Effects of a Monoenergetic Electron Beam on the Plasma Sheath in the Presence of Secondary Electron Emission

The properties of a plasma sheath are investigated numerically by using a fluid model in which a monoenergetic electron beam is taken into account. To suit the realistic environment, secondary electrons emitted from the wall surface due to collision between the electrons and the wall surface is considered. The result reveals that the effective emission coefficient depends on the emission generated by a monoenergetic electron beam when the temperature of a monoenergetic electron beam is high. The effective emission coefficient of secondary electrons changes monotonically with the increase of emission coefficient generated by beam electrons. Using the Sagdeev pseudopotential method, a modified Bohm criterion can be obtained. It is found that the ion Bohm velocity increases with increasing beam electron energy and emission coefficient generated by a monoenergetic electron beam. Moreover, when the emission coefficient generated by a monoenergetic electron beam is small, the wall potential decreases with increasing beam electron energy and concentration. When the emission coefficient generated by a monoenergetic electron beam is large, the opposite is true. It is also shown that a monoenergetic electron beam can cause an increase in the critical effective secondary electron emission coefficient, and the increase is nonmonotonic.