Reversing the dust charge polarity in a plasma afterglow

The charge of a dust grain is normally negative in a laboratory plasma, but we have found that when the plasma power is extinguished, the grain's charge can reverse and become positive. This charge reversal was observed in our experiments with polymer microspheres in a capacitively coupled RF plasma, and the positive charging is attributed to the dust collecting ions that stream in the presence of a DC electric field, in a cathode sheath where there is a deficit of electrons, in the afterglow. In a different afterglow experiment, we obtained the opposite outcome, with negatively charged dust. The latter result was obtained by reversing the lower electrode’s polarity during the early afterglow to make that electrode anodic, while electrons were still present.