Charging of aerosol particles in the near free-molecule regime

Abstract.

The charging of small neutral and charged particles suspended in weakly ionized plasma is investigated under the assumption that the Coulomb + image forces give rise to the ion transport in the carrier plasma and define the rate of charging processes. Our approach is based on a BGK version of the kinetic equation [1,2] describing the ion transport in the presence of force fields created by the particle charge and the image force. A special type of the perturbation theory (with respect to the reciprocal Knudsen number) is used for calculating the rate of ion deposition onto neutral and charged particles. As the starting approximation, the free-molecule ion distribution with a floating ion flux is used for evaluating the collision term in the Boltzmann equation. The value of the ion flux as a function of the particle size is then fixed self-consistently from the solution of the Boltzmann equation with the approximated collision term. The expression for the ion flux J(a) to the spherical particle of radius a is derived in the form $J = \xi(a) J_{fm}$, where J _fm is the free-molecule flux (no carrier plasma) and $\xi(a)$ is a correction factor taking into account the ion-molecular collisions. The latter is shown to never exceed unity and to depend weakly on the particle-ion interaction.