological applications. The jet consists of a quartz capillary surrounded by two ring electrodes that are capacitively coupled to an RF generator. The outer capillary is fed with pure argon. No reactive admixture of precursor to argon was added for this study. Depending on gas flow rate and applied RF power, the plasma jet can be operated in three distinctive discharge regimes (stochastic, stationary or self-organized modes). In the stochastic regime, erratic filaments develop in the region between the electrodes under most conditions. With decreasing RF power and flow rate the stationary mode is established and spatio-temporally stable filaments appear. The self-organization regime is an exclusive stable mode between stochastic and stationary regime. It is characterized by a number of equidistant filaments rotating in azimutal direction with a constant velocity along the inner wall of the jet capillary. The optical setup consists of an ICCD camera with a macro objective. The spectral characteristics of the objective lenses allow mainly photons from the visible range to expose the chip. The images were taken side-on from the active zone between the electrodes at an exposure time of 20 μs which leads to evaluable emission from the plasma. The observed periodic structure of low and intensive integral emission in all three modes indicates an appearance of striations. An estimation of the r- and z-size of these striations results in 50 to 100 μm (r) and 150 to 300 μm (z) depending on the particular mode and could be correlated to measurements of the electron concentration.