The Bohm criterion of plasma sheath with different species

A fluid model has been used to study the Bohm criterion of the plasma sheath with different species. The charge particle includes electrons, ions, negative ions and secondary electrons from the wall striked by the electrons. Numerical calculation results are obtained through quasi-Newton method. It is found that secondary electron emission(SEE) can increase the critical ion Mach number of the plasma sheath. The critical ion Mach number decreases with the increase of the temperature of the electrons emitted. Negative ions reduce this critical number. In addition, it is obvious that the SEE affects the ion Mach number when the density of negative ions is small, but the ion Mach number is mainly affected by negative ions when the density of negative ions is high in the presence of secondary electron emission and negative ions.