Double and triple layers in dusty plasmas

Summary form only given, as follows. The results of experiments on dusty plasmas in RF excited discharges are reported. These discharges use graphite electrodes and helium gas fills around 1 Torr. Laser scattering data was recorded on a system with 4:1 area ratio graphite electrodes, mounted vertically. This configuration eventually produces a thin dust layer at the edge of the larger electrode sheath. As the dust density of this disk increased, the electrode sheath expanded (indicating a higher sheath potential drop) and a similar thick sheath appeared on the side of the dusty layer facing the large electrode. The thickness of the dust sheath on the plasma side was much smaller. The results indicate that the dust increases the impedance of the plasma locally. This causes an increase in the electrode sheath voltage and a corresponding increase in the dust sheath voltage. The disk of dust acts like a suspended insulator that is charged to different potentials on its two sides, thus creating two double layers. In the early stages, the disk sides can be at equal potential, resulting in a triple space charge layer.