• Amit Kumar

      Articles written in Pramana – Journal of Physics

    • CAMAC based continuous/transient digitizer for long duration discharge

      Amit Kumar Srivastava Rachana Rajpal H D Pujara

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      With the development of fusion research, in the next generation tokamak like SST1 and ITER the tokamak discharge duration time of the order of 1000 sec is planned. At the same time acquisition of rapid changes of plasma parameters during discharge is required and this demands for higher sampling rate to acquire the data. Hence a large size of RAM (memory) is required to fulfill the above requirements but normally CAMAC based digitizer has limited on-board RAM which can be emptied only after the buffer is completely filled. Therefore, acquisition period is dependent on sampling frequency of the signal as well as on existing size of RAM. This drawback of above is overcome in this design of CAMAC based digitizer where we are using combination of FIFO memory and RAM to get continuous lossless acquisition as well as transient acquisition. This paper includes design of 4-channel CAMAC digitizer with 32 K samples RAM per channel for on-board storage and 8 K samples FIFO per channel for continuous acquisition. The module can be operated in different modes like monitoring, transient acquisition and continuous lossless acquisition with selectable sampling rate

    • Magnetic structure of molecular magnet Fe[Fe(CN)6]·4H2O

      Amit Kumar S M Yusuf

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      We have studied the magnetic structure of Fe[Fe(CN)6]·4H2O, prepared by precipitation method, using neutron diffraction technique. Temperature dependent DC magnetization study down to 4.2 K shows that the compound undergoes from a high temperature disordered (paramagnetic) to an ordered magnetic phase transition at 22.6 K. Rietveld analysis of neutron diffraction pattern at 60 K (in its paramagnetic phase) revealed a face centred cubic structure with space group Fm3m. The structure contains three-dimensional network of straight Fe3+-C≡N-Fe3+ chains along the edges of the unit cell cube. Fe3+ ions occupy 4a (0, 0, 0) and 4b (1/2, 1/2, 1/2) positions. Fe3+(0, 0, 0) is surrounded octahedrally by six nitrogen atoms and Fe3+ (1/2, 1/2, 1/2) is surrounded octahedrally by six carbon atoms. Magnetic Rietveld refinement of neutron diffraction data at 11 K shows a ferromagnetic coupling between the two inequivalent Fe3+ sites. Refinement yielded an ordered moment of 4.4(6) and 0.8(6) μB per Fe ion located at (0, 0, 0) and (1/2, 1/2, 1/2), respectively. Ordered moments are found to align along the face diagonal. The observed net moment from low temperature neutron diffraction study is consistent with DC magnetization results.

    • Impact of metal oxide nanoparticles on unsteady stagnation point flow of the hybrid base fluid along a flat surface


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      This paper deals with a detailed investigation of the effects of various metal oxide nanoparticles on unsteady stagnation point flow of a hybrid base fluid impinging on a flat surface. The ‘single-phase’ nanofluid model, i.e., the Tiwari and Das model, is considered for the study. We consider water and ethylene glycol in 1:1 ratio as the base fluid and four different types of metal oxides, namely, CuO, TiO$_2$, ZnO and MgO as the nanoparticles. Using similarity transformations, the conservation equations are transformed into self-similar ordinary differential equations. Dual and unique similarity solutions are obtained for certain set of values of parameters. The analysis explores many important findings. Dual self-similar solutions exist up to a certain critical value of the decelerating unsteady parameter and the critical value is independent of the type of metal oxide nanoparticles considered. The strongest surface drag force is observed for the nanofluid with CuO nanoparticles, while the weakest is for the nanofluid with MgO nanoparticles. The heat transfer rate is highest for the nanofluid with CuO nanoparticles and lowest for the nanofluid with TiO$_2$ nanoparticles. Also, the boundary layer is thickest for the nanofluid with MgO nanoparticles.

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