Rabindranath Pal

Accessibility of very low qa (VLQ) and ultra-low qa (ULQ) discharges in the SINP tokamak

Attempts to operate a tokamak at qa < 2 are normally found to terminate with a major disruption. In the SINP tokamak, however, it has been found possible to operate at qa < 1 with comparative ease. In this letter, an attempt is made to quantify the rate of current rise required to set up discharges having qa < 2 and qa < 1 by comparing the risetime with the resistive diffusion timescale. It has been observed that discharges with qa less than a particular value can be obtained if the time taken to cross the previous qa barrier is less than the resistive diffusion time. When the resistive diffusion time is less than the time taken to cross a particular qa barrier, it becomes extremely difficult to cross the next lower qa barrier

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.

Runaway electron studies in the startup phase of very low edge safety factor (qa) (VLQ) discharges in the SINP tokamak

Two interesting features of runaway behaviour are observed in the startup phase of the very low qa (VLQ) (1 < qa < 2) discharges in the SINP tokamak. As qa was reduced by lowering the toroidal field values, the hard X (HX) ray intensity (which indicates the presence of runaway electrons) fell considerably. The time at which the peak in the HX ray intensity occurs also shifts ahead in time, indicating premature dumping of the runaway electrons. This seems to aid the development of the rest of the discharge as exhibited by the diamagnetic loop signal.

Investigation of long-range temporal correlation in electron cyclotron resonance produced linear magnetized plasma of the MaPLE device

Long range temporal correlation of the low frequency fluctuations is investigated in a linear electron cyclotron resonance produced magnetized plasma at different radial positions and filling gas pressures. These fluctuations turn from chaotic to coherent and again chaotic as one moves radially outwards from the center towards the edge region. The power spectrum of these fluctuations shows three distinct frequency regions characterized by their power exponents. Long range temporal correlation of these fluctuations is investigated by estimating the self similarity parameter (Hurst exponents) using rescaled range (R/S) statistics as well as from power spectrum analysis. Dependence of this long range temporal correlation on filling gas pressure has also been investigated.

Breakdown and preionization experiments in the SINP tokamak

Breakdown phenomena in the SINP tokamak are studied experimentally for different values of filling pressure (p), toroidal electric field (E), toroidal magnetic field and vertical magnetic field by using a hot filament preionization system. Breakdown had been achieved for a wide range of E/p, starting from 100 up to 4000 V.cm-1.torr-1. In the present experiment it has been observed for the first time that there exists an optimum value of the electric field on either side of which the breakdown time increases. The experimental results have been explained in terms of a model by Hutchinson and Strachan (1974), invoking particle drift compensation by runaway electrons during the breakdown phase in the tokamak. Previously, in this kind of runaway electron dominated breakdown a monotonic increase of the breakdown time: with E was observed. The effect of the preionization strength has also been studied with the help of a radio frequency (RF) system. The breakdown time could be shortened to within a few ionization times ( approximately=15 mu s) by RF preionization

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.

Simultaneous measurements of plasma parameters in the Saha Institute of Nuclear Physics Tokamak using a new low cost optoisolator

In the Saha Institute of Nuclear Physics Tokamak (SINP-TOKAMAK), simultaneous measurement of signals from some diagnostics, like, electrostatic probes, etc., is not possible because pickup noises due to the formation of ground and other loops affect them quite severely. To avoid these loops and also to isolate some diagnostics from the ground, we developed a new type of optical isolator using a very low cost optical isolator chip (MC2TE made by Motorola) in a novel configuration. We continuously kept the light emitting diode on and used a feedback arrangement at the receiving side of the photoemitted signal for achieving good isolation for very low signals with reasonably high frequency response and good linearity. The method is highly cost effective in low budget tokamak experiments.

Experimental observation of electron-acoustic wave propagation in laboratory plasma

In the field of fundamental plasma waves, the direct observation of electron-acoustic wave (EAW) propagation in laboratory plasmas remains a challenging problem, mainly because of heavy damping. In the Magnetized Plasma Linear Experimental device, the wave is observed and seen to propagate with the phase velocity ∼ 1.8 times the electron thermal velocity. A small amount of cold, drifting electrons, with the moderate bulk to cold temperature ratio ( ≈ 2 − 3), is present in the device. It plays a crucial role in reducing the damping. Our calculation reveals that the drift relaxes the stringent condition on the temperature ratio for wave destabilization. Growth rate becomes positive above a certain drift velocity even if the temperature ratio is moderate. The observed phase velocity agrees well with the theoretical estimate. Experimental realization of the mode may open up a new avenue in the EAW research.

Formation of annular plasma downstream by magnetic aperture in the helicon experimental device

In the Helicon eXperimental (HeX) device, the geometric aperture is fixed, but the position of the magnetic aperture can be varied. Working with Argon gas in the pressure range of 1 − 10 × 10 − 4 mbar, an annular plasma (density ∼ 10 16 m − 3) is formed downstream, always in front of the magnetic aperture. This occurs irrespective of the relative position of the geometric aperture or the presence of a radial electric field. This is in contrary to the earlier proposition made by others that a radial electric field is necessary to produce a hollow plasma profile. Instead, the ionization of neutrals in the radially outer region by the tail electrons, rotating fast due to gradient-B drift in the azimuthal direction, seems to account for the observed off-axis density peaking in the present experiment. This also explains the variation of the plasma annulus diameter seen here by changing the input radio frequency power ( 100 − 800 W ).