The effects of ion collisionality on the plasma sheath are revealed by a two‐fluid model. In contrast to previous work, the ion–neutral collision cross section is modeled using a power law dependence on ion energy. Exact numerical solutions of the model are used to determine the collisional dependence of the sheath width and the ion impact energy at the wall. Approximate analytical solutions appropriate for the collisionless and collisionally dominated regimes are derived. These approximate solutions are used to find the amount of collisionality at the center of the transition regime separating the collisionless and collisional regimes. For the constant ion mean‐free‐path case, the center of the transition regime for the sheath width is at a sheath width of five mean‐free paths. The center of the transition regime for the ion impact energy is at a sheath width of about one‐half of a mean‐free path.

## REFERENCES

*Introduction to Plasma Physics and Controlled Fusion*(Plenum, New York, 1984), 2nd ed., Vol. 1, pp. 290–295.

*Glow Discharge Processes: Sputtering and Plasma Etching*(Wiley, New York, 1980), p. 109.

*Ordinary Differential Equations: A Computational Approach*(Prentice-Hall, Englewood Cliffs, New Jersey, 1979), pp. 111–116.

*Numerical Recipes: The Art of Scientific Computing*(Cambridge U.P., Cambridge, 1986), pp. 572–577.