SINP Tokamak Research Articles

Interplay of transitions between oscillations with emergence of fireballs and quantification of phase coherence, scaling index in a magnetized glow discharge plasma in a toroidal assembly

Interplay of transition of floating potential fluctuations in a glow discharge plasma in the toroidal vacuum vessel of SINP tokamak has been observed. With variation in the strength of the vertical and toroidal magnetic fields, regular and inverted relaxation oscillations as well as sinusoidal oscillations are observed with the slow and fast time scale of the relaxation oscillations reversing their nature at a high value of vertical magnetic field strength. However for small value of toroidal magnetic field the transitions follow relaxation → chaotic oscillations with the chaotic nature prevailing at higher values of toroidal magnetic field. Evolution of associated anode fireball dynamics under the action of increasing vertical, toroidal as well as increasing vertical field at a fixed toroidal field (mixed field) of different strength has been studied. Estimation of phase coherence index for each case has been carried out to examine the evidence of finite nonlinear interaction. A comprehensive study of the dynamics of the fireball is found to be associated with the values of phase coherence index. The index is found to take maximum values for the case of toroidal, mixed field when there is an existence of power/energy concentration in a large region of frequency band. A detailed study of the scaling region using detrended fluctuation analysis (DFA) by estimating the scaling exponent has been carried out for increasing values of discharge voltage (DV), vertical, toroidal as well as the mixed field (toroidal plus vertical). A persistence long range behaviour associated with the nature of the anode glow has been investigated in case of higher values of toroidal, mixed field whereas increasing DV, vertical magnetic field lead to a perfectly correlated dynamics with values of scaling exponent greater than unity.

Hysteresis of fluctuation dynamics associated with a fireball in a magnetized glow discharge plasma in a currentless toroidal assembly

Floating potential fluctuations associated with an anode fireball in a glow discharge plasma in the toroidal vacuum vessel of the SINP tokamak are found to exhibit different kinds of oscillations under the action of vertical magnetic field of different strengths. While increasing the vertical magnetic field, the fluctuations have shown transitions as: chaotic oscillation → inverse homoclinic transition → intermittency → chaotic oscillation. However, on decreasing the magnetic field, the fluctuations are seen to follow: chaotic oscillations → homoclinic transition → chaotic oscillation; that is the intermittent feature is not observed. Fireball dynamics is found to be closely related to the magnetic field applied; results of visual inspection with a high speed camera are in close agreement with the fluctuations, and the fireball dynamics is found to be closely related to the transitions. The statistical properties like skewness, kurtosis, and entropy of the fluctuations are also found to exhibit this hysteresis behaviour.

Comparative study of runaway electron diffusion in the rise phase of low q a and normal q a discharges in the SINP tokamak

The behaviour of runaway electrons in the SINP tokamak, which can be operated in a normal edge safety factor (qa) (NQ) discharge configuration as well as in a low qa (LQ) configuration, was experimentally investigated, during the initial plasma generation phase. An energy analysis of the runaway electron dynamics in the rise phase of the SINP tokamak discharges was also made. A comparison of the runaway electron diffusion coefficients in NQ and LQ is carried out in this paper.

Radial transport of the nonthermal electrons in runaway discharges in the SINP tokamak from analysis of hard x-ray fluctuations

X-ray bursts from nonthermal electrons (∼10 keV to 1 MeV) emanating from the core plasma and the limiter are measured simultaneously in runaway dominated discharges in the SINP tokamak using a tangentially viewing NaI(Tl) detector and a detector that views the limiter areas. The radial transport coefficient of the nonthermal electrons has been derived from the cross-correlation function of the two x-ray intensities. Two phases of the plasma discharges with distinct transport of the nonthermal electrons are found in these experiments. The stochastic magnetic field fluctuations obtained from the transport coefficients are found to have a relationship with the autocorrelation functions of the x-ray emissivity signals.

Intermittency in the heat and particle transports in the SINP tokamak scrape-off layer

The intermittent heat and particle transports have been studied in the scrape-off layer of the SINP tokamak. Properties of the plasma structures, responsible for the intermittency, have been measured by the conditional averaging technique. The probability distribution functions of the fluctuations, including temperature fluctuations, obey non-Gaussian statistics. Wavelet analysis has shown that the cross-correlation between two probes is also intermittent in time and is connected to the passage of plasma structures. The structures decay in density as they move radially outward but their temperature is found to decay more rapidly.

Novel approach to Abel inversion

Simple yet versatile, physically valid emissivity functions for peaked and hollow profiles with only two determinable parameters are proposed for performing Abel inversion. The advantages of the proposed functions have been explored. The inversion is very fast, accurate, convenient, and viable, in contrast to the existing methods. The validation of these functions has been confirmed by using simulated data under various conditions. The error in the process has been computed and found to depend on the functional form of the model emissivity. A comprehensive comparison has been drawn with the existing method and it has been found to offer a definite advantage over the existing technique in some respects, especially for real time applications. Limitation of this technique has also been discussed. The soft x-ray and visible light emissivity profile of SINP tokamak has been successfully obtained by using this method.

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, jΦ-profile modification as well as energy confinement time improvement occurs prior to Er-profile modification.

Observation of high energy ion tail in the SINP tokamak plasma

The present study examines the possibility of acceleration of ions by large scale magnetohydrodynamic activities in the SINP tokamak. The analysis is done with a retarding field ion energy analyzer looking into the plasma in the tangential direction. The experiment shows the presence of high energy ions escaping the confinement zone along with the bulk low energy ions. This process seems to have a close connection with electron acceleration due to the electric field of the m∕n=2∕1 tearing modes.

Intermittent transport in the scrape-off layer of the SINP tokamak

Plasma structures, localized in the two-dimensional poloidal cross section propagating radially and poloidally, have been detected in the scrape-off layer of the SINP tokamak. Structures with densities both higher and lower than the density of the background plasma are present, with electron temperatures higher and lower, respectively, than that of the background plasma. These are responsible for the large and sporadic bursts of particle flux to the wall, amounting to 30%–40% of the total cross-field flux. These intermittent structures also cause the probability distribution function of the fluctuations to deviate from the Gaussian behavior, especially away from the limiter.

Negative current profile measurements in tokamaks using internal Rogowskii coil

An internal Rogowskii coil (IRC) has been developed to measure current profiles within SINP tokamak discharges. The present article has highlighted measurements using the IRC in SINP tokamak discharges exhibiting extension in phase after the discharge decays to approximately half its peak value. The IRC current profile results have been compared to that obtained from an internal Langmuir-magnetic probe system and have been observed to measure current profile directly (without employing any reconstruction methods) as well as the localized fluctuations much better than the latter. Interesting observation of negative currents in such Ohmic discharges is reported for the first time. The presence of negative currents has been observed to help in improving the confinement of the discharges. These results are important especially in relation to recent observations of “current holes,” in large fusion devices (such as JET and JT-60U), enhancing confinement properties.

Effect of poloidal velocity shear on the edge fluctuations in the SINP tokamak

The fluctuations of potential, density and electron temperature have been measured in the edge of the SINP tokamak using Langmuir probes. A naturally occurring extended E × B velocity shear region has been found to exist near the limiter. The poloidal and radial correlation lengths decrease in this region, indicating a spatial decorrelation induced by the velocity shear. The phase angles of the fluctuating signals are also different in the shear region. The turbulent transport drops, causing a steepening of the density and temperature profiles, resembling the characteristics of the high confinement modes in tokamaks.

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 fields in SINP–Tokamak can contribute significantly to the observed asymmetry.

Temperature fluctuations and turbulent transport at the edge of the SINP tokamak

Density, temperature and potential fluctuations have been measured in the edge region and the scrape off layer (SOL) of the SINP tokamak. The triple probe technique has been used to obtain the instantaneous value of these quantities simultaneously and virtually at the same spatial position. The fluctuation levels thus found are significant (T̃e/Te≈20–35% and ñ/n≈35–55%) and the spectra are broadband. The effect of temperature fluctuation on the turbulent transport has also been investigated and found to be considerable. The measured diffusion coefficient comes out to be about twice the Bohm diffusion coefficient. Radial profiles show sharp density and temperature gradients and there exists a self-generated Er×B velocity shear layer near the limiter. The results have been compared with the existing theories of edge turbulence in tokamaks and the rippling mode driven by the resistivity gradient has been identified as the most probable mechanism.

Investigations of low q a discharges in the SINP tokamak

Low edge safety factor discharges including very low q a (1<q a <2) and ultra low q a (0<q a <1) have been obtained in the SINP tokamak. It has been observed that accessibility of these discharges depends crucially on the fast rate of plasma current rise. Several interesting results in terms of different time scales like T q a · τ R etc have been obtained using a set of softwares developed at SINP. From fluctuation analysis of the external magnetic probe data it has been found that MHD instabilities m=1, n=1 and m=2, n=1 etc. play major role in the evolution of these discharges. To investigate the internal details of these discharges, an internal magnetic probe system has been developed using which current density j φ and other related parameters have been estimated. By carrying out a resistive stability analysis, evidence of the above-mentioned MHD instabilities have again been found. The physical processes lying behind the accessibility and evolution of the low q a discharges have been thoroughly investigated.

Measurement of plasma diamagnetism in the SINP tokamak by a flux loop system inside the vacuum vessel

Plasma diamagnetism has been measured in the SINP tokamak by a toroidal flux loop placed inside the vacuum vessel. The flux due to the strong toroidal field has been compensated for by a coplaner annular loop which encircles but does not contain the plasma column. The influence of the eddy currents in the vacuum vessel and the conducting shell in these loops has been calculated analytically by a circuit model using the theory of linear networks and compensated accordingly. This method has been shown to yield an almost exact compensation for toroidal flux (∼0.01%) as well as pickups from other fields. Typical results with plasma shots have been presented.

Origin of the inversion of up-down potential asymmetry inthe magnetized toroidal plasma

The up-down potential asymmetry in the toroidal magnetic plasma iscaused by the separation of ions and electrons due to the toroidicity of theconfining magnetic field and/or due to the effect of flow along the verticalmagnetic field. In the SINP tokamak an interesting phenomenon of inversion ofup-down asymmetry during a discharge has been observed experimentally. Theorigin of this up-down potential asymmetry and its inversion is attributed totwo oppositely directed vertical velocities, causing the separation of chargedspecies.

Measurement of temperature fluctuations and anomalous transport in the SINP tokamak

Temperature fluctuations have been measured in the edge region of the SINP tokamak. We find that these fluctuations have a comparatively high level (30–40%) and a broad spectrum. The temperature fluctuations show a quite high coherence with density and potential fluctuations and contribute considerably to the anomalous particle flux.

Observation of floating potential asymmetry in the edge plasma of the SINP tokamak

Edge plasma properties in a tokamak is an interesting subject of study from the view point of confinement and stability of tokamak plasma. The edge plasma of SINP-tokamak has been investigated using specially designed Langmuir probes. We have observed a poloidal asymmetry of floating potentials, particularly the top-bottom floating potential differences are quite noticeable, which in turn produces a vertical electric field (Ev). This Ev remains throughout the discharge but changes its direction at certain point of time which seems to depend on applied vertical magnetic field (Bv).

Ion Temperature Measurement in the Edge Region of SINP Tokamak by a Retarding Field Analyser

Based on the principle of retarding field analyser an ion energy analyser has been developed for the measurement of ion temperature in the edge plasma region of the SINP tokamak. This has been used to determine the dependence of the ion temperature on the safety factor q in the normal q region (q=2 to 4), plasma current, plasma electron temperature and also the Spitzer resistivity.

Real time measurement of plasma position in the SINP tokamak

An electronic hardware has been developed for the real time determination of the position of the plasma column in the SINP tokamak. The method is based on the Fourier integration of the poloidal magnetic field using cos θ and sin θ coils. The spurious pickups in these coils due to time varying magnetic fields have been explained by a model as being due to eddy currents in the vacuum vessel and the conducting shell. These pickups have been compensated in the hardware by simulating the effects of the eddy currents by linear networks. This diagnostic provides outputs proportional to the horizontal and vertical displacements of the plasma column at three toroidal locations simultaneously. An output proportional to the plasma current is also available. Performance evaluation shows that for both plasma current and displacements, the hardware outputs are in good agreement with the values computed by software.