Charging of dust grains in a plasma with negative ions

The effect of negative ions on the charging of dust particles in a plasma is investigated experimentally. A plasma containing a very low percentage of electrons is formed in a single-ended Q machine when SF6 is admitted into the vacuum system. The relatively cold Q machine electrons (T-e approximate to 0.2 eV) readily attach to SF6 molecules to form SF6- negative ions. Calculations of the dust charge indicate that for electrons, negative ions, and positive ions of comparable temperatures, the charge (or surface potential) of the dust can be positive if the positive ion mass is smaller than the negative ion mass and if epsilon, the ratio of the electron to positive ion density, is sufficiently small. The Q machine plasma is operated with K+ positive ions (mass 39 amu) and SF6- negative ions (mass 146 amu), and also utilizes a rotating cylinder to dispense dust into the plasma column. Analysis of the current-voltage characteristics of a Langmuir probe in the dusty plasma shows evidence for the reduction in the (magnitude) of the negative dust charge and the transition to positively charged dust as the relative concentration of the residual electrons is reduced. Some remarks are offered concerning experiments that could become possible in a dusty plasma with positive grains. (c) 2006 American Institute of Physics.