The role of the ambipolar field and the local approximation inapplicability in determination of the electron distribution function at high pressures

It is demonstrated that the condition for applicability of the local approximation in solving the kinetic equation for electrons includes not only smallness of the electron kinetic relaxation length as compared with the characteristic plasma volume, but also smallness of the ambipolar filed as compared with the current (heating) field. Therefore, at the discharge periphery, where the ambipolar field exceeds the longitudinal one, the local approximation cannot be used for calculating the electron distribution function even at high gas pressures.