Debajyoti Saha

Investigation of complexity dynamics of inverse and normal homoclinic bifurcation in a glow discharge plasma

Order-chaos-order was observed in the relaxation oscillations of a glow discharge plasma with variation in the discharge voltage. The first transition exhibits an inverse homoclinic bifurcation followed by a homoclinic bifurcation in the second transition. For the two regimes of observations, a detailed analysis of correlation dimension, Lyapunov exponent, and Renyi entropy was carried out to explore the complex dynamics of the system.

Intrinsic noise induced coherence resonance in a glow discharge plasma

Experimental evidence of intrinsic noise induced coherence resonance in a glow discharge plasma is being reported. Initially the system is started at a discharge voltage (DV) where it exhibited fixed point dynamics, and then with the subsequent increase in the DV spikes were excited which were few in number and with further increase of DV the number of spikes as well as their regularity increased. The regularity in the interspike interval of the spikes is estimated using normalized variance. Coherence resonance was determined using normalized variance curve and also corroborated by Hurst exponent and power spectrum plots. We show that the regularity of the excitable spikes in the floating potential fluctuation increases with the increase in the DV, up to a particular value of DV. Using a Wiener filter, we separated the noise component which was observed to increase with DV and hence conjectured that noise can play an important role in the generation of the coherence resonance. From an anharmonic oscillator equation describing ion acoustic oscillations, we have been able to obtain a FitzHugh-Nagumo like model which has been used to understand the excitable dynamics of glow discharge plasma in the presence of noise. The numerical results agree quite well with the experimental results.

Experimental evidence of intermittent chaos in a glow discharge plasma without external forcing and its numerical modelling

Intermittent chaos was observed in a glow discharge plasma as the system evolved from regular type of relaxation oscillations (of larger amplitude) to an irregular type of oscillations (of smaller amplitude) as the discharge voltage was increased. Floating potential fluctuations were analyzed by different statistical and spectral methods. Features like a gradual change in the normal variance of the interpeak time intervals, a dip in the skewness, and a hump in the kurtosis with variation in the control parameter have been seen, which are strongly indicative of intermittent behavior in the system. Detailed analysis also suggests that the intrinsic noise level in the experiment increases with the increasing discharge voltage. An attempt has been made to model the experimental observations by a second order nonlinear ordinary differential equation derived from the fluid equations for an unmagnetized plasma. Though the experiment had no external forcing, it was conjectured that the intrinsic noise in the experiment could be playing a vital role in the dynamics of the system. Hence, a constant bias and noise as forcing terms were included in the model. Results from the theoretical model are in close qualitative agreement with the experimental results.

Irregular-regular-irregular mixed mode oscillations in a glow discharge plasma

Floating potential fluctuations of a glow discharge plasma are found to exhibit different kinds of mixed mode oscillations. Power spectrum analysis reveals that with change in the nature of the mixed mode oscillation (MMO), there occurs a transfer of power between the different harmonics and subharmonics. The variation in the chaoticity of different types of mmo was observed with the study of Lyapunov exponents. Estimates of correlation dimension and the Hurst exponent suggest that these MMOs are of low dimensional nature with an anti persistent character. Numerical modeling also reflects the experimentally found transitions between the different MMOs.

A localized cathode glow in the presence of a bar magnet and its associated nonlinear dynamics

A localized glowing, fireball like structure, appears near the cathode surface of a glow discharge plasma device when it is subjected to a magnetic dipole field produced by a bar magnet placed outside the plasma chamber. It is seen that the plasma density in the localized glow region and the luminous intensity of this structure increases with the increase in the magnetic field strength. The effect of such localized glow region on the plasma floating potential fluctuation dynamics is investigated. Floating potential fluctuations show that the emergence of such localized structure leads the system towards nonlinear dynamical regimes. Increasing the magnetic field strength reveals a transition from order to chaos via period doubling bifurcation. This transition is analyzed by using bifurcation diagram, phase space plots, power spectrum plots, Hilbert Huang transform, and by estimating the largest Lyapunov exponent. The interaction of plasma with a dipole magnetic field produces a non-monotonic potential stru...

Investigation and quantification of nonlinearity using surrogate data in a glow discharge plasma

Detection of nonlinearity has been carried out in periodic and aperiodic floating potential fluctuations of DC glow discharge plasma by generating surrogate data using iterative amplitude adjusted Fourier transform method. We introduce “delay vector variance” analysis (DVV) for the first time, which allows reliable detection of nonlinearity and provides some easy to interpret diagram conveying information about the nature of the experimental floating potential fluctuations (FPF). The method of false nearest neighbourhood is deployed on the FPFs to find a good embedding so as to be acquainted with the precise knowledge of m, which is desirable for carrying out DVV analysis. The emergence of nonlinearity with increase in discharge voltage has been ensured by taking into consideration the total energy present in different band of frequencies excited due to nonlinear processes. Rejection of null hypothesis has been verified by performing the rank test method that confirms the presence of nonlinearity quantitatively.

Hysteresis of fluctuation dynamics associated with a fireball in a magnetized glow discharge plasma in a currentless toroidal assembly

Floating potential fluctuations associated with an anode fireball in a glow discharge plasma in the toroidal vacuum vessel of the SINP tokamak are found to exhibit different kinds of oscillations under the action of vertical magnetic field of different strengths. While increasing the vertical magnetic field, the fluctuations have shown transitions as: chaotic oscillation → inverse homoclinic transition → intermittency → chaotic oscillation. However, on decreasing the magnetic field, the fluctuations are seen to follow: chaotic oscillations → homoclinic transition → chaotic oscillation; that is the intermittent feature is not observed. Fireball dynamics is found to be closely related to the magnetic field applied; results of visual inspection with a high speed camera are in close agreement with the fluctuations, and the fireball dynamics is found to be closely related to the transitions. The statistical properties like skewness, kurtosis, and entropy of the fluctuations are also found to exhibit this hyste...

Reformation of hydrocarbons using non-thermal plasma at atmospheric pressure: discharge characteristics and associated nonlinear dynamics

This paper studies the atmospheric non-thermal plasma generated in a hydrocarbon reforming system. Here, discharge is excited in a typically designed electrode configuration and has been carried out with four different combination pairs of electrodes. The plasma discharge is attained by applying high ac (pulsed) voltages between electrodes. The voltage?current characteristics have been studied in the system with respect to different electrode materials such as steel (SS304), copper and nickel. A Langmuir probe is used to measure fluctuations in the ion saturation region with all electrode configurations. The Hilbert transform technique and empirical mode decomposition have been used to understand the nonlinear dynamics of the fluctuation signals.

Investigation of coherent modes and their role in intermittent oscillations using empirical mode decomposition

The paper presents an application of empirical mode decomposition (EMD) for the analysis of intermittent chaotic fluctuations from a glow discharge plasma. Here, the nature of the oscillations changes from an initial relaxation oscillation to a final chaotic oscillatory state via intermittent chaos. The time series data have been decomposed into several intrinsic mode functions (IMFs) using EMD. Furthermore, the estimation of the variance of the IMFs and the correlation of these IMFs with the original time series help us to identify the presence of coherent modes in the fluctuations. Through this analysis, we could clearly observe that initially during the relaxation oscillations the system was dominated by one type of coherent mode, whereas in the final chaotic state, it was dominated by another coherent mode. In the intermediate case, i.e., intermittent chaotic state, both the coherent modes are seen to be present. We have also used rescaled range ( R / S ) statistical method to identify the coherent mo...