Comment on ‘Kinetic theory of the presheath and the Bohm criterion’

Recently, Baalrud and Hegna published a paper (2011 Plasma Sources Sci. Technol. 025013) claiming that the well-known kinetic Bohm criterion (BC) is based on two errors and suggested a new BC using exclusively positive-exponent velocity moments. It is shown that the paper conflicts with accepted sheath theory and that the v−2 velocity moment cannot be avoided in a rigorous asymptotic theory.