Generation scenarios of anodic structures and experimental realization of turbulence in unmagnetized plasma

Abstract

In the present work, we report development of a DC glow discharge plasma (GDP) set-up to study controlled evolution of anodic structures having distinctive geometry, size and layers, generated in front of a positively biased electrode, submerged in unmagnetized plasma. For such an anodic structure, we have also investigated the condition under which the turbulence is triggered. Characteristic of these structures, generated in front of a positively biased electrode, depends on multiple parameters such as the ratio of anode to cathode size, electrode separation, gas pressure, biasing configuration such as anode bias, cathode bias and grounding schemes. We attempted to classify different anodic structures observed experimentally, as anode glow, fireball, anode spot, double layer and multiple double layers (MDLs) based on its physical characteristics. Among these structures the present investigation is focused on MDLs. The number of layers, observed in MDLs varied from as high as six to as low as zero, by controlling the operating discharge parameters, externally. Diagnostics were carried out using Langmuir probe. The analysis of floating potential fluctuations corresponds to a multiple anodic structure showed emergence of turbulence, at its critical stage, satisfying condition for self-organized criticality (SOC). This was identified with three slopes observed in the power spectrum, resembling the sand-pile model. Though, the GDP is completely different from that of the magnetically confined plasma, the nature of turbulence observed with SOC, is very similar to that observed in the scrape of layer of fusion devices. Therefore, the present investigation could provide new approach to study turbulence of similar nature, under an experimental condition that is free from the complexities of complicated field geometries used in confinement devices.