DYNAMIC N-BODY SIMULATIONS OF COULOMB SCATTERING IN PLASMA SHEATHS

N-body simulations are used to assess the importance of discrete ion-ion Coulomb interactions in an electron-free plasma sheath, where ordinary Debye screening is ineffective. The angular dispersion theta(rms) of the ion flux incident on the cathode is found to have a power-law dependence on a parameter OMEGA measuring the ''graininess'' of the sheath electric field, and an explanation for the observed exponent in this relation is postulated in terms of diffusion theory. In the Coulomb logarithm, a lower cutoff b(min) equal to a fraction of the mean ion-ion distance at the plasma-sheath boundary is determined, while the upper cutoff b(max) appears to be no more than about ten times the sheath thickness d; no sustained logarithmic growth of theta(rms)2 with the lateral extent a over which Coulomb interactions are taken into account was observed. For sheath parameters representative of the low-pressure weakly ionized discharges used in very-large-scale-integration manufacturing processes, the dispersion due to Coulomb scattering in the sheath is typically theta(rms) less-than-or-similar-to 0.1-degrees.