• N Kumar

      Articles written in Pramana – Journal of Physics

    • Thermal dynamics-based mechanism for intense laser-induced material surface vaporization

      N Kumar S Dash A K Tyagi Baldev Raj

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      Laser material processing involving welding, ablation and cutting involves interaction of intense laser pulses of nanosecond duration with a condensed phase. Such interaction involving high brightness radiative flux causes multitude of non-linear events involving thermal phase transition at soild–liquid–gas interfaces. A theoretical perspective involving thermal dynamics of the vaporization process and consequent non-linear multiple thermal phase transitions under the action of laser plasma is the subject matter of the present work. The computational calculations were carried out where titanium (Ti) was treated as a condensed medium. The solution to the partial differential equations governing the thermal dynamics and the underlying phase transition event in the multiphase system is based on non-stationary Eulerian variables. The Mach number 𝑀 depicts significant fluctuations due to thermal instabilities associated with the laser beam flux and intensity. A conclusive amalgamation has been established which relates material surface temperature profile to laser intensity, laser flux and the pressure in the plasma cloud.

    • Manipulation of microparticles and red blood cells using optoelectronic tweezers

      R S Verma R Dasgupta N Kumar S Ahlawat A Uppal P K Gupta

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      We report the development of an optoelectronic tweezers set-up which works by lightinduced dielectrophoresis mechanism to manipulate microparticles. We used thermal evaporation technique for coating the organic polymer, titanium oxide phthalocyanine (TiOPc), as a photoconductive layer on ITO-coated glass slide. Compare to the conventional optical tweezers, the technique requires optical power in 𝜇W range and provides a manipulation area of a few mm2. The set-up was used to manipulate the polystyrene microspheres and red blood cells (RBCs). The RBCs could be attracted or repelled by varying the frequency of the applied AC bias.

    • Long term performance evaluation of the TACTIC imaging telescope using ∼400 h Crab Nebula observation during 2003–2010

      A K Tickoo R Koul R C Rannot K K Yadav P Chandra V K Dhar M K Koul M Kothari N K Agarwal A Goyal H C Goyal S Kotwal N Kumar P Marandi K Venugopal K Chanchalani N Bhatt S Bhattacharyya C Borwankar N Chouhan S R Kaul A K Mitra S Sahaynathan M Sharma K K Singh C K Bhat

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      The TeV atmospheric Cherenkov telescope with imaging camera (TACTIC) 𝛾-ray telescope has been in operation at Mt. Abu, India since 2001 to study TeV 𝛾-ray emission from celestial sources. During the last 10 years, apart from consistently detecting a steady signal from the Crab Nebula above ∼1.2 TeV energy, at a sensitivity level of ∼5.0𝜎 in ∼25 h, the telescope has also detected flaring activity from Mrk 421 and Mrk 501 on several occasions. Although we used Crab Nebula data partially, in some of the reported results, primarily for testing the validity of the full data analysis chain, the main aim of this work is to study the long term performance of the TACTIC telescope by using consolidated data collected between 2003 and 2010. The total on-source data, comprising ∼402 h, yields an excess of ∼(3742±192) 𝛾-ray events with a statistical significance of ∼19.9𝜎 . The off-source data, comprising ∼107 h of observation, is found to be consistent with a no-emission hypothesis, as expected. The resulting 𝛾-ray rate for the onsource data is determined to be ∼(9.31±0.48) h-1. A power law fit (d𝛷/d𝐸 = $f_0E^{−\Gamma}$) with $f_0 \tilde (2.66 \pm 0.29) \times 10^{−11}$ cm-2 s-1 TeV-1 and $\Gamma \tilde$ 2.56 ± 0.10 is found to provide reasonable fit to the inferred differential spectrum within statistical uncertainties. The spectrum matches reasonably well with that obtained by other groups. A brief summary of the improvements in the various subsystems of the telescope carried out recently, which has resulted in a substantial improvement in its detection sensitivity (viz., ∼5𝜎 in an observation period of ∼13 h as compared to ∼25 h earlier) are also presented in this paper. Encouraged by the detection of strong 𝛾-ray signals from Mrk 501 and Mrk 421 on several occasions, there is considerable scope for the TACTIC telescope to monitor similar TeV 𝛾-ray emission activity from other active galactic nuclei on a long-term basis.

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