• MANASH KUMAR PAUL

      Articles written in Pramana – Journal of Physics

    • Langmuir probe study in the nonresonant current drive regime of helicon discharge

      Manash Kumar Paul Dhiraj Bora

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      Characterization of the current drive regime is done for helicon wave-generated plasma in a torus, at a very high operating frequency. A radiofrequency-compensated Langmuir probe is designed and used for the measurement of plasma parameters along with the electron energy distributions in radial scans of the plasma. The electron energy distribution patterns obtained in the operational regime suggest that Landau damping cannot be responsible for the efficient helicon discharge in the present study. A typical peaked radial density profile, high plasma temperature and absence of an appreciable amount of energetic electrons for resonant wave–particle interactions, suggest that the chosen operational regime is suitable for the study of nonresonant current drive by helicon wave. Successful and significant current drive achieved in our device clearly demonstrates the capability of nonresonant current drive by helicon waves in the present operational regime.

    • Antenna coupling study for ICWC plasma characterization in TEXTOR

      Manash Kumar Paul A Lyssoivan R Koch G Van Wassenhove M Vervier G Bertschinger R Laengner B Unterberg G Sergienko V Philipps T Wauters the TEXTOR Team

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      Ion cyclotron wall conditioning (ICWC) discharges, in pulsed-mode operation, were carried out in the limiter tokamak TEXTOR to explore safe operational regimes for the experimental parameters for possible ICWC-discharge cleaning in International Thermonuclear Experimental Reactor (ITER) at half field. Antenna coupling properties obtained during the ion cyclotron range of frequencies (ICRF) wall conditioning experiments performed in helium–hydrogen mixture in TEXTOR were analysed in relation to the obtained ICWC-plasma characterization results. Satisfactory antenna coupling in the mode conversion scenario along with reproducible generation of ICRF plasmas for wall conditioning, were achieved by coupling radio frequency (RF) power from one or two ICRF antennas. The plasma breakdown results obtained in the TEXTOR tokamak have been compared with the predictions of a zero-dimensional RF plasma production model. The present study of ICWC emphasizes the beneficial effect of application of an additional (along with toroidal magnetic field) stationary vertical ($B_{V} \ll B_{T}$) or oscillating poloidal magnetic field ($B_{P} \ll B_{T}$) on antenna coupling and relevant plasma parameters.

    • On multiple complex structure formations in expanding hollow cathode discharge

      SUBHOJIT BOSE SHAMIK CHAKRABORTY APARNA NATH MANASH KUMAR PAUL

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      A hollow cathode source is designed to generate multiple double layer (MDL) structures by the trapping of charged particles in the absence of any auxiliary plasma source or magnetic field, while the glowd is charge switches between two distinctly different stable regimes. The generation and dynamics of localised and intensely luminous MDL formations are experimentally investigated in low-frequency sheath oscillation inhibited glow discharge by modified Langmuir probes in a linear vacuum vessel. The localised complex structures grow axially into visibly glowing coaxial formations along with structural changes in MDL formations, on increasing the applied voltage in nitrogen plasma at 0.1 mbar pressure. The discharge gradually expands to fill up the entire vessel through redistribution of energetic charged particles and formation of multiple sheaths on increasing the applied voltage across the electrodes. The present experimental study reveals interesting information about the formation of thesheath, stability of the plasma sheath and charged particle dynamics during the discharge transformations. The paper relates the plasma parameter variations with the nonlinear time series analysis of the discharge current and floatingpotential fluctuations through the reconstructed phase space in different discharge domains to analyse the dynamics of MDL formations and harmonic generation through sheath oscillations during the hollow cathode dischargeregimes. The radial motion of the charged particles during the glow discharge between the semi-transparent gridded cylindrical cathode and the central anode might be useful for a better understanding of the underlying basic sheath physics related to particle acceleration in plasma as well as application in plasma sputtering.

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