Plasma parameters deduced from cylindrical probe measurements: determination of the electron density at the ion saturation current

This work is devoted to the study of the electron density measured by means of a Langmuir probe at the ion saturation current. Investigations are performed in a microwave expanding plasma in an Ar-N(2) gas mixture. We show that in such a plasma the electron energy distribution function (EEDF) is not a Maxwell-Boltzmann distribution function and we study the ion velocity at the sheath edge and the sheath thickness around the probe in that case. On the basis of previous works given in the literature and by considering a general expression for the EEDF (no assumption concerning the distribution function), we develop a model describing the ion motion in the sheath around the probe by considering both the sheath expansion and the effect of ion - neutral collision processes. This model is tested in Ar-N(2) gas mixtures using different experimental conditions. Results show a good agreement between the electron density values measured in ion saturation current conditions using this model and values measured using other classical methods. These investigations mainly show the efficiency of the method.