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Miniaturized non-thermal atmospheric pressure plasma jet—characterization of self-organized regimes

The study reports for the first time on self–organization effects in a radio frequency (RF) plasma generated with a miniaturized non-thermal atmospheric pressure plasma jet. The source is configured as a capacitively coupled RF jet (27.2 MHz) with two outer ring electrodes around a quartz capillary (d = 4.0 mm) between which a gas mixture flows at typical rates of 0.05—5 slm. The application background of this source is the deposition of thin films with a PECVD process. Therefore, thin film producing agents can be added in small quantities downstream the active discharge region. Commonly, the time-resolved observation of the discharge development reveals that the discharge consists of distinct discharge filaments that appear stochastically and evolve alongside the wall of the capillary. This stochastic mode can be easily found under most situations. However, under special conditions, a quasi-laminar flow is established and a controlled number of equidistant filaments develop which form fixed discrete rotating patterns (locked mode). In this paper, a systematic study is performed using Ar as process gas to define the range of existence of the locked mode. The temporal discharge behaviour is studied by performing a low frequency analysis on the optical emission of the plasma. RF power, gas flow rate and electrode distance are interpreted as scaling parameters that are responsible for the self-organization in the non-thermal atmospheric pressure plasma jet. The appearance of the different discharge regimes is described on a phenomenological basis and the collective behavior of the discharge filaments is explained based on the thermal interference of the discharge channels with the gas flow inside the capillary.