The role of collisional absorption is supposed to be important in the beginning of the interaction between a laser pulse and matter. Later on, collective absorption phenomenons take place. The scope of this paper is to present first results about the so-called AC-run-away effect, defined as a “sudden transition from collisional absorption regime to a collective one.” For this purpose, we use a simple model based on ballistic theory. The delicate point concerning the Coulomb logarithm is also studied: we propose for it a convenient cutoff and an ensemble averaged Coulomb logarithm fit formula. Another application of the model presented here is the normal skin effect. We show some sensible differences between the present approach and those based on more common and rough collision frequency formulas.