On the importance of slow ions in the kinetic Bohm criterion

Between a plasma and a solid target lies a positively charged sheath of several Debye lengths $λ _D$ width, typically much smaller than the characteristic length scale $L$ of the main plasma. This scale separation implies that the asymptotic limit $ɛ = λ _D / L \rArr 0$ is useful to solve for the plasma-sheath system. In this limit, the Bohm criterion must be satisfied at the sheath entrance. A new derivation of the kinetic criterion, admitting a general ion velocity distribution, is presented. It is proven that, for $ɛ \rArr 0$, the distribution of the velocity component normal to the target, $v_x$, and its first derivative must vanish for $|v_x| \rArr 0$ at the sheath entrance. These two conditions can be subsumed into a third integral one after it is integrated by parts twice. A subsequent interchange of the limits $ɛ \rArr 0$ and $|v_x| \rArr 0$ is invalid, leading to a divergence which underlies the misconception that the criterion gives undue importance to slow ions.