No abstract

Blue core (BC) is a special spectral phenomenon in argon helicon plasma, with intense blue lights from ion emission around the radial center of discharge tube. In this paper, the characteristics of BC in argon (Ar) helicon plasma were investigated experimentally from aspects of discharge mode transitions, plasma spatial distributions, and spectral features. It is found that the BC generally appears at strong magnetic field (480 G or above in this work) with high plasma density in wave mode, accompanied by exponentially rising of ion line intensity. The electron density and temperature, the neutral density and temperature, and the line emission intensity show a radial profile with a central peak in BC mode. The steep gradient of ion line intensity (corresponding to the ion density) defines a clear boundary of the core. Further, a pressure balance model was developed to investigate the influence of neutral depletion on BC formation. The neutral density is depleted significantly from 7.24×1013 to 0.38 × 1013 cm−3 at magnetic field of 600 G in BC mode, while to 3.13 × 1013 cm−3 at magnetic field of 250 G in normal wave (NW) mode. The ionization rate in BC reaches as high as 70% compared with 9.6% of that in NW mode. The ionization rate and the ion line intensity show similar radial profiles, indicating the BC phenomenon is closely related to the distribution of peaked ion density and hollowed neutral density. Fundamentally, the central electron heating and strong magnetic field contribute to the centrally local high ionization rate and strong neutral heating. The severe neutral depletion with prominent central heating is considered to be the immediate cause of appearance of blue core.

The nonlinear phenomenon is very popular in dielectric barrier discharge (DBD) plasmas. There are at least three kinds of spatial and temporal nonlinear phenomena appearing synchronously or asynchronously in DBDs, i.e. self-organized patterns, striations and chaos. This paper describes the recent research and progress in understanding the nature of these nonlinear phenomena. Patterns are macroscopic structures with certain spatial and/or temporal periodicities generated through selforganization of microscopic parameters. The physics of patterns in DBDs is mainly associated with lateral dynamic behaviors or the lateral non-local effect of charged particles resulting in the lateral development or non-uniformity of discharge. Striations are ionization waves with unique properties determined by transport phenomena, ionization processes and electron kinetics in current-carrying plasmas. The physics of striations in DBDs is mainly associated with the advances in non-local electron kinetics in spatially inhomogeneous plasmas. Chaos is a kind of random and non-periodic phenomenon occurring in a determined dynamic system, following a series of certain rules while exhibiting random locomotion, and is regarded as an intrinsic and ubiquitous phenomenon in a nonlinear dynamic system. An evolution trajectory including period-doubling bifurcation to chaos was observed in DBDs or DBD-derived plasmas. In a common sense, it is believed that the formation of all the three nonlinear phenomena in a DBD system should be related to the non-local transversal and/or longitudinal dynamics of space charges (i.e. non-local effect) or the localized electric field interaction. Future work is still needed on the underlying physics and should be directed to pursuing the unification of these nonlinear phenomena in DBD.

In this paper, a 2D fluid model is employed to investigate the radial evolution of the discharge structures in a helium dielectric barrier discharge with 100 ppm nitrogen impurity. By elevating the applied voltage amplitude (V am ), the discharge exhibits some distinctive radial evolution features that comprise three aspects: (1) the lateral migration of the peripheral filament, i.e. outward filament migration nearby the electrode edge; (2) the reduced intervals between two successive filaments; (3) the growth in the number of filaments. It is revealed that the radial position of the peripheral filament is basically consistent with that of the initial local intense discharge, whose location is closely related to the surface charge distribution during the initial breakdown. An increase in V am reduces the duration of the current pulse, and hence, the displacement of space charges is restrained. When more charges are restrained in the gap rather than being attached to the dielectric surfaces, the surface charge distribution becomes more uniform, which contributes to the lateral migration of the peripheral filament. Meanwhile, the lateral uniformity of Penning ionization rate is improved with V am increasing, and the seed electron level in the intervals becomes comparable to that in the filamentary channels, leading to a more uniform radial seed electron profile that attenuates the electric field distortion. As a result, the intervals between two adjacent filaments are shortened. With the lateral migration and reduced intervals as V am increases, we observe the growth in the number of filaments, and the improvement of the radial discharge uniformity.

In this paper, we report the experimental results on the characteristics of plasma patterns with different planar electrode shapes in dielectric barrier glow discharge. The formation and the evolution of the discharge patterns at different voltages were investigated. The results show that the plasma patterns in this glow-barrier system form at the beginning of the discharge pulse. The limited size of planar electrodes and the electric field distribution are important factors for the pattern formation.