Debjyoti Basu

The MaPLE device of Saha Institute of Nuclear Physics: construction and its plasma aspects.

The Magnetized Plasma Linear Experimental (MaPLE) device is a low cost laboratory plasma device at Saha Institute of Nuclear Physics fabricated in-house with the primary aim of studying basic plasma physics phenomena such as plasma instabilities, wave propagation, and their nonlinear behavior in magnetized plasma regime in a controlled manner. The machine is specially designed to be a versatile laboratory device that can provide a number of magnetic and electric scenario to facilitate such studies. A total of 36 number of 20-turn magnet coils, designed such as to allow easy handling, is capable of producing a uniform, dc magnetic field of about 0.35 T inside the plasma chamber of diameter 0.30 m. Support structure of the coils is planned in an innovative way facilitating straightforward fabrication and easy positioning of the coils. Further special feature lies in the arrangement of the spacers between the coils that can be maneuvered rather easily to create different magnetic configurations. Various methods of plasma production can be suitably utilized according to the experimental needs at either end of the vacuum vessel. In the present paper, characteristics of a steady state plasma generated by electron cyclotron resonance method using 2.45 GHz microwave power are presented. Scans using simple probe drives revealed that a uniform and long plasma column having electron density approximately 3-5x10(10) cm(-3) and temperature approximately 7-10 eV, is formed in the center of the plasma chamber which is suitable for wave launching experiments.

A novel approach for mitigating disruptions using biased electrode in Aditya tokamak

Disruptions, induced in Aditya tokamak by hydrogen gas puffing, are successfully mitigated through stabilization of magnetohydrodynamic (MHD) modes by applying a bias voltage to an electrode placed inside the last-closed flux surface prior to the gas injection. Above a threshold voltage sheared Er???B? rotation of the plasma generated by the edge biasing leads to substantial reduction in the growth of MHD modes (m/n?=?3/1, 2/1), which causes avoidance of disruptions through prevention of mode overlapping and subsequent ergodization of magnetic field lines.

Oblique cone formation in Ge by Ar+ sputtering

Abstract Sharp oblique cones are only formed on polished germanium surfaces by 30 keV Ar + bombardment, in the presence of tungsten seed atoms; substantiating Wehners hypothesis that higher melting point seed material is the crucial factor for come formation. The morphology of the cone is characterized by the formation of a sloped ridge, consistent with the observations of earlier workers. Moreover, some strange wing-like structures are associated with these cones.

Suppression of electric and magnetic fluctuations and improvement of confinement due to current profile modification by biased electrode in Saha Institute of Nuclear Physics tokamak

Improvement of plasma confinement is achieved in normal qa discharges of SINP-tokamak by introducing a biased electrode inside the last closed flux surface. All the important features of high confinement mode are observed biasing the electrode negatively with respect to the vacuum vessel. Arrays of electric and magnetic probes introduced in the edge plasma region reveal suppression of electric and magnetic fluctuations over distinct frequency ranges as well as modification of the toroidal current profile due to biasing. Further analysis identifies the electrostatic fluctuations to be due to drift mode and the magnetic fluctuations may be of slow compressional Alfven waves. Both get suppressed due to current profile modification during biasing, hence leading to the improvement of plasma confinement.

The angular dependence of sputtering yields of Ge and Ag

Abstract The angular dependence of physical sputtering yields of Ge and Ag was investigated for 30 keV Ar+ and Kr+ ion bombardment. It is found that Ag exhibits a weaker angular dependence, though its sputtering yield is higher than that of Ge. The results are compared to the existing theoretical and empirical formulas.

Disruption avoidance in the SINP-Tokamak by means of electrode-biasing at the plasma edge

Control of plasma disruption by a biased edge electrode is reported in SINP-Tokamak. The features that characterize a plasma disruption are reduced with increasing bias potential. The disruption can be completely suppressed with the concomitant stabilization of observed MHD modes that are allegedly precursors of the disruption. An m = 3/n = 1 tearing mode, which apparently causes disruption can be stabilized when a negative biasing potential is applied near the edge. These changes in the disruptive behavior with edge biasing are hypothesized to be due to changes in the current density profile.

Modification of edge current profile and improved confinement induced by biased electrode in the very low qa (VLQ) discharges of SINP ? tokamak

A better energy-confined state with modified current density profile is achieved by fast biasing (applied voltage rise time ~ the ion?ion collision time) an electrode placed inside the limiter in very low qa discharges of the Saha Institute of Nuclear Physics?tokamak (SINP?Tokamak) (Lahiri et al 1996 Nucl. Fusion 36 254). The plasma current increases and lasts for an extended period. Profile modification of the radial electric field (Er) in the edge region is also observed in the better-confined state. However, j?-profile modification as well as energy confinement time improvement occurs prior to Er-profile modification.

Transmission electron microscope studies of tantalum irradiated by MeV energy α particles

Abstracts are not published in this journal

Dependence of up–down asymmetry on the edge safety factor in the Saha Institute of Nuclear Physics–Tokamak (SINP–Tokamak)

Inside the limiter of Saha Institute of Nuclear Physics–Tokamak (SINP–Tokamak) [A. N. Sekar Iyengar, S. K. Majumdar, J. Basu, R. K. Paul, R. Pal, and S. Chowdhury, Pramana, J. Phys. 39, 181 (1992)] plasma potential was investigated using Langmuir probe arrays with poloidal and radial resolution and as a function of the safety factor at the limiter. A poloidal asymmetry was observed in floating potential in the edge region. Quantitatively, its up–down difference exhibits an almost square of safety factor divided by toroidal magnetic field-like scaling. A radial electric field, also observed in this region, was seen to increase with the edge safety factor. The plasma seems to be collisional here with the mean free path shorter than the connection length, where Pfirsch–Schluter current causes a qualitatively similar up–down asymmetry. However, its magnitude was seen to be much lower than the observed results. It has been shown that the presence of radial and/or toroidal component of the electric and magnetic...

Development of a simple electrolytic jet thinning apparatus for the preparation of thin metal foils for transmission electron microscopy

A very simple and inexpensive electrolytic jet thinning apparatus for rapid thinning of metal foils for transmission electron microscopy has been described. Regarding the specimen holder assembly, either a polytetrafluoroethylene (PTFE) holder or a platinum tipped locking tweezer‐type holder, can be attached electrically and mechanically to a micromanipulator to be used in the same jet thinning apparatus. This gives a unique feature to our unit, in which the specimen remains above the electrolyte during the polishing process. Also the constructional simplicity of the unit provides easy drainage of the electrolyte for lengthy polishing and a simple way of stopping the thinning process. Transmission electron micrographs of bombarded and unbombarded specimens electrothinned in the unit are presented herein.