On the nature of “collisionless” Landau damping

The essential role of collisions in Landau damping is illustrated with heuristic but quantitatively accurate arguments. They show that above a critical (and very weak) collisionality the linearization of the Vlasov equation for the perturbation and the quasilinear description of the evolution of the background distribution function are simultaneously justified, and yield a closed and internally consistent model. We argue that phase mixing is not the same as irreversibility, but greatly enhances the efficiency of collisions in causing it.