Dan Gheorghe Dimitriu

Common physical mechanism for concentric and non-concentric multiple double layers in plasma

Experimental results are presented on the generation and dynamics of concentric as well as non-concentric multiple double layers in hot filament type discharge plasma. These results emphasize striking similarities between the generation and dynamics of both types of multiple double layers, i.e. similar potential steps corresponding to the ionization energy in used gas, similar hysteresis cycles in the current–voltage characteristic of the exciting electrode, similar dynamic behaviour, etc. The results show that a common physical mechanism is at the origin of both phenomena, in which electron-neutral impact excitation and ionization reactions play a key role.

Type I intermittency related to the spatiotemporal dynamics of double layers and ion-acoustic instabilities in plasma

Anodic double layer instabilities occur in low-temperature diffusion filament-type discharge plasma by applying a certain positive bias with respect to the plasma potential to an additional electrode. Periodic nonlinear regimes, characterized by proper dynamics of double layers, are sustained if excitation and ionization rates in front of the electrode reach the value for which current limitation effects appear in the static current-voltage characteristic. It was experimentally shown that under specific experimental conditions these ordered spatiotemporal phenomena can evolve into chaotic states by type I intermittency. This transition was verified by the evolution of time series, fast Fourier transform amplitude plots, three–dimensional reconstructed state spaces, power laws, and flickering phenomena spectrum, as well as by the return map and tangent bifurcation.

Experimental and theoretical investigations of plasma multiple double layers and their evolution to chaos

We report experimental results showing that a discharge plasma, which self-structures as concentric multiple double layers, is capable of evolving towards chaos through a cascade of spatio-temporal sub-harmonic bifurcations when the applied constraint (in the form of voltage applied on a supplementary electrode immersed in the plasma) is gradually increased. By considering that the plasma particles (electrons, ions and neutrals) move on fractal curves, a theoretical model able to explain both the plasma self-structuring and the scenario of evolution to chaos is constructed. The experimental results were found to be in good agreement with those obtained from the theoretical model.

Experimental observation of multiple double Layers structures in Plasma-part II: nonconcentric multiple double Layers

In certain experimental conditions, in front of a positively biased electrode immersed into a diffusion plasma, a concentric multiple double layers structure can appear. If the electrode is large or asymmetric (one dimension is larger than the others), this structure appears nonconcentrically, as many spherical luminous structures attached to the electrode. Each of these structure consists of a positive nucleus (an ion-rich plasma) bordered by an electrical double layer. The structures are almost equally distributed on the electrode because of the negative external shell of the double layer.

Experimental and theoretical evidence for the chaotic dynamics of complex structures

This paper presents the experimental results on the formation, dynamics and evolution towards chaos of complex space charge structures that emerge in front of a positively biased electrode immersed in a quiescent plasma. In certain experimental conditions, we managed to obtain the so-called multiple double layers (MDLs) with non-concentric configuration. Our experiments show that the interactions between each MDLs constituent entities are held responsible for the complex dynamics and eventually for its transition to chaos through cascades of spatio-temporal sub-harmonic bifurcations. Further, we build a theoretical model based on the fractal approximation (scale relativity theory) in order to reproduce the experimental results (plasma self-structuring and scenario of evolution to chaos). Comparing the experimental results with the theoretical ones, we observe a good correlation between them.

Quantum-mechanical study and spectral analysis on some derivatives of Rhodamine in solutions.

Abstract Rhodamine derivatives (B and 6G) are heterocyclic compounds, related to florone. They are widely used as dyes with numerous biotechnological applications. The quantum-mechanical, electro-optical and spectral properties of the isolated molecules in the ground state were determined using molecular modeling programs. The visible electronic absorption spectra of 6G and B Rhodamine were recorded in solvents with different macroscopic parameters. Dipolar moments and polarizabilities of the studied Rhodamines in the excited state were determined by solvatochromic study.

New method to determine the optical rotatory dispersion of inorganic crystals applied to some samples of Carpathian Quartz.

A new method to determine the optical rotatory dispersion (ORD) in the visible range, based on a channeled spectrum obtained with a uniax inorganic crystal introduced between two crossed polarizers with its optical axis parallel to the light propagation direction is detailed in this paper. When the studied inorganic crystals are transparent, this method permits the estimation of the optical rotatory dispersion in the visible range, for which the cheap polarizers are available. The speed of the measurements is very high, because the estimations are made from the channeled spectrum obtained for a single arrangement of the optical components. By using a computer, ORD is quickly determined for the visible range. The results obtained by this method for some Carpathian Quartz samples are consistent with those from literature. The proposed method can be also applied in UV and IR spectral ranges, when the anisotropic layers are transparent and the linearly polarized radiations can be obtained.

Solvatochromic Characterization of Sudan Derivatives in Binary and Ternary Solution

ABSTRACT Sudan derivatives are widely used as dyes for foods, textiles, plastics, paper, and cosmetics. The wavenumbers in the maximum of the electronic absorption bands of two Sudan derivatives in solvents with varying physicochemical properties were correlated with microscopic and macroscopic parameters of solution components. The molecular parameters of the Sudan derivatives in the excited state were estimated using solvatochromic and quantum-mechanical calculations.

On the interaction between two fireballs in low-temperature plasma

We report experimental results and theoretical modeling showing the interaction between two fireballs excited on two positively biased electrodes immersed in a low-temperature plasma. This interaction leads to a synchronized dynamics of the two fireballs, its frequency depending on the plasma density, the voltages applied on the electrodes, and the distance between the two electrodes. By considering that the plasma particles (electrons, ions, neutrals) move on fractal curves, a theoretical model describing the interaction between the two fireballs is developed. The results of the theoretical model were found to be in good agreement with the experimental results.

Experimental investigations of the nonlinear dynamics of a complex space-charge configuration inside and around a grid cathode with hole

By negatively biasing a metallic grid with a small hole, down to a critical value of the applied potential a complex space-charge structure appears inside and around the grid cathode. The static current–voltage characteristic of the discharge shows one or two current jumps (the number of current jumps depending on the working gas pressure), one of them being of hysteretic type. Electrical probe measurements show a positive potential inside the grid cathode with respect to the potential applied on it. This is interpreted as being due to the hollow cathode effect. Thus, the inner fireball appears around the virtual anode inside the grid cathode. For more negative potentials, the electrons inside the cathode reach sufficient energy to penetrate the inner sheath near the cathode, passing through the hole and giving rise to a second fireball-like structure located outside the cathode. This second structure interacts with the negative glow of the discharge. The recorded time series of the discharge current oscillations reveal strongly nonlinear dynamics of the complex space-charge structure: by changing the negative potential applied on the grid cathode, the structure passes through different dynamic states involving chaos, quasi-periodicity, intermittency and period-doubling bifurcations, appearing like a competition of different routes to chaos.