Experimental observation and numerical investigation of filamentary structures in magnetized plasmas

Abstract

Filamentary structures in low-pressure, low-temperature plasmas are produced when strong magnetic fields are applied parallel to the electric field defined by parallel electrodes. Filamentary structures are regions within the plasma that have distinct properties such as optical brightness and extend along the magnetic field lines. In our experiments, an argon, radio frequency discharge is exposed to a strong background magnetic field in the magnetized dusty plasma experiment at Auburn University. Different forms of filamentary structures emerge in the plasma such as columns, target, or spiral-like structures. To investigate the origin and the characteristics of these patterns, we have developed a three-dimensional fluid model that can reproduce the experimental observations which enables us to investigate the underlying mechanisms of the filamentation process.

Variation of filamentation phenomenon in strongly magnetized plasma with various discharge parameters

“…[ 24–29 ] Formation of these self‐organized patterns in low‐pressure magnetized plasmas (filamentation phenomenon) has been observed for over two decades. [ 24–29 ] These patterns are extended in the discharge along the magnetic axis, channelling from one sheath region to another. So far, filamentation phenomenon has been reported to appear at very low pressure () plasmas that are exposed to strong magnetic fields ().…”

“…The image sums up 500 frames of 8 μm diameter dust particles suspended in an argon plasma at a pressure, p = 16.6 Pa (125 mTorr), a radio frequency (rf) input power, P RF = 1.6 W, at 13.56 MHz, and a magnetic field of B = 2.5 T. The image reveals the competition between the previously reported dust gridding phenomenon (i.e., the rectangular grid pattern formed by the dust particle motion, shown as white lines in the image) [21][22][23] and the light grey, spiral structures observed in the visible light glow of the plasma (i.e., filamentary structures). [24][25][26][27][28][29] Therefore, in order to understand the dynamics of the dust particles in strongly magnetized plasmas, it is essential that we understand the underlying plasma dynamics. This paper is motivated by these observations and focuses on furthering the understanding of this pattern formation in the background plasma at high magnetic fields.…”

“…Filamentary patterns are self-organized structures within a strongly magnetized plasma that differ from the rest of plasma in terms of parameters such as density, optical brightness, and electron temperature. [24][25][26][27][28][29] Formation of these self-organized patterns in low-pressure magnetized plasmas (filamentation phenomenon) has been observed for over two decades. [24][25][26][27][28][29] These patterns are extended in the discharge along the magnetic axis, channelling from one sheath region to another.…”

“…The formation and properties of the filamentary patterns in magnetized plasmas can be affected by various parameters such as neutral gas pressure, magnetic field, type of the discharge gas, electrodes conductivity, and the geometry of the plasma chamber. [25,27,29] Schwabe et al [25] experimentally investigated the variation of filamentation phenomenon with gas pressure and magnetic field strength and showed the strong dependence of the phenomenon on the ion Hall parameter (the ratio of the ions mean-free-path to their gyro-radius). The effect of the ion Hall parameter along with the impact of electrode conductivity and gas type on filamentation phenomenon was also identified in numerical simulations of the phenomenon by Menati et al [27,29] Despite all the efforts, introducing a comprehensive theoretical framework that successfully describes the formation mechanism of filamentary structures in strongly magnetized plasmas and their dependency on the aforementioned parameters is still an ongoing research study.…”

“…[30,31] In addition to that, due to the complicated nature of the filamentary structures, application of optical measurement methods such as laser induced fluorescence (LIF) is limited. [25][26][27][28][29] Therefore, numerical and theoretical investigation of the magnetized plasma remains one of the most effective ways of studying this phenomenon.…”