R. Manchanda

GRB 010921: Localization and Observations by the High Energy Transient Explorer Satellite

On 2001 September 21 at 05:15:50.56 UT, the French Gamma Telescope (FREGATE) on the High Energy Transient Explorer (HETE) detected a bright gamma-ray burst (GRB). The burst was also seen by the X-detector on the Wide-field X-ray Monitor (WXM) instrument and was therefore well localized in the X-direction; however, the burst was outside the fully coded field of view of the WXM Y-detector, and therefore information on the Y-direction of the burst was limited. Cross-correlation of the HETE and Ulysses time histories yielded an Interplanetary Network (IPN) annulus that crosses the HETE error strip at an ~45° angle. The intersection of the HETE error strip and the IPN annulus produces a diamond-shaped error region for the location of the burst having an area of 310 arcmin2. Based on the FREGATE and WXM light curves, the duration of the burst is characterized by t90 = 34.2 s in the WXM 4-25 keV energy range, and 23.8 and 21.8 s in the FREGATE 6-40 and 32-400 keV energy ranges, respectively. The fluence of the burst in these same energy ranges is 4.8 × 10-6, 5.5 × 10-6, and 11.4 × 10-6 ergs cm-2, respectively. Subsequent optical and radio observations by ground-based observers have identified the afterglow of GRB 010921 and determined an apparent redshift of z = 0.450.

Spectral-Lag Relations in GRB Pulses Detected with HETE-2

Using a pulse-fit method, we investigated the spectral lags between the traditional gamma-ray band (50–400 keV) and the X-ray band (6–25 keV) for 8 GRBs with known redshifts (GRB 010921, GRB 020124, GRB 020127, GRB 021211, GRB 030528, GRB 040924, GRB 041006, and GRB 050408), detected with the WXM and FREGATE instruments aboard the HETE-2 satellite. We found several relations for individual GRB pulses between the spectral lag and other observables, such as the luminosity, pulse duration, and peak energy, Epeak. The obtained results are consistent with those for BATSE, indicating that the BATSE correlations are still valid at lower energies (6–25 keV). Furthermore, we found that the photon energy dependence for the spectral lags can be reconciled with the simple curvature effect model. We discuss the implications of these results from various points of view.

Characterization of helium discharge cleaning plasmas in ADITYA tokamak using collisional-radiative model code

The wall conditioning of the ADITYA tokamak is usually done, by first producing an electron cyclotron resonance (ECR) plasma inside the vessel and then superimposing a pulsed ohmic discharge on the ECR background. Sometimes, helium gas is used as a working gas. In this article, the spectral line emissions of neutral helium for the two different plasmas—namely, the ECR and the pulsed discharge cleaning (PDC) plasmas—have been analyzed using a collisional-radiative (CR)-model code to estimate the electron density and temperature. We are able to match the experimentally obtained relative intensity ratios with those predicted by the model under the assumption of ionizing plasma condition if the possible effects of the metastable states are not ignored. This has been done by using the populations of two metastable levels (2S1 and 2S3) as independent parameters in addition to the ground states of neutrals and ions in the CR model under a quasisteady-state approximation. It is further seen that, it is the metast...

Space- and time-resolved visible-emission spectroscopy of Aditya-tokamak discharges using multi-track spectrometer

A diagnostic system based on a multi-fiber input high resolution spectrograph has been set up on the Aditya tokamak (Bhatt et al 1989 Ind. J. Pure Appl. Phys. 27 710) for utilizing the passive light emission to measure different kinds of plasma flow and to identify the location of emissions of hydrogen and impurities along with their temperatures. Eight simultaneous vertically collimated lines-of-sight from a top port view a poloidal cross-section of the plasma. This arrangement simplifies the analysis of spectra in terms of making the Zeeman splitting easier to account for, since each chord passes through a region of nearly constant toroidal magnetic field (BT). This paper describes the complete set-up, the wavelength and intensity calibrations performed and the initial results including the impurity emissivity profiles and simultaneous flow measurements in the inboard and outboard regions of the Aditya tokamak.

Investigation of oxygen impurity transport using the O4+ visible spectral line in the Aditya tokamak

Intense visible lines from Be-like oxygen impurity are routinely observed in the Aditya tokamak. The spatial profile of brightness of a Be-like oxygen spectral line (2p3p 3D3?2p3d 3F4) at 650.024?nm is used to investigate oxygen impurity transport in typical discharges of the Aditya tokamak. A 1.0?m multi-track spectrometer (Czerny?Turner) capable of simultaneous measurements from eight lines of sight is used to obtain the radial profile of brightness of O4+ spectral emission. The emissivity profile of O4+ spectral emission is obtained from the spatial profile of brightness using an Abel-like matrix inversion. The oxygen transport coefficients are determined by reproducing the experimentally measured emissivity profiles of O4+, using a one-dimensional empirical impurity transport code, STRAHL. Much higher values of the diffusion coefficient compared with the neo-classical values are observed in both the high magnetic field edge region and the low magnetic field edge region of typical Aditya ohmic plasmas, which seems to be due to fluctuation-induced transport. The diffusion coefficient at the limiter radius in the low-field (outboard) region is typically ? twice as high as that at the limiter radius in the high-field (inboard) region.

Evolution of the periodicities in 2S 0114+650

Physical Research Laboratory, Navrangpura, Ahmedabad 380009, IndiaAbstractWe have analysed nine years of data from the All Sky Monitor on the Rossi X-rayTiming Explorer for 2S 0114+650 to study the evolution of its spin, binary andsuper-orbital periods. The spin history of the neutron star in this system exhibitstorque reversals lasting ∼ 1 yr. The newly discovered super-orbital period has re-mained stable over the 9-yr span, making 2S 0114+650 the fourth known systemto exhibit stable super-orbital modulation. We compare its super-orbital periodevolution with those of the other three such systems.Key words: X-rays: binary - stars: neutron - accretion: accretion discs - stars:individual (2S 0114+650)

Influence of Wall Conditioning on ADITYA Plasma Discharges

ADITYA (R0 = 75 cm, a = 25 cm), an ohmically heated circular limiter tokamak is regularly being operated to carry out several experiments related to controlled thermonuclear fusion research. In recent operational campaign, various experiments have been carried out to enhance the discharge performance as well as improve the plasma parameters. A comparative plasma discharges study with SiC and Graphite limiter was carried out to increase the plasma heating and reduce runaways. Excellent plasma heating has been observed in many discharges using Graphite limiter. Good repeatability of low hard X-rays, high temperature discharges was obtained. The control of plasma impurities and hydrogen recycling is very much essential for high performance discharges. The wall conditioning in ADITYA tokamak is carried out by hydrogen glow discharge cleaning (GDC), Pulse discharge cleaning and electron cyclotron resonance (ECR) discharge cleaning techniques with and without lithium wall coating. GDC assisted Lithiumization was found to be the most effective technique for substantial reduction in Ha and low Z (CIII & O-I) impurities. The partial pressure of mass number 18 (H2O) and 28 (N2/C2H4/CO) were regularly monitored before plasma discharge operation. Furthermore, experiment on optimization of pulse gas feed was helped in reducing wall loading and recycling. However, hard X-rays suppression with the application of multiple gas puff has been successfully achieved during negative converter operation, which led to the extension of plasma pulse length up to ~ 250 ms. All the supporting facts and operation aspects are reported.

Direct electron heating experiment on the Aditya tokamak using fast waves in the ion cyclotron resonance frequency range

Second harmonic heating experiments using fast waves are carried out on the Aditya tokamak in the ion cyclotron resonance frequency (ICRF) range with the help of a 200?kW, 20?40?MHz RF heating system, which is developed indigenously. Significant direct electron heating is observed in a hydrogen plasma. The rise in electron temperature is prompt with the application of RF power and the increment in electron temperature increases linearly with RF power. A corresponding increase in plasma beta and hence an increase in stored diamagnetic energy are also observed in the presence of RF power. The low-Z impurity radiation and electron density do not increase significantly with RF power. The direct electron heating by fast wave in Aditya is also predicted by the ion cyclotron resonance heating code TORIC.

Improvement of Plasma Performance with Lithium Wall Conditioning in Aditya Tokamak

Lithiumization of the vacuum vessel wall of the Aditya tokamak using a lithium rod exposed to glow discharge cleaning plasma has been done to understand its effect on plasma performance. After the Li-coating, an increment of ~100 eV in plasma electron temperature has been observed in most of the discharges compared to discharges without Li coating, and the shot reproducibility is considerably improved. Detailed studies of impurity behaviour and hydrogen recycling are made in the Li coated discharges by observing spectral lines of hydrogen, carbon, and oxygen in the visible region using optical fiber, an interference filter, and PMT based systems. A large reduction in O I signal (up to ~40% to 50%) and a 20% to 30% decrease of Hα signal indicate significant reduction of wall recycling. Furthermore, VUV emissions from O V and Fe XV monitored by a grazing incidence monochromator also show the reduction. Lower Fe XV emission indicates the declined impurity penetration to the core plasma in the Li coated discharges. Significant increase of the particle and energy confinement times and the reduction of Zeff of the plasma certainly indicate the improved plasma parameters in the Aditya tokamak after lithium wall conditioning.

Drift-Alfven waves induced optical emission fluctuations in Aditya tokamak

In Aditya tokamak [S. B. Bhatt et al. Indian J. Pure Appl. Phys. 27, 710 (1989)], an increase in the Hα and C2+ intensity fluctuations from the edge region is observed with an increase in the magnetohydrodynamic (MHD) activity. Very small fluctuation amplitudes of Hα and C2+ intensity are observed in discharges where there is no MHD activity compared to the discharges with MHD activity. These fluctuations in the Hα and C2+, measured by optical filter—photomultiplier tube combination—are modulated by Mirnov oscillations having a dominant peak with a common frequency ∼7–10 kHz. Further investigation reveals the presence of strong coherent fluctuations in density and floating potential at same frequency as well. These observations indicate the existence of a nonelectrostatic instability, which may be based on the coupled mode of the drift mode and the Alfven mode. The coherent density fluctuations give rise to the experimentally observed coherent Hα and C2+ intensity fluctuations.