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J. Laimer, H. Reicher, H. Störi:
"Atmospheric pressure plasma jet operated at narrow gap spacings";
Vacuum, 84 (2009), 104 - 107.

The atmospheric pressure plasma jet (APPJ) makes use of a dielectric-barrier-free radio-frequency (RF)
glow discharge for the production of a non-equilibrium plasma. Usually the APPJ is operated in the alpha
mode at gap spacings in the mm range, where the alpha sheath thickness is in the order of 200-300 mm.
Narrow gap spacings are experimentally not yet investigated, but it is expected that the bulk region of
the alpha discharge should disappear and the discharge should exhibit a sheath-only structure.
In order to provide experimental evidence for such situations, APPJs with gap spacings down to 0.1 mm
are investigated. The electric properties of the APPJ are studied by measuring the current and voltage
characteristics. Time-averaged images of the front view of the discharge are taken with a digital camera.
By using an image-intensified gateable video camera the time development of the discharge is studied
with nanosecond resolution.
It was possible to sustain alpha discharges at gap spacings down to 100 mm, whereby the voltage needed
decreases down to an rms voltage of 70 V. A weak indication for a laterally oscillating sheath in the
100 mm gap was found.