Nonlinear drag force in dusty plasmas

The condition for large angle scattering in ion-grain collisions, which determine the nonlinear part of the ion drag force, coincides with the condition for the grain screening to be nonlinear (eφ/Ti>1 where φ is the electrostatic grain potential and Ti is the ion mean energy), a condition met in many laboratory experiments. A self-consistent treatment of the nonlinear ion-drag force therefore requires a correct treatment of nonlinear screening. A model to account for the nonlinearity is presented and the drag force calculated, showing that it can be substantially larger than that for linear screening. An important physical feature found in the present investigation is that nonlinear screening corresponds to almost full screening inside a finite radius and that large angle scattering occurs only inside this radius. The nonlinear ion-drag force is found to have a strong dependence on the grain size, ratio of ion to electron temperatures, and ion density