• Volume 59, Issue 1

      July 2002,   pages  1-154 and L155-L161

    • Bianchi type I inflationary universe in general relativity

      Raj Bali Vimal Chand Jain

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      In this paper, we have investigated Bianchi type I inflationary universe in the presence of massless scalar field with a flat potential. To get an inflationary solution, we have considered a flat region in which potential V is constant. The inflationary scenario of the model is discussed in detail.

    • Critical exponents in the transition to chaos in one-dimensional discrete systems

      G Ambika N V Sujatha

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      We report the numerically evaluated critical exponents associated with the scaling of generalized fractal dimensions during the transition from order to chaos. The analysis is carried out in detail in the context of unimodal and bimodal maps representing typical one-dimensional discrete dynamical systems. The behavior of Lyapunov exponents (LE) in the cross over region is also studied for a complete characterization.

    • Fragment production in 16O+80Br reaction within dynamical microscopic theory

      Rajeev K Puri Jaivir Singh Suneel Kumar

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      We analyze the formation of fragments in O—Br reaction at different incident energies between E/A=50 MeV and 200 MeV. This study is carried out within the quantum molecular dynamics (QMD) model coupled with recently advanced simulated annealing clusterization algorithm (SACA). For comparison, we also use the conventional minimum spanning tree (MST) method. Our detailed study shows that the SACA can detect the final stable fragment configuration as early as 60 fin/c which is marked by a dip in the heaviest fragment. On the other hand, the MST method needs several hundred fm/c to identify the final stable distribution. A comparison of the charge distribution with experimental data shows that the SACA is able to reproduce the data very nicely whereas (as reported earlier) the MST method fails to break the spectator matter into intermediate mass fragments. Furthermore, our results with SACA method indicate the onset of multi-fragmentation around 75 MeV/A which is again in good agreement with experimental findings.

    • Effects of non-local thermodynamic equilibrium conditions on numerical simulations of inertial confinement fusion plasmas

      N K Gupta B K Godwal

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      Effects of non-local thermodynamic equilibrium (non-LTE) condition on emission and hydrodynamics of typical inertial confinement fusion (ICF) plasmas are studied. The average degree of ionization at high temperatures is seen to be much lower compared to the values obtained from Thomas-Fermi scaling or Saha equation for high-Z element like gold. LTE and non-LTE predictions for emitted radiation from laser-driven gold foil are compared with the experimental results and it is seen that non-LTE simulations show a marked improvement over LTE results. The effects of one group and multigroup, LTE and non-LTE approximations of radiation transport on hydrodynamic parameters are studied for laser-driven aluminium and gold foils. It is further seen that non-LTE and multigroup effects play an important role in predicting conversion efficiency of laser light to X-rays.

    • Beam optics of the folded tandem ion accelerator at BARC

      S Santra P Singh

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      The beam optics of the 6 MV folded tandem ion accelerator, that has recently been commissioned at Bhabha Atomic Research Centre, Mumbai, is presented. Typical beam trajectories for proton and 12C beams under different conditions, are shown. The constraints on the design due to the use of the infrastructure of the Van de Graaff accelerator, which existed earlier, are discussed.

    • Highly efficient deep ultraviolet generation by sum-frequency mixing in a BBO crystal pair

      Gopal C Bhar Pathik Kumbhakar Udit Chatterjee Anil K Chaudhary Alexander Kokh

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      Generation of deep ultraviolet radiation at 210 nm by Type-I third harmonic generation is achieved in a pair of BBO crystals with conversion efficiency as high as 36%. The fundamental source is the dye laser radiation pumped by the second harmonic of a Q-switched Nd: YAG laser. A walk-off compensated configuration with the BBO crystal pair has enabled us to realize such a high conversion efficiency in the interaction.

    • Effect of Nd3+ concentration on CW and pulsed performance of fiber-coupled diode laser pumped Nd:YVO4 laser at 1064 nm

      Pranab K Mukhopadhyay K Ranganathan Jogy George SK Sharma TPS Nathan

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      The effect of Nd3+ concentration on the CW and Q-switched laser performances at 1064 nm from Nd: YVO4 has been studied under diode laser pumping in identical laser configuration. The Nd3+ concentrations used were 1, 2 and 3 at.% in YVO4 crystals. Under the CW operations we have compared the thermal lensing effect, slope efficiencies and also the beam quality at the fourth-order degeneracy configuration. Q-switching was done with the help of an acousto-optic modulator and we have compared the pulses obtained from Nd: YVO4 laser with different doping concentration. It was found that the 1 at.%-doped crystal is the best, offering highest optical-to-optical conversion efficiency (55%), lowest fractional heat load (24%), highest pulse energy (80 µJ) and shortest pulse width (20 ns). It was also found that there was not much difference in performances for 2 and 3 at.%-doped crystals both in CW and Q-switched configurations.

    • Control of a coupled map lattice model for vortex shedding in the wake of a cylinder

      G Balasubramanian DJ Olinger MA Demetriou

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      The flow behind a vibrating flexible cable at low Reynolds numbers can exhibit complex wake structures such as lace-like patterns, vortex dislocations and frequency cells. These structures have been observed in experiments and numerical simulations, and are predicted by a previously developed low-order coupled map lattice (CML). The discrete (in time and space) CML models consist of a series of diffusively coupled circle map oscillators along the cable span. Motivated by a desire to modify the complex wake patterns behind flexible vibrating cables we have studied the addition of control terms into the highly efficient CML models and explored the resulting dynamics. Proportional, adaptive proportional and discontinuous non-linear (DNL) control methods were used to derive the control laws. The first method employed occasional proportional feedback. The adaptive method used spatio-temporal feedback control. The DNL method used a discontinuous feedback linearization procedure, and the controller was designed for the resulting linearized system using eigenvalue assignment. These techniques were applied to a modeled vortex dislocation structure in the wake of a vibrating cable in uniform freestream flow. Parallel shedding patterns were achieved for a range of forcing frequency-forcing amplitude combinations studied to validate the control theory. The adaptive proportional and DNL methods were found to be more effective than the proportional control method due to the incorporation of a spatially varying feedback gain across the cylinder span. The DNL method was found to be the most efficient controller of the low-order CML model. The required control level across the cable span was correlated to the 1/1 lock-on behavior of the temporal circle map.

    • Fabrication of polystyrene hollow microspheres as laser fusion targets by optimized density-matched emulsion technique and characterization

      KK Mishra RK Khardekar Rashmi Singh HC Pant

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      Inertial confinement fusion, frequently referred to as ICF, inertial fusion, or laser fusion, is a means of producing energy by imploding small hollow microspheres containing thermonuclear fusion fuel. Polymer microspheres, which are used as fuel containers, can be produced by solution-based micro-encapsulation technique better known as density-matched emulsion technique. The specifications of these microspheres are very rigorous, and various aspects of the emulsion hydrodynamics associated with their production are important in controlling the final product. This paper describes about the optimization of various parameters associated with density-matched emulsion method in order to improve the surface smoothness, wall thickness uniformity and sphericity of hollow polymer microspheres. These polymer microshells have been successfully fabricated in our lab, with 3–30 µm wall thickness and 50–1600 µm diameters. The sphericity and wall thickness uniformity are better than 99%. Elimination of vacuoles and high yield rate has been achieved by adopting the step-wise heating of W1/O/W2 emulsion for solvent removal.

    • Self-assembled growth of nanostructural Ge islands on bromine-passivated Si(111) surfaces at room temperature

      Amai K Das BN Dev B Sundaravel EZ Luo JB Xu IH Wilson

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      We have deposited relatively thick (∼60 nm) Ge layers on Br-passivated Si(111) substrates by thermal evaporation under high vacuum conditions at room temperature. Ge has grown in a layer-plus-island mode although it is different from the Stranski-Krastanov growth mode observed in epitaxial growth. Both the islands and the layer are nanocrystalline. This appears to be a consequence of reduction of surface free energy of the Si(111) substrate by Br-passivation. The size distribution of the Ge nanoislands has been determined. The Br-Si(111) substrates were prepared by a liquid treatment, which may not produce exactly reproducible surfaces. Nevertheless, some basic features of the nanostructural island growth are reasonably reproducible, while there are variations in the details of the island size distribution.

    • Estimation of dynamic properties of attractors observed in hollow copper electrode atmospheric pressure arc plasma system

      S Ghorui S N Sahasrabudhe P S S Murthy A K Das N Venkatramani

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      Understanding of the basic nature of arc root fluctuation is still one of the unsolved problems in thermal arc plasma physics. It has direct impact on myriads of thermal plasma applications being implemented at present. Recently, chaotic nature of arc root behavior has been reported through the analysis of voltages, acoustic and optical signals which are generated from a hollow copper electrode arc plasma torch. In this paper we present details of computations involved in the estimation process of various dynamic properties and show how they reflect chaotic behavior of arc root in the system.

    • Effect of weight fraction of different constituent elements on the total mass attenuation coefficients of biological materials

      Karamjit Singh Charanjeet Singh Parjit S Singh Gurmel S Mudahar

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      The mass attenuation coefficients, µm, of biological materials have been studied as a function of weight fraction of constituent elements (hydrogen, carbon, oxygen and nitrogen). A considerable change in µm is seen only in low energy region whereas no change is observed with the increasing percentage of constituent elements in high energy region up to 10 MeV. The results have been presented in graphical form.

    • Fractal patterns on the onset of coherent structures in a coupled map lattice

      G Ambika Kamala Menon

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      We report the formation of Cantor set-like fractals during the development of coherent structures in a coupled map lattice (CML). The dependence of these structures on the size of the lattice as well as the first three dimensions of the associated fractal patterns are analyzed numerically.

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