A. N. Sekar Iyengar

Large Mach number ion acoustic rarefactive solitary waves for a two electron temperature warm ion plasma

An exact analytical form of Sagdeev pseudopotential has been derived for a two electron temperature warm ion plasma, from which ion acoustic rarefactive solitary wave solutions could be investigated for a wide range of different plasma parameters, viz., ion temperature (σ), cold to hot electron temperature ratio (β), and initial cold electron concentration (μ). Explicitly large Mach numbers have been obtained for increasing hot to cold electron temperature ratios, and an analytical condition for the upper bound of the Mach number has been derived for such a rarefactive solitary wave. It is found that the width of these waves obey Korteweg–de Vries soliton‐type behavior only for small amplitudes (i.e., eφ/Teff<1) while for large amplitudes, the width of the rarefactive solitary waves increases with increasing amplitude.

Comparative study of nonlinear properties of EEG signals of normal persons and epileptic patients

Background Investigation of the functioning of the brain in living systems has been a major effort amongst scientists and medical practitioners. Amongst the various disorder of the brain, epilepsy has drawn the most attention because this disorder can affect the quality of life of a person. In this paper we have reinvestigated the EEGs for normal and epileptic patients using surrogate analysis, probability distribution function and Hurst exponent. Results Using random shuffled surrogate analysis, we have obtained some of the nonlinear features that was obtained by Andrzejak et al. [Phys Rev E 2001, 64:061907], for the epileptic patients during seizure. Probability distribution function shows that the activity of an epileptic brain is nongaussian in nature. Hurst exponent has been shown to be useful to characterize a normal and an epileptic brain and it shows that the epileptic brain is long term anticorrelated whereas, the normal brain is more or less stochastic. Among all the techniques, used here, Hurst exponent is found very useful for characterization different cases. Conclusion In this article, differences in characteristics for normal subjects with eyes open and closed, epileptic subjects during seizure and seizure free intervals have been shown mainly using Hurst exponent. The H shows that the brain activity of a normal man is uncorrelated in nature whereas, epileptic brain activity shows long range anticorrelation.

Realization of SOC behavior in a dc glow discharge plasma

Experimental observations consistent with Self Organized Criticality (SOC) have been obtained in the electrostatic floating potential fluctuations of a dc glow discharge plasma. Power spectrum exhibits a power law which is compatible with the requirement for SOC systems. Also the estimated value of the Hurst exponent (self similarity parameter), H being greater than 0.5, along with an algebraic decay of the autocorrelation function, indicate the presence of temporal long-range correlations, as may be expected from SOC dynamics. This type of observations in our opinion has been reported for the first time in a glow discharge system.

Parametric investigation of nonlinear fluctuations in a dc glow discharge plasma

Glow discharge plasmas exhibit various types of self-excited oscillations for different initial conditions like discharge voltages and filling pressures. The behavior of such oscillations associated with the anode glow has been investigated using nonlinear techniques like correlation dimension, largest Lyapunov exponent, etc. It is seen that these oscillations go to an ordered state from a chaotic state with an increase in input energy, i.e., with discharge voltages implying occurrence of inverse bifurcations. These results are different from the other observations wherein the fluctuations have been observed to go from ordered to chaotic state.

Anomalous width variations for ion acoustic rarefactive solitary waves in a warm ion plasma with two electron temperatures

Anomalous width–amplitude variations were observed in large amplitude rarefactive solitary waves which show increasing width with increasing amplitude, contrasting the usual reciprocal relation between the square of the width and the amplitude, beyond a certain value of the plasma parameters [S. S. Ghosh, K. K. Ghosh, and A. N. Sekar Iyengar, Phys. Plasmas, 3, 3939 (1996)]. For the limiting maximum amplitude, the “increasing width” solitary wave tends to a double layer-like solution. The overall variation was found to depend crucially on the specific parameter space. From a detailed investigation of the above behavior, a plausible physical explanation has been presented for such increases in the width. It is found that the ions’ initial kinetic energies and the cold electron concentration within the perturbed region play a significant role in determining the observed width–amplitude variation. This contradicts the investigation of Sayal, Yadav, and Sharma [Phys. Scr. 47, 576 (1993)].

Continuous wavelet transform based time-scale and multifractal analysis of the nonlinear oscillations in a hollow cathode glow discharge plasma

Continuous wavelet transform (CWT) based time-scale and multifractal analyses have been carried out on the anode glow related nonlinear floating potential fluctuations in a hollow cathode glow discharge plasma. CWT has been used to obtain the contour and ridge plots. Scale shift (or inversely frequency shift), which is a typical nonlinear behavior, has been detected from the undulating contours. From the ridge plots, we have identified the presence of nonlinearity and degree of chaoticity. Using the wavelet transform modulus maxima technique we have obtained the multifractal spectrum for the fluctuations at different discharge voltages and the spectrum was observed to become a monofractal for periodic signals. These multifractal spectra were also used to estimate different quantities such as the correlation and fractal dimension, degree of multifractality, and complexity parameters. These estimations have been found to be consistent with the nonlinear time series analysis.

Noise-invoked resonances near a homoclinic bifurcation in the glow discharge plasma

Stochastic resonance (SR) and coherence resonance (CR) have been studied experimentally in discharge plasmas close to a homoclinic bifurcation. For the SR phenomenon, it is observed that a superimposed subthreshold periodic signal can be recovered via stochastic modulations of the discharge voltage. Furthermore, it is realized that even in the absence of a subthreshold deterministic signal, the system dynamics can be recovered and optimized using noise. This effect is defined as CR in the literature. In the present experiments, induction of SR and CR is quantified using the absolute mean difference and normalized variance techniques, respectively.

Effect of cooler electrons on a compressive ion acoustic solitary wave in a warm ion plasma — Forbidden regions, double layers, and supersolitons

It is observed that the presence of a minority component of cooler electrons in a three component plasma plays a deterministic role in the evolution of solitary waves, double layers, or the newly discovered structures called supersolitons. The inclusion of the cooler component of electrons in a single electron plasma produces sharp increase in nonlinearity in spite of a decrease in the overall energy of the system. The effect maximizes at certain critical value of the number density of the cooler component (typically 15%–20%) giving rise to a hump in the amplitude variation profile. For larger amplitudes, the hump leads to a forbidden region in the ambient cooler electron concentration which dissociates the overall existence domain of solitary wave solutions in two distinct parameter regime. It is observed that an inclusion of the cooler component of electrons as low as < 1% affects the plasma system significantly resulting in compressive double layers. The solution is further affected by the cold to hot ...

Long range temporal correlation in the chaotic oscillations of a dc glow discharge plasma

Long range temporal correlations in the fluctuations of the plasma floating potentials (measured using a Langmuir probe) are investigated in a dc glow discharge plasma. Keeping the neutral pressure constant, the discharge voltage was varied and at the formation of the plasma, quasi periodic oscillations were excited and on further increase of the discharge voltage they became chaotic (irregular) beyond a threshold voltage. We compared the Lyapunov exponent with the Hurst exponent obtained from R/S statistics which showed an opposite behaviour at the transition point. These results are perhaps new since we have not come across such comparative analysis for chaotic oscillations in a glow discharge plasma before.

Accessibility of very low qa (VLQ) and ultra-low qa (ULQ) discharges in the SINP tokamak

Attempts to operate a tokamak at qa < 2 are normally found to terminate with a major disruption. In the SINP tokamak, however, it has been found possible to operate at qa < 1 with comparative ease. In this letter, an attempt is made to quantify the rate of current rise required to set up discharges having qa < 2 and qa < 1 by comparing the risetime with the resistive diffusion timescale. It has been observed that discharges with qa less than a particular value can be obtained if the time taken to cross the previous qa barrier is less than the resistive diffusion time. When the resistive diffusion time is less than the time taken to cross a particular qa barrier, it becomes extremely difficult to cross the next lower qa barrier