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Electrical and spectroscopic investigations of an atmospheric-pressure plasma jet ( APPJ) using variable flow rates of helium gas have shown that an alteration of the gas flow rate changes the operation region of stable uniform discharges. By decreasing the gap spacing to values below 400 μm and decreasing the gas flow rate, the uniform discharges can be operated at higher current densities and, consequently, higher radio-frequency power. Time-resolved optical emission spectroscopy has shown that, at low gas flow rates in narrow gaps (below 400 μm), back diffusion is prominent, which influences the stability of the discharge in the jet. An attempt for scaling up an APPJ by doubling its surface area has shown that, with increasing the surface area, the power dissipation cannot be increased, thus limiting the possibility to scale up such type of plasma sources.