Non-equilibrium atmospheric pressure radio-frequency glow-like discharges

At atmospheric pressure uniform radio-frequency (RF) glow-like discharges can be generated in helium in gap spacings down to 0.1 mm. In larger gaps an α discharge is present, which exhibits a decreasing bulk plasma region with decreasing gap spacing. At gap spacings < 0.5 mm the discharge changes to a sheath-only structure. At gap spacings > 0.4 mm two regimes can be observed, a normal regime with partial coverage and an abnormal regime with full coverage of the electrodes. In all cases impedance measurements have been performed and compared with equivalent circuit models. There is a good agreement for the α discharges in the normal and abnormal regime, but still a lack of knowledge in the case of the sheath-only structure.