Vijay Shankar

The effect of ion-neutral collisions on the dynamics of potential edge biasing in a tokamak plasma is studied. Under the influence of such collisions the ions and electrons drift differently thereby creating a non-ambipolar mechanism that impacts the radial electric field created by the biasing. This is similar to the effect of electron and ion mobilities that has been investigated in the past (Shankar et al 2024 Nucl. Fusion 64 076041). In this work, we carry out a consolidated investigation of the edge biasing dynamics in the presence of both ion-neutral collisions and electron/ion mobility contributions. Our detailed numerical simulations of an extended set of model equations reveal that both these contributions permit a different level of turbulence to exist. However, the extent of their individual impacts and influence on the spectral characteristics of the turbulence differ as a function of the plasma parameters. We present a detailed analysis of the physical origin of the stabilizing contribution due to ion-neutral collisions, followed by a comparison with similar effects arising from electron/ion mobility contributions. The practical implications of our findings for edge control in low-temperature tokamaks are discussed.

We present an improved model for the study of edge biasing in a tokamak plasma that incorporates electron and ion mobility contributions. The non-ambipolar nature of the drifts due to the electron/ion mobility terms influences the space charge separation due to edge biasing and affects plasma dynamics in the edge and SOL regions in a significant manner. In contrast to earlier studies, the present model enables simulation studies at higher biasing voltages. The inclusion of mobility enhances/decreases the effect of negative/positive biasing. The radial profiles of plasma density, electron temperature, radial electric field, and its shear for positive as well as negative biasing are investigated as a function of mobility.

The impact of positive and negative edge biasing on the interchange plasma turbulence in the edge and scrape-off layer (SOL) regions of a tokamak is assessed by comparing numerical simulation results with and without any biasing. The concomitant modifications in the radial transport and power spectra are also analyzed. It is found that a positive bias shifts the peak of the ky-spectra toward a lower ky value in the edge region, but no such shift takes place in the SOL region. A negative bias produces a negligible shift. The heat and particle fluxes in the edge region are seen to increase with the radial electric field shear in the region where a flow reversal takes place. Elsewhere, the fluxes decrease with an increase in the shear. The heat and particle loads per unit time on the limiter/divertor material plates decrease with the biasing voltages in comparison to those w/o biasing, but the load intensities increase. It is found that the blob fraction decreases with the radial electric field shear, but at lower radial positions around the biasing region, it increases from −16 to +64 V.

Numerical simulations of the interchange plasma turbulence in the presence of medium-Z impurities (N2, Ne, and Ar) seeding have been performed using BOUT++. These simulation results are used to study the impurity transport mechanism in the edge and scrape-off layer (SOL) regions. An analytical relation for the impurity ion density with the vorticity, sources and sinks, and mass to charge ratio has also been derived. Simulation shows that Ar+ moves more strongly inward compared to N+ and Ne+ that has been verified from the analytical relation. The most abundant species move in both the inward and outward directions, but on average, they mainly move outward. These behaviors have been confirmed using cross-correlation techniques. The inward transport or negative flux of the impurity ions is found directly associated with the monopolar density holes in the presence of the electron temperature gradient, whereas the outward transport is associated with the plasma blobs. The inward impurity transport has been analyzed using hole fraction analysis. The percentages of Ar+, Ne+, and N+ ions of their total impurity densities transported inward mainly through the avalanche events of the turbulent plasma are presented.

A theoretical study is made of the effect of edge biasing on the dynamics of the interchange turbulence in the edge and scrape-off layer regions. A linear analysis of a set of model fluid equations shows that biasing stabilizes the small k _y modes. The model equations are next solved numerically, using the BOUT++ framework, to explore the nonlinear dynamics in the presence of positive or negative bias and compared to results in the absence of bias. Positive biasing is found to lead to a larger increment in plasma density and temperature as compared to negative biasing. It is further observed that cross-correlation between density and poloidal electric field at different radial positions decreases for positive biasing and in the case of negative biasing it is almost similar to that of no biasing. Plasma density and poloidal electric field fluctuations have been investigated which show that the density fluctuations increase (decrease) for positive (negative) biasing but the radially outward flux for these biasing cases always decreases mainly due to the decrease of cross-correlation between density and poloidal electric field fluctuations.

We present numerical simulation studies on impurity seeding using nitrogen, neon, and argon gases. These impurity gases are ionized by the electron impact ionization. These ions can be at multiply ionized states, recombine again with the plasma electrons, and radiate energy. The radiation losses are estimated using a non-coronal equilibrium model. A set of 2D model equations to describe their self-consistent evolution are derived using interchange plasma turbulence in the edge and SOL regions and solved using BOUT++. It is found that impurity ions (with single or double-positive charges) move in the inward direction with a velocity ∼0.02c _s so that these fluxes are negative. These fluxes are analyzed for different strengths of an effective gravity that help to understand the impurity ion dynamics. Increased gravity shows an accumulation of certain charged species in the edge region. The radiation loss is seen to have a fluctuation in time with frequency 5–20 kHz that closely follows the behavior of the interchange plasma turbulence. The simulation results on the radiated power and its frequency spectrum compare favourably with observations on the Aditya-U tokamak. The negative fluxes of the impurity ions, their dynamics in the edge region, and the fluctuating nature of the radiation loss are the most important results of this work.

Nonlinear coherent dense plasma structures—‘blobs’—that form in the turbulent edge plasma of a tokamak play an important role in the anomalous nature of the plasma transport in that region. A plasma blob is normally formed when another kind of coherent structure—a radially elongated streamer structure—breaks due to differential stretching in the radial and poloidal directions. We investigate the nature of such a blob formation in the scrape-off layer region by taking into account electron temperature effects. It is found that the shear related to the poloidal gradient of the poloidal electric field plays a major role. A blob is predicted to form when this shear exceeds the interchange mode growth rate within the radially elongated region. Our theoretical estimate of this extended criterion for blob formation is validated from three-dimensional numerical simulation results using the BOUT++ framework and could be useful in the interpretation of blob formation in the presence of finite electron temperature gradient that is applicable in the H and L mode discharges.