The influence of ambipolar electric field on the EDF formation and the electron processes in partially ionized plasmas

Summary form only given. The local approximation for EDF is widely used for the obtaining plasma characteristics with participation of electrons. This means that the terms corresponding to spatial gradients and to the ambipolar field can be omitted in the Boltzmann equation and EDF can be factorized as the product of the electron density, which depends on the spatial coordinates and time, and the EDF depending on the local electric field strength at a given point. Accordingly, the characteristics for the different electron processes also depend on local electric field. On the other hand, the kinetic equation contains total electric field at a given point. In other words, electrons "feel" total electric field which is sum of heating, ambipolar, high-frequency electric fields, etc. This means that when ambipolar field exceeds the heating one, applicability of the local approximation for EDF seems to be ambiguously. In present work analysis of applicability criteria of the local approximation for EDF has been proceeded. It has been shown that standard criterion have to be completed by the relation between the ambipolar and heating electric fields (E
_heat
>>E
_amb
). But, it should be stressed that this condition is not fulfilled at high pressures at the discharge periphery and at practically any point of plasma volume at low gas pressures. Computational simulation for argon gas has been proceeded to prove these assumptions. The model have been included processes such as the direct ionization, the electronic excitation from ground-state atoms, the step-wise ionization from metastables, Penning ionization, and the deexcitation. The calculations have been performed in the wide range of gas pressures from 1 to 100 Torr. It was shown that the EDF becomes nonlocal on the discharge periphery at high pressures and on whole plasma volume at low gas pressures. It results in complex behavior of the differential electron fluxes on the phase plane. Furthermore, the spatial excitation rate profiles have been find to present nonmonotonic behavior in dependence of gas pressures. These effects was analyzed and explained.