Charge reversal and Coulomb expansion of a dust cloud in an afterglow plasma

The charging and Coulomb expansion of a dust cloud was studied experimentally, under afterglow conditions. A cloud of polymer microspheres was electrically levitated in a plasma under RF power, which was suddenly turned off. The dust particle charge, which was negative during steady plasma operation, reversed polarity in the afterglow due to a lack of electrons. We found that the magnitude of the positive charge was enhanced, to many thousands of elementary charges, due to ions streaming at their mobility-limited velocity, in the presence of a DC electric field. About 2 ms after switching off the RF power, the positively charged dust particles were the only charged species remaining in the chamber. They fell downward. We found that their fall could be slowed by reversing the electric field direction, at t = 2 ms, so that they could be observed for about 190 ms, before impact at the bottom of the chamber. During this extended observation time, the dust particle cloud expanded horizontally, due to the interparticle repulsion of like-charged particles. We quantified this expansion, and we found that at early times it is modeled accurately by the equation of state for a uniformly charged thin disk.