Filamentation and Formation of Imposed Patterns in Low-Pressure Magnetized Electric Discharges: A Numerical Approach

Abstract

Presence of strong magnetic fields (B ≥ 1.0 T) in magnetized dusty plasma experiments can result in a variety of phenomena in the background plasma and in the dust cloud. In the background plasma, the magnetic field induces a new type of filamentary structures that are extended in the plasma parallel to the external magnetic field. Filamentary structures are defined as regions within the plasma that have distinct properties such as optical brightness and appear in the plasma in different forms such as columns and target or spiral like structures. Some of the effects of strong magnetic field on dust particles include rotation of dust particles around the filaments and the formation of imposed, ordered structures in the dust cloud due to placing a wire mesh in the plasma bulk (gridding phenomenon). Gridding of a dust cloud suspended in a magnetized plasma is defined as the flow of the dust particles along paths which have the same shape and size of a metal mesh embedded in the bulk of the discharge. Both filamentation and gridding phenomena are primarily observed at high magnetic fields (B ≥ 1.0 T) and low pressure (P ≤ 100 m Torr)/low-temperature electric discharges (electron temperatures of few electron-volts and room temperature ions) and therefore they are thought to be originating from the same underlying physics. To investigate the origin and the characteristics of these phenomena, a 3-dimensional (3D) fluid model has been developed that can reproduce the experimental observations and enables us to investigate the physics of the filamentation and gridding phenomena. In this 3D model, plasma fluid equations are solved along with Poisson’s equation. The simulation using this model revealed that filamentation is the consequence of the difference between the reduced diffusion of the electrons and ions across the magnetic field. In the presence of strong magnetic fields, electrons mainly flow parallel to the field lines while the less magnetized ions can also have a limited cross-field diffusion which results in non-ambipolar diffusion of the plasma. This non-ambipolar diffusion is thought to be the underlying physics behind filamentation phenomenon.