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J. Laimer, H. Störi, Q. ul Ain:
"Characterization of rf discharges in narrow gaps of non-thermal atmospheric pressure plasma jets";
Poster: 13th International Conference on Plasma Surface Engineering (PSE 2012), Garmisch-Partenkirchen/Germany; 11.09.2012.

One possible method for generating non-equilibrium plasmas at atmospheric pressure is the so-called atmospheric pressure plasma jet (APPJ), which features a radio-frequency (rf) discharge between bare metallic electrodes. While this source is usually operated at gap spacings in the order of 1 mm, it has been shown that uniform discharges can be sustained in helium down to gap spacings of 0,1 mm.
At higher gas flow rates the maximum rf power for uniform discharges is almost independent of gap spacing. However, in very narrow gaps (≤ 0.2 mm) the situation is different. Much more power can be dissipated in the jet when the gas flow rate is reduced. In a 0.1 mm gap uniform discharges can be even sustained to rf powers at least 5 times as high than usual, hence, no transition to a non-uniform gamma discharge was observed.
In the present study plasma impedances were derived from time-resolved measurements of discharge voltage, current and phase angle. Additionally, time-resolved optical emission spectroscopy was performed to detect excited species in respect to the driving rf voltage.