Void formation in the cathode sheath region of a dc dusty plasma

Dusty plasma supports the formation of various dynamical structures including linear and nonlinear waves, wakes, voids, etc. under different discharge conditions. Among these, void is a spectacular dust free empty domain in the dust cloud with a characteristic sharp boundary maintained by the balance between two counteracting force on the dust particles. In the present investigation, we report the formation of highly stable and reproducible void in the cathode sheath region of a specially designed DC dusty plasma experimental setup having circular disk shaped anode and a tray shaped cathode attached with a circular confinement ring. Initially, the glow discharge plasma using Argon gas at a pressure of 0.12 mbar has been produced between anode and grounded cathode by applying 280 V. Then mono-dispersed Melamine Formaldehyde (MF) dust particles having diameter of 10.66 µm are introduced into the plasma using a dust dispenser. While falling down, the dust particles get negatively charged and trapped inside the confinement rings subsequently forms stable strongly coupled coulomb crystal. Increasing the pressure, the homogeneous Coulomb crystal forms void at the center of the dust cloud. The void radius increases from 1.5 mm to 9.0 mm over the pressure range of 0.12 mbar to 0.14 mbar. Further, it is found that the inter-particle distance near the edge of the void decreases with the increase in pressure. The void diameter obtained experimentally over the discharge condition is then compared with the analytical model associated with force balance condition on the dust particles and found a good agreement between them. (author)