Ion drag on an isolated particulate in a low-pressure discharge

Classical scattering theory is used to calculate momentum-transfer cross sections between positive ions and isolated particulates in glow discharges for several model particle sheath potentials. Cross sections calculated from a potential profile obtained using self-consistent orbit (Poisson-Vlasov) theory, an attractive screened Coulomb potential, and an attractive cutoff Coulomb potential are compared. At high ion energies, all model potentials result in the same cross-section dependence on ion energy. At low ion energies, however, the cutoff Coulomb potential gives a cross section too low by about a factor of 10. The screened Coulomb potential and Poisson-Vlasov theory potential result in momentum-transfer cross sections equal to within 30% over the entire range of ion energies. An approximate analytic expression for the momentum transfer rate coefficient is given for order of magnitude estimation of the ion drag force.