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[en] Neutral beam heated tokamak plasma may have higher ion temperature than electron temperature in scrapeoff layer (SOL) region. In this case, ion temperature can modify the SOL sheath dynamics mainly by increasing ion loss into the limiter/divertor plate. It can also increase total diamagnetic drift current. As the sheath dynamics and diamagnetic current have a direct influence on the plasma stability, the SOL plasma turbulence and transport will be altered. In this work, for the first time, we have studied the SOL turbulence and transport including ion temperature dynamics. Our model consists of flux driven two dimensional (2D) density, potential, electron and ion temperatures (fourfield) fields based on interchange instability. The four fields have been solved numerically. Simulation shows that plasma sustains higher ion temperature than the electron temperature in the turbulent saturated state. Ion temperature fluctuation is also higher compared to the electron temperature. Statistical properties of density, potential, particle, and energy fluxes and a comparison with experimental results are presented. A specific attention has been given to identify the effect of ion temperature in this region by comparing the numerical simulation results with the three field model (density, potential, and electron temperature) results. It is found that ion temperature reduces the SOL width and is closer to the experimentally observed value. It also reduces zonal flows, and amplitudes of inward particle and electron energy fluxes. (author)
[en] Turbulence in Scrape-off layer (SOL) of tokamak plasma has been studied numerically using interchange modes with the help of electron continuity, quasineutrality, and ion energy equations. Electron temperature is assumed uniform. We have studied dynamics of seeded plasma blob and plasma turbulence to identify the role of ion temperature and its gradient. The ion temperature elongates the blob poloidally and reduces its radial velocity. Initial dipole nature of the plasma blob potential breaks and generates few more dipoles during its propagation in the SOL. Plasma turbulence simulation shows poloidally elongated density and ion temperature structures that are similar to the seeded blob simulation studies. Fluctuations of the density and ion temperature have been presented as function of scale lengths of the density and ion temperature. Reduction of the SOL width and increase of radial electric field have been measured in the presence of the ion temperature. Particle and energy transports have been also presented as the function of the density and ion temperature scale lengths.
[en] Neutral gas in scrape-off layer of tokamak plasma plays an important role as it can modify the plasma turbulence. In order to investigate this, we have derived a simple two-dimensional (2D) model that consists of electron continuity, quasi-neutrality, and neutral gas continuity equations using neutral gas ionization and charge exchange processes. Simple 1D profile analysis predicts neutral penetration depth into the plasma. Growth rate obtained from the linear theory has been presented. The 2D model equations have been solved numerically. It is found that the neutral gas reduces plasma fluctuations and shifts spectrum of the turbulence towards lower frequency side. The neutral gas fluctuation levels have been presented. The numerical results have been compared with Aditya tokamak experiments
[en] The observation of intermittency in the turbulent scrape-off layer plasma of ADITYA tokamak was first reported about one and a half decade ago. In the last decade or so, several aspects of intermittency-like phenomena have been observed on tokamaks and other fusion devices throughout the world. A review of the research carried out at the Institute for Plasma Research (IPR) is presented, which closely follow the research trend on intermittency-like phenomena in plasmas worldwide. We also present our analysis of particle flux data in order to test the recently proposed fluctuation theorem, which states that the probability of 'entropy consuming' flux events falls off exponentially with the averaging time. This theorem, proposed in the context of small systems, is applied to macroscopic system like tokamak edge plasma by invoking an 'effective temperature' of the bath of drift waves from which, plasma objects take energy and carry out work of transporting matter
[en] Two-dimensional (2D) interchange turbulence in the scrape-off layer of tokamak plasmas and their subsequent contribution to anomalous plasma transport has been studied in recent years using electron continuity, current balance, and electron energy equations. In this paper, numerically it is demonstrated that the inclusion of ion energy equation in the simulation changes the nature of plasma turbulence. Finite ion temperature reduces floating potential by about 15% compared with the cold ion temperature approximation and also reduces the radial electric field. Rotation of plasma blobs at an angular velocity about 1.5×105 rad/s has been observed. It is found that blob rotation keeps plasma blob charge separation at an angular position with respect to the vertical direction that gives a generation of radial electric field. Plasma blobs with high electron temperature gradients can align the charge separation almost in the radial direction. Influence of high ion temperature and its gradient has been presented.
[en] Formation of a density blob and its motion in the edge and scrape-off layer (SOL) of a tokamak plasma have been simulated using two-dimensional, two-field, fluid model equations. The simulation results show that density blobs form in the edge or in the edge-to-SOL transition region where the poloidal velocity shear is maximum. From the numerical data, a condition for density blob formation has been obtained. Dynamics of the detached blob in the edge and SOL regions has been studied. It is observed that not all the blobs that form in the edge or edge-to-SOL transition region are capable of ejection deep into the SOL. A condition for their ejection is also discussed. Radial particle transport associated with the blobs in the SOL has been calculated. It is found that about 60% of the total radial particle flux is carried out by these blobs
[en] A novel two-dimensional (2D) fluid model is proposed for investigating flux driven plasma turbulence in the tokamak edge and scrape-off layer (SOL). Unlike most previous turbulence simulations of this region, our model treats the two regions in a consolidated manner with a smooth transition region in between. Our unified 2D model is simpler and less computer intensive than three-dimensional (3D) models, but captures most features of 3D edge and 2D SOL turbulence. It also illustrates the influence of tokamak edge turbulence on the SOL transport, something not captured by the earlier 2D SOL simulations. Existence of an equilibrium radial electric field in the edge and SOL regions is confirmed, this electric field is found to change sign in the edge-to-SOL transition region. Turbulence in the edge is characterized by radially elongated streamers and zonal flows. The streamer structures occasionally break to form blobs. We obtain a phenomenological condition for the breaking of streamers. Formations of density blob and its dynamics in the edge and SOL regions have been studied. It is found that blobs are created in the edge region where the radial electric field changes sign. All the blobs, which form in this region are not ejected deep into the SOL. Only few of them are ejected. The ejection condition has been discussed. In the SOL region, the effective diffusion co-efficient has been calculated from the simulation results and is found to be consistent with most tokamak experimental values. Statistical properties of the particle transport obtained from this simulation are compared with the earlier flux driven 2D SOL turbulence simulation results and also with Aditya tokamak results. (author)
[en] (1) The stability of ideal ballooning-peeling MHD modes is examined in the presence of external magnetic field perturbations. It is demonstrated that external field perturbation can increase the threshold of MHD modes. It is also shown that this effect should be much more important under ITER conditions than in the present devices like D-III D. (2) We also report a simple self-consistent theoretical model of multi-scale interaction of edge-localized modes (ELMs) such as the ideal ballooning-peeling modes interacting with zonal magnetic fields and zonal flows. The dynamics of self-consistent zonal flows in relaxation of ELMs is unimportant when beta exceeds beta critical and the edge pedestal is unstable to ideal mode. The secondary instabilities of zonal fields are used to estimate saturation level and energy flux induced by ballooning-peeling mode turbulence. (3) The linear instabilities of non-ideal curvature driven modes, including the influence of trapped electrons and electron inertia in the weakly collisional edge of hot reactor like plasmas, are investigated. Fluid theory for the weakly collisional edge tokamak plasma in the presence of trapped electrons has been used. Even when the plasma beta is less than its critical value, a robust non-ideal curvature driven instability persists in the presence of electron inertia and trapped electrons effects. (author)
[en] This paper presents the control system overview, hardware, software and network for Data acquisition and Control system for steady state neutral beam injector (NBIDACS) to be used for heating of plasma in steady state superconducting tokamak (SST-1). The task for NBIDACS is not only to safely deliver 1.7 MW of neutral beams at 55 keV H deg. a period of 1000 s with 16.7% duty cycle but also to acquire the data related to house keeping of the system and its auxiliaries and diagnostics which determine the quality and parameters of the beam. Major issues concerning the design of the system stem from operation duty cycle of 1000 s ON/5000 s OFF. This calls for use of intelligent techniques not only for managing a large amount (100 MB) of data per shot but also to obtain failsafe, reliable control system and to archive the recorded data
[en] An experiment aimed at investigating the cause of fluctuation suppression by gas puffing in the edge and scrape-off layer plasma of the ADITYA tokamak is reported. The fluctuation suppression is accompanied by a flattening of the radial profiles of the floating potential, plasma density and electrostatic Reynolds stress. Both the fluctuation suppression and the simultaneous reduction in particle flux occur throughout the observed region of the plasma edge. The bolometer data also show an increase in the edge radiation from 20% to 40% of the input power during the gas puff. These results indicate a significant role of edge cooling in the fluctuation suppression mechanism and are shown to be well explained in terms of a concomitant reduction in the growth rate of the interchange instability associated with the electron temperature gradient.