• A K Das

      Articles written in Pramana – Journal of Physics

    • A simpler and elegant algorithm for computing fractal dimension in higher dimensional state space

      S Ghorui A K Das N Venkatramani

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      Chaotic systems are now frequently encountered in almost all branches of sciences. Dimension of such systems provides an important measure for easy characterization of dynamics of the systems. Conventional algorithms for computing dimension of such systems in higher dimensional state space face an unavoidable problem of enormous storage requirement. Here we present an algorithm, which uses a simple but very powerful technique and faces no problem in computing dimension in higher dimensional state space. The unique indexing technique of hypercubes, used in this algorithm, provides a clever means to drastically reduce the requirement of storage. It is shown that theoretically this algorithm faces no problem in computing capacity dimension in any dimension of the embedding state space as far as the actual dimension of the attractor is finite. Unlike the existing algorithms, memory requirement offered by this algorithm depends only on the actual dimension of the attractor and has no explicit dependence on the number of data points considered.

    • Magnetic field modulation spectroscopy of rubidium atoms

      S Pradhan R Behera A K Das

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      The magnetically modulated saturation absorption profile is studied for a wide range of external DC magnetic field. The salient features of Doppler-free signal generated by laser frequency modulation and atomic energy level modulation are compared. The DC offset of the signal profile is found to be unstable as the external DC magnetic field is changed. The technical difficulty of tuning laser frequency under locked condition over a large frequency span is discussed along with possible solutions.

    • Arc plasma devices: Evolving mechanical design from numerical simulation

      S Ghorui A K Das

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      Wide ranges of technological applications involve arc plasma devices as the primary plasma source for processing work. Recent findings exhibit the existence of appreciable thermal non-equilibrium in these so-called thermal plasma devices. Commercially available magnetohydrodynamic codes are not capable of handling such systems due to unavailability of non-equilibrium thermodynamic and transport property data and self-consistent models. A recipe for obtaining mechanical design of arc plasma devices from numerical simulation incorporating two-temperature thermal non-equilibrium model is presented in this article with reference to the plasma of the mixture of molecular gases like nitrogen and oxygen. Such systems are technologically important as they correspond to the plasma devices operating with air, oxygen plasma torches in cutting industries and plasma devices using nitrogen as shielding gas. Temperature field, associated fluid dynamics and electrical characteristics of a plasma torch are computed in a systematic manner to evaluate the performance of a conceived design using a two-fluid CFD model coupled with a two-temperature thermodynamic and transport property code. Important effects of different nozzle designs and plasma gases obtained from the formalism are discussed. Non-equilibrium thermo-dynamic properties are computed using modified two-temperature Saha equations and transport properties are computed using standard Chapman–Enskog approach.

    • Spectral narrowing of coherent population trapping resonance in laser-cooled and room-temperature atomic gas

      S Pradhan S Mishra R Behera N Kawade A K Das

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      We have investigated coherent population trapping (CPT) in laser-cooled as well as room-temperature (with and without buffer gas) rubidium atoms. The characteristic broad signal profile emerging from the two-photon Raman resonance for room-temperature atomic vapour is consistent with the theoretical calculation incorporating associated thermal averaging. The spectral width of the dark resonance obtained with cold atoms is found to be broadened, compared to roomtemperature vapour cell, due to the feeble role played by thermal averaging, although the cold atomic sample significantly overcomes the limitation of the transit time broadening. An alternative way to improve transit time is to use a buffer gas, with which we demonstrate that the coherent population trapping signal width is reduced to < 540 Hz.

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