R. Jha

Evidence of Lévy stable process in tokamak edge turbulence

The time series of floating potential and poloidal electric field fluctuations in the edge plasma of ohmically heated ADITYA tokamak [Phys. Plasmas 4, 4292 (1997)] are analyzed for self-similarity. It is observed that the distribution function of a sum of n data points converges to a self-similar distribution of Levy scale index, α=1.1–1.3 for n⩽40 and α=1.8–2.0 for larger n. This shows that the scaling properties of small scale fluctuations are non-Gaussian and those of large scale fluctuations are Gaussian. Implication of this observation to our understanding of plasma transport is discussed.

Observation of vortex-like coherent structures in the edge plasma of the ADITYA tokamak

Vortex-like coherent structures are observed in the edge plasma of ohmically heated ADITYA tokamak [ Phys. Rev. Lett. 69, 1375 (1992)]. The structures are observed on statistical basis when the floating potential fluctuations are analyzed using conditional averaging technique. The structures, which have dipole nature, experience stretching until their radial isolation across the limiter is destroyed. The potential fluctuation also shows non-Gaussian statistics indicating intermittency in broadband turbulence of the edge plasma.

Study of electromagnetic fluctuations in high beta plasma of a large linear device

Observation of electromagnetic fluctuations in lower hybrid range of frequencies is reported in a large volume linear plasma device. The instability is observed in the plasma core when a narrow multifilamentary source is used and it is absent when a broad source is used. This instability is observed in high beta plasma and it is characterized by broadband turbulent spectra with central frequency ω=5×104u2002s−1 and wave number k⊥=0.2u2002cm−1 and satisfies the condition k⊥ρe≤1, where ρe is the electron Larmor radius. When increasing the axial magnetic field reduces plasma beta, the instability weakens in magnitude and magnetic component is totally suppressed at plasma beta less than 0.5. Several possible explanations are considered and it is indicated that either the pressure gradient modified by energetic electrons or the electron temperature gradient may be responsible for the instability.

Radiation power measurement on the ADITYA tokamak

The radiation power loss and its variation with plasma density and current are studied in the ADITYA tokamak. The radiation power loss varies from 20% to 40% of the input power for different discharges. The radiation fraction decreases with increasing plasma current but it increases with increasing line-averaged central density. The radiated power behavior has also been studied in discharges with short pulses of molecular beam injection (MBI) and gas puff (GP). The increase in radiation loss is limited to the edge chords in the case of GP, but it extends to the core region for MBI fueling. The MBI seems to indicate reduction in the edge recycling. It is observed that during the density limit disruption, the radiated power loss is more in the current quench phase as compared with the thermal quench phase and comes mainly from the plasma edge.

Theory of coupled whistler-electron temperature gradient mode in high beta plasma: Application to linear plasma device

This paper presents a theory of coupled whistler (W) and electron temperature gradient (ETG) mode using two-fluid model in high beta plasma. Non-adiabatic ion response, parallel magnetic field perturbation (δBz), perpendicular magnetic flutter (δB⊥), and electron collisions are included in the treatment of theory. A linear dispersion relation for whistler-electron temperature gradient (W-ETG) mode is derived. The numerical results obtained from this relation are compared with the experimental results observed in large volume plasma device (LVPD) [Awasthi et al., Phys. Plasma 17, 42109 (2010)]. The theory predicts that the instability grows only where the temperature gradient is finite and the density gradient flat. For the parameters of the experiment, theoretically estimated frequency and wave number of W-ETG mode match with the values corresponding to the peak in the power spectrum observed in LVPD. By using simple mixing length argument, estimated level of fluctuations of W-ETG mode is in the range of ...

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 ∼7–10u2002kHz. 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.

Nondestructive test of brazed cooling tubes of prototype bolometer camera housing using active infrared thermography.

The active infrared thermography technique is used for assessing the brazing quality of an actively cooled bolometer camera housing developed for steady state superconducting tokamak. The housing is a circular pipe, which has circular tubes vacuum brazed on the periphery. A unique method was adopted to monitor the temperature distribution on the internal surface of the pipe. A stainless steel mirror was placed inside the pipe and the reflected IR radiations were viewed using an IR camera. The heat stimulus was given by passing hot water through the tubes and the temperature distribution was monitored during the transient phase. The thermographs showed a significant nonuniformity in the brazing with a contact area of around 51%. The thermography results were compared with the x-ray radiographs and a good match between the two was observed. Benefits of thermography over x-ray radiography testing are emphasized.

Estimation of Effective Responsivity of AXUV Bolometer in ADITYA Tokamak by Spectrally Resolved Radiation Power Measurement

The radiation emission from ADITYA Tokamak is routinely measured using AXUV bolometers [K. Tahiliani et al., Plasma Phys. Control. Fusion 51, 085004 (2009)] and the total radiation power loss is estimated from these measurements assuming constant responsivity. This assumption is valid for the current flattop phase of the discharge, where the contribution from long wavelength radiation (> 620 Å) is expected to be small and the AXUV responsivity is almost constant. It is likely that in disruptive discharges, with significant edge radiation, a part of the unaccounted power is in the long wavelength range. A better approach is to experimentally determine an effective responsivity by spectrally resolving the radiation power loss and assigning appropriate weights to spectral ranges [S.D. Gray et al., Rev. Sci. Instrum. 75, 376 (2004)]. For this purpose, we have installed a multichannel filtered bolometer camera in ADITYA Tokamak. The wide angle view camera houses three single channel AXUV bolometers, of which two view the plasma through different ultraviolet filters and one has an unfiltered view. All the bolometers have the same poloidal view and are located adjacently in the toroidal direction. The initial results of the spectrally resolved bolometer measurements show that the radiation in the spectral range > 1200 Å is significant fraction of the total radiation during the disruptive phase, but doesn’t contribute much during the flattop region. An effective average responsivity has been estimated for AXUV bolometer for ADITYA. c

Bolometers for Fusion Plasma Diagnostics

The thermonuclear fusion is one of the most seriously pursued alternative sources of energy for the future of mankind. The fusion energy is safer and cleaner compared to fission energy, produces no greenhouse gases and, the nuclear fuels are evenly distributed throughout the globe. Nuclear fusion is responsible for heat and radiation generated by the Sun. In the Sun, two atoms of hydrogen fuse together to produce helium. It has been determined that the fusion of the two hydrogen isotopes, namely deuterium (D) and tritium (T) that produces 17.6 MeV (mega electron-volt) of fusion energy, is feasible in a laboratory setting (Wesson, 2004). The D-T fusion however takes place at fairly high temperature of 1030 keV or, (1-3) uf0b4uf020107 K, which is necessary for deuterium and tritium nuclei to come close together to overcome electrostatic repulsion. At these thermonuclear temperatures, the atoms get stripped of all the electrons and form a plasma (electrically charged gas). Such plasmas can be confined in a desired region by using strong magnetic fields. The magnetic fields force the particles to spiral along the field lines thus confining them. The most promising magnetic confinement systems are toroidal in shape. Among the toroidal shaped plasma devices, tokamak is the most advanced one. Presently, ITER (International Thermonuclear Experimental Reactor) is the largest tokamak under construction at Cadarache, France, and JET (Joint European Torus) in Culham, UK is the largest operating tokamak. Other non-magnetic confinement systems are also being investigated. For example, the laser induced inertial confinement systems.