DynamicalN-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 ${\mathrm{\theta }}_{\mathrm{rms}}$ of the ion flux incident on the cathode is found to have a power-law dependence on a parameter Ω 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 ${\mathit{b}}_{\min }$ equal to a fraction of the mean ion-ion distance at the plasma-sheath boundary is determined, while the upper cutoff ${\mathit{b}}_{\max }$ appears to be no more than about ten times the sheath thickness d; no sustained logarithmic growth of ${\mathrm{\theta }}_{\mathrm{rms}}^2$ 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 ${\mathrm{\theta }}_{\mathrm{rms}}$≲0.1°.