Comment on ‘Information hidden in the velocity distribution of ions and the exact kinetic Bohm criterion’

Abstract

Tsankov and Czarnetzki (2017 Plasma Sources Sci. Technol. 055003) showed that measurements with a wall-mounted plasma monitor can allow noninvasive access to spatially resolved plasma parameters, including the ion velocity distribution. On the basis of experimental data and numerical modeling, they proposed a new form of collisionally modified kinetic Bohm criterion. In this work, we point out that the kinetic Bohm criterion may be written in a form that is exactly equivalent to the original criterion but does not pose the much-debated problem of divergence for slow ions. This form enables one to show that the kinetic Bohm criterion, as well as the Bohm criterion for monoenergetic ions, has a distinct mathematical meaning. Collisionally modified Bohm criteria do not possess the same, or any other, mathematical meaning. Hence, there are no mathematical grounds for speaking of a collisionally-modified Bohm criterion and no non-arbitrary way to introduce it. Therefore, the use of the term ‘exact kinetic Bohm criterion’ in the paper by Tsankov and Czarnetzki is hardly justified.