• G RAVI

      Articles written in Pramana – Journal of Physics

    • Anomalous magnetic hysteresis loops and smallHc1 values in highTc superconductors

      A K Grover C Radhakrishnamurty P Chaddah G Ravi Kumar G V Subbarao

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      We have studied the hysteresis loops of RBa2Cu3O7 (R=Gd, Ho and Y) and detected anomalies in some of them. The observed anomalies support a recent prediction by Ravi Kumar and Chaddah based on an extension of Bean’s model. The anomalies indicate lowHc1 values and we have confirmed this by studying the onset of low-field hysteresis in less than 10 Oe at 77 K for these highTc superconductors.

    • Measurement and understanding of magnetization in AC and DC fields and the determination of intragrainHc1 in highTc RBa2Cu3O7 superconductors

      A K Grover C Radhakrishnamurty P Chaddah G Ravi Kumar G V Subba Rao

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      We present DC and low frequency AC magnetization measurements on various RBa2Cu3O7 superconductors. We identify features intrinsic to these compounds, and establish the features originating from intergranular links in sintered pellets. The isothermal magnetization curves, and the temperature dependence of magnetization in field-cooled and zero field-cooled states are shown to be consistent with the calculations done following a recent extension of Bean’s model. Low field anomalies predicted within this model are observed, and yieldHc1 values of a few Oe. These values are shown to be consistent with the temperature variation of magnetization. A comparison is made with the other existing data and it is demonstrated that earlier quoted values ofHc1 are gross overestimates.

    • Magnetic field dependence of critical current density in YBa2Cu3O7

      P K Mishra G Ravi Kumar P Chaddah B A Dasannacharya M K Malik

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      We have measured the transport critical current densityJcof sintered YBa2Cu3O7, in various applied fields up to 185 Oe at 77 K. We find a sharp decay ofJcwith magnetic field. We show that this sharp decay is consistent with the low field hysteresis results of Groveret al. We argue that the observed field dependence is not caused by intragranular weak links.

    • A new linear plasma device for the study of plasma waves in the electron magnetohydrodynamics regime

      GARIMA JOSHI G RAVI S MUKHERJEE

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      A new, user-friendly, linear plasma device has been developed in our laboratory where a quiescent ($\Delta n/n \approx 1%$), low temperature (1–10 eV), pulsed (3–10 ms) plasma can be produced over a large uniform region of 30–40 cm diameter and 40 cm length. Salient features of the device include the flexibility of tuning the plasma density in the range of $10^{10}$ to $10^{12} \rm{cm}^{−3}$ and capability of scanning the plasma and field parameters in two dimensions with a precision of < 1 mm. The plasma is produced by a multifilamentary cathode and external magnetic field by Helmholtz coils, both designed and constructed in-house. The plasma parameters can be measured by Langmuir probes and electromagnetic field parameters by miniature magnetic probes and Rogowski coils. The plasma produced is uniform and essentially unbounded for performing experiments on waves and turbulence. The whole device can be operated single-handedly by undergraduate or graduate students. The device can be opened, serviced, new antennas/probes installed and ready for operation in a matter of hours. Some results on the excitation of electromagnetic structures in the context of electron magnetohydrodynamics (EMHD) are also presented to demonstrate the suitability of the device for carrying out such experiments.

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