Paper PS2-WeA4
Kinetic Theory of the Sheath near Electron Emitting Surfaces

Wednesday, October 31, 2012, 3:00 pm, Room 25

It has long been known that electron emission from a surface significantly affects the sheath surrounding that surface. Typical fluid theory of a planar sheath with emitted electrons assumes that the plasma electrons follow the Boltzmann relation and the emitted electrons have zero energy at the surface and predicts a potential drop of 1.03 T_e across the sheath when the surface is allowed to float. By removing the assumption that all plasma electrons entering the sheath are reflected back into the bulk plasma (i.e. the Boltzmann relation) and considering those electrons lost to the wall, the predicted sheath potential is reduced to 0.91 T_e. Analysis of this type has been published by various authors, but our work presents a more accurate model. We performed an analytical study of sheath and presheath structure making use of a kinetic description of the emitted and plasma electron densities in the self-consistent electric field. It is shown that kinetic theory predicts that the sheath potential depends on the ratio of temperatures of plasma and emitted electrons (Θ_e). For Θ_e = 5 (for example, for 0.2 eV emitted electron temperature and 1.0 eV plasma electron temperature), theory predicts a sheath potential of half that predicted by fluid theory. The effects of a bi-Maxwellian Electron Energy Distribution Function (EEDF) were considered. These predictions were compared to measurements made in a non-magnetized plasma confined by a multidipole chamber which has a bi-Maxwellian EEDF. A barium-tungsten dispenser cathode was used as a planar emitted electron source. The inflection point in the limit of zero emission emissive probe technique was used to measure the plasma potential in the sheath and presheath near the emitting surface. These results were compared to the kinetic theory and the effects of the bi-Maxwellian EEDF were taken into account.