Floating potential of spherical probes and dust grains in collisional plasmas

Numerical results of the floating potential of an isolated spherical dust grain or spherical Langmuir probe immersed in an electropositive or electronegative collisional plasma are presented. The positive ions are assumed to be cold so that the ion motion is radial; the electrons and negative ions are Maxwellian. Collisions between the positive ions and neutrals are modelled by a constant mobility. Curves of floating potential (in absolute magnitude) to electron temperature against dust (Langmuir probe) radius to electron Debye length are obtained over a wide range of collisionality and electronegativity. It is found that if there are sufficient collisions in the sheath the floating potential and surface charge are increased. Also, the floating potential increases with decreasing negative ion to electron density and temperature ratios. For typical conditions used in dusty plasma research, collisions are expected to be important for micron-sized dust grains for pressures greater than about 6 Pa in argon.