A general method is devised for calculating the image force (a second approximation) acting on electrons very close to the surface. The image force for ions is thus greater than for electrons when these particles are at a given distance from the surface. Perturbation method for the study of image forces.-Within distances of a few Angstroms from the surface, the classical image force requires modification because the "effective reflecting plane" which determines the location of the image, changes its position as any given electron or ion approaches the surface. Laue distance equal to (kT2πe 2n 1) 12, n 1 being the concentration at the boundary between the space charge sheath and the image sheath (corresponding to the saturation current). Throughout both the space charge and the image sheaths, the concentration of electrons or ions (whichever is larger) is given by n=exp(x sx)8πx s(x+x L) 2 where x s, the Schottky distance, is e 24kT and x L is the v. Electrc image forces near plane metallic surfaces.-Still closer to the surface there is an image force sheath in which the distribution of ions and electrons is governed mainly by the image force f=(e2x) 2. 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. The potential distribution of the weak fields that reach into the plasma are governed by the Debye-Hückel theory characterized by the Debye distance λ D. Near the walls are space charge sheaths in which particles of only one sign are usually present. The interior of the enclosure constitutes a typical plasma with n e=n p. 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 n en pn a=K where the equilibrium constant K can be calculated from the ionizing potential of Cs vapor.
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