Ion drag force in collisional plasmas

A simple hydrodynamic model is used to calculate the ion drag force acting on a small grain in highly collisional weakly anisotropic plasma. First, it is shown that as long as the grain does not absorb ions on its surface, the result from this model coincides with the highly collisional limit of the more general kinetic approach proposed by Ivlev et al. and predicts an increase of the ion drag force with ion collisionality. Next, the effect of ion absorption on the grain is investigated. It turns out that ion absorption reduces the amplitude of the ion drag force and under certain conditions the force can even reverse direction (i.e., it acts in the direction opposite to the ion drift). The physics behind this interesting effect and its possible consequences are briefly discussed. Finally, the effect of plasma production and loss in the vicinity of the grain is investigated. The most important result here is that with increasing relative ionization rate a transition from "negative" to "positive" ion drag force can occur. The transitional ionization rates have been estimated for plasma production due to electron impact ionization and two different plasma loss mechanisms (electron-ion volume recombination and ambipolar diffusion loss).