Filament Morphology in Highly Magnetized Capacitively Coupled Low Temperature Plasmas

Abstract

Due to the small charge-to-mass ratio of dust particles, it is often necessary to use large magnetic fields of B ≥ 1 T, in order to observe the influence of magnetic forces in laboratory dusty plasmas. However, when experiments are performed at high magnetic fields in capacitively coupled, low temperature, radio frequency discharges, the background plasma is often observed to form filamentary structures between the electrodes that are aligned to the external magnetic field which disrupt the uniformity of the plasma and adversely impact our dust experiments. The filament structures are observed as bright columns distinct from the background plasma that, when viewed from above, can take several distinct morphological shape geometries from circular, to multi-armed spirals. The morphology of the structures are dependent on plasma conditions including the neutral gas pressure and the strength of the magnetic field. Filaments and their morphologies are similarly observed in plasmas of different neutral gas types and experiments performed in the Magnetized Dusty Plasma Experiment (MDPX) device have identified and characterized these filamentary structures. Moreover, there is strong evidence that each spatial mode has a threshold condition that is dependent on the ion Hall parameter – which is a function of magnetic field, neutral pressure, and ion mass. The criteria for the formation of the filaments and their shape evolution are shown to be somewhat consistent with predictions of numerical simulations.