• Rashmi

      Articles written in Pramana – Journal of Physics

    • A simple and efficient levitation technique for noncontact coating of inertial confinement fusion targets

      K K Mishra R K Khardekar Rashmi Chouhan R K Gupta

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      A simple and very efficient gas jet levitation technique for levitating inertial confinement fusion (ICF) targets has been developed. A low velocity gas jet through diverging nozzle generates precisely controlled low Reynolds number flow pattern, capable of levitating polymer microballoons up to 2500 µm diameter. Different shaped diverging nozzle are investigated, satisfactory levitation is achieved with simple conical shapes. With this setup microballoon can be levitated for hours with excellent stability, continuous rotation and at the desired height (reproducible with in less than 100 µm). The height of stabilization depends upon cone angle of diverging nozzle and velocity of levitating gas. This technique is very robust and highly insensitive to external disturbances like nonuniform temperature fields and vibrations.

      This setup is very economical to fabricate, easy to operate and can be used efficiently in various spray coating application involving plastic and metallic layers on microballoons.

    • 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.

    • A model for the pressure dependence of diffusion in condensed matter

      Kamal Kapoor Rashmi Kumari Narsingh Dass Rajendra Kumar

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      In the present paper, a model has been used to develop a simple relation to study the pressure dependence of self-diffusion in solids and liquids that has two adjustable parameters. The computation done in each substance is found to be in very good agreement with the experimental data. It is interesting to note that the present relation is also capable of giving the activation volume in solids and liquids. The activation volume computed in the solids is found to be in very good agreement with the data available.

    • Degradation process in organic thin film devices fabricated using P3HT

      Rashmi Ashok K Kapoor Upendra Kumar V R Balakrishnan P K Basu

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      The stability of regioregular poly(3-hexylthiophene 2,5-diyl) (P3HT) thin films sandwiched between indium tin oxide (ITO) and aluminium (Al) electrodes have been investigated under normal environmental conditions ($25^{\circ}$C and RH$\sim 45-50$%). Electrical and optical properties of ITO/P3HT/Al devices have been studied over a period of 30 days. Mobility 𝜇 of the order of $10^{-4}$ cm2/V-s has been obtained from the $V^{2}$ law in the as- deposited P3HT ¯lms. Scanning electron microscopy (SEM) investigations show blistering of Al contacts in devices with a poly(3,4-ethylenedioxythiophene) (PEDOT) interlayer on application of voltage whereas no blistering is seen in devices without PEDOT. The results have been explained in terms of trap generation and propagation and the moisture-absorbing nature of PEDOT.

    • Laser interaction with low-density carbon foam

      S Chaurasia S Tripathi D S Munda G Mishra C G Murali N K Gupta L J Dhareshwar A K Rossall G J Tallents Rashmi Singh D K Kohli R K Khardekar

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      Experiments were performed with a 15 J/500 ps Nd:glass laser ($\lambda = 1064$ nm) focussed to an intensity > 1014 W/cm2 . X-ray emissions from carbon foam and 5% Pt-doped carbon foam of density 150–300 mg/cc were compared with that of the solid carbon targets. The thickness of the carbon foam was 15 𝜇m on a graphite substrate. X-ray emission was measured using semiconductor X-ray diodes covered with various filters having transmissions in different X-ray spectral ranges. It covered X-ray spectrum of 0.8–8.5 keV range. The X-ray emission in the soft X-ray region was observed to increase to about 1.8 times and 2.3 times in carbon foam and Pt-doped foam, respectively with respect to solid carbon. In hard X-rays, there was no measurable difference amongst the carbon foam, Pt-doped carbon foam and solid carbon. Scanning electron microscope (SEM) analysis demonstrates that foam targets smoothens the crater formed by the laser irradiation.

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