The Nature of Adsorbed Films of Caesium on Tungsten. Part I. The Space Charge Sheath and the Image Force

State of thermal equilibrium in an enclosure having tungsten walls and containing caesium vapor.—In a large heated enclosure having tungsten walls and containing Cs vapor in thermal equilibrium, there are ions, electrons and atoms, with concentrations given by $\frac{n_e{n}_p}{n_a}=K$ where the equilibrium constant $K$ can be calculated from the ionizing potential of Cs vapor. The interior of the enclosure constitutes a typical plasma with $n_e=n_p$. Near the walls are space charge sheaths in which particles of only one sign are usually present. The potential distribution of the weak fields that reach into the plasma are governed by the Debye-Hückel theory characterized by the Debye distance ${\lambda }_D$. The sheaths contain an excess (or deficiency) of ions and there is a corresponding sheath adsorption which is related to a negative surface tension (spreading force $F$) by Gibbs' adsorption equation.