An investigation of the collisionless discharge in the presence of an electron beam

Abstract

The properties of a plane collisionless plasma are investigated by using the Tonks-Langmuir model with the assumption of an electron beam in addition to Maxwellian bulk electrons. The case of very energetic beam electrons, as well as the case when the beam electrons acquire the greatest part of their energy in the sheath and the presheath regions, are considered under the condition that the discharge is terminated by a floating wall. In the case of a very energetic beam the plasma can be terminated by a double layer appearing close to the wall or, provided the beam density is high enough with respect to the bulk electron density, by a quasineutral double layer like structure. In the case of a beam of a small initial energy, however, the double layer and double layer like structures appear far from the wall even for very small beam densities. The last result opens a question about the relevance of the initial assumptions made on the electron velocity distribution in non-energetic and very energetic beams. The plasma flow to the boundary was calculated for the cases when the initial assumptions may be regarded as quite regular, and the results were compared with those ones obtained under the assumption of monoenergetic ions at the plasma-sheath boundary.