Simple model for fine particle (dust) clouds in plasmas

•Simple model of fine particle clouds in plasmas is proposed.•The model enables independent control of the size and the density.•The system with cylindrical or spherical symmetry under microgravity is treated.•The model is based on the enhanced charge neutrality in fine particle clouds.

In the cloud of fine particles (dusts) in plasmas, the charge neutrality can be much enhanced due to large charge numbers of fine particles. The required condition is not difficult to satisfy even when their charge density is substantially smaller than electrons or ions. Based on this fact, a simple model of fine particle clouds is proposed and the cloud radius is related to the half-width, the radius where the density of surrounding plasmas drops by half, in cylindrically and spherically symmetric cases under microgravity. When fine particles are gradually introduced with parameters of surrounding plasma especially the half-width being fixed, the size of clouds first increases and then saturates at the value determined by the plasma half-width, giving a possibility to control the size and density of clouds independently.