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

TL;DR

This paper challenges the conventional kinetic Bohm criterion's applicability in laboratory plasmas, proposing an alternative based on velocity moments that aligns better with experimental and simulation data.

Contribution

It demonstrates that the traditional kinetic Bohm criterion often fails in laboratory conditions and introduces a moment-based criterion that is more accurate and consistent with observations.

Findings

Conventional kinetic Bohm criterion is violated in typical laboratory plasmas.

A moment-based criterion aligns with experimental and simulation data.

The new criterion reduces to fluid theory expectations in the appropriate limit.

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.