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 b0.5 mm the discharge changes to a sheath-only
structure. At gap spacings N0.4 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.
