I reviewed many literatures said that if we have plasma with multicomponent species. The plasma contacted with solid surface can satisfy Bohm criterion that
\begin{align*} \sum_{is=1}^{n} \frac{n_{is}c_{s,is}^{2}}{n_{e}v_{is}^{2}} & \leq 1, \end{align*} where $n_{is}$, $c_{s,is}$, $v_{is}$ are ion density, sound speed and ion velocity for ion s$th$ species in sheath edge, and $n_{e}$ is electron density. I would like to know that how can I derive the above equation for a simple case, such as two ion species?
