Reply to Comment on "Kinetic Theory of the Presheath and the Bohm Criterion"

Abstract

In his comment, Riemann defends the conventional kinetic Bohm criterion on the basis that the underlying approximations become rigorous in the limit \lambda_D/l \rightarrow 0, where \lambda_D is the Deybe length and l is a collision length scale for the dominant collision process. Here, we expand on our previous arguments showing that the basic assumptions for the justified use of the conventional kinetic Bohm criterion are typically not satisfied in laboratory plasmas. We corroborate our argument with experimental data, as well as data from numerical simulations, showing that the conventional criterion is violated in common situations. In contrast, a formulation based on positive velocity moments of the kinetic equation provides a criterion that both agrees with the experimental data and reduces to the traditional expectations from fluid theory in the appropriate limit.