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We propose a direct means of identifying and tracking plasma blobs in the tokamak edge and scrape-off layer (SOL) regions through evaluation of a quantity Q that is a measure of the relative contribution from the square of the sheared strain and the square of the vorticity at any spatial location. The sheared strain and vorticity are related to the various velocity gradients in the plasma fluid. The blob formation from the breakup of a streamer structure can be determined from the criterion Q > 0. A blob is associated with a high negative value of Q that is indicative of a stable vortex flow of the local fluid trajectories. The validity of this criterion, obtained from a linear eigenvalue analysis of the stability of fluid trajectories, is supported by nonlinear fluid simulations that clearly identify blobs with regions where Q Q can be experimentally determined from measurements of local electric field shears using gas puff imaging or Langmuir probe techniques, and the proposed criterion can provide a direct means of tracking plasma blobs. The rotation or spin of a blob in the edge and SOL regions can also be characterized by Q.

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.

Since the 2018 IAEA-FEC conference, in addition to expanding the parameter horizons of the ADITYA-U machine, emphasis has been given to dedicated experiments on inductively driven particle injection (IPI) for disruption studies, runaway electron (RE) dynamics and mitigation, plasma rotation reversal, radiative-improved modes using Ne and Ar injection, modulation of magneto–hydrodynamic modes, edge turbulence using periodic gas puffs and electrode biasing (E-B). Plasma parameters close to the design parameters of circular plasmas with H2 and D2 as fuel have been realized, and the shaped plasma operation has also been initiated. Consistent plasma discharges having I P ∼ 100–210 kA, t ∼ 300–400 ms, n e ∼ 3–6 × 1019 m−3, core T e ∼ 300–500 eV were achieved with a maximum B T of ∼1.5 T. The enhanced plasma parameters are the outcome of repeated cycles of baking (135 °C), followed by extensive wall conditioning, which includes pulsed glow discharge cleaning in H, He and Ar–H mixture, and lithiumization. A higher confinement time has been observed in D2 compared to H2 plasmas. Furthermore, shaped plasmas are attempted for the first time in ADITYA-U. A first of its kind inductively driven particle injection for disruption mitigation studies has been developed and operated. The injection of solid particles into the plasma core leads to a fast current quench. Two pulses of electron cyclotron resonance wave at 42 GHz are launched in a single discharge: one pulse is used for pre-ionization and the second for heating. In a novel approach, a positively biased electrode is used to confine REs after discharge termination. E-B is also used for controlling the rotation of drift-tearing modes by changing the plasma rotation. Cold pulse propagation and signatures of detachment are observed during the injection of short gas puffs. A correlation between the plasma toroidal rotation and the total radiated power has been observed with neon gas injection-induced improved confinement modes.

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.

Anomalous plasma transport in the boundary region of a tokamak plasma is commonly associated with the formation and evolution of coherent density structures known as blobs. Recently, a theory for a universal mechanism of plasma blob formation has been put forward. It is based on a breaking process of a radially elongated streamer due to poloidal and radial velocity shears. The theory is well supported by two-dimensional and three-dimensional numerical simulation results but lacks experimental validation. In this work, we report the first ever experimental validation of this universal criterion by testing it against NSTX data on blobs obtained using the gas-puff imaging diagnostic. It is found that the criterion is widely satisfied in most L-mode discharges and may explain the significantly larger number of blob events. We also validate the theoretical criterion against ADITYA Langmuir probe data taken in the scrape-off layer region.

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.

The first Indian tokamak ADITYA, operated over 2 decades with circular poloidal limiter has been upgraded to a tokamak named ADITYA Upgrade (ADITYA-U) for attaining shaped-plasma operations with open divertor in single and double-null configurations. The experimental research in ADITYA-U has made significant progress, since last FEC 2016. After installation of PFC and standard tokamak diagnostics in ADITYA-U, the Phase-I plasma operations has been initiated from December 2016 with graphite toroidal belt limiter. Ohmically heated circular plasmas supported by filament pre-ionization with plasma parameters, Ip ~ 80 – 95 kA, duration ~ 80 – 180 ms with maximum toroidal field ~ 1T and chord-averaged electron density ~ 2.5 x 10^19 m^-3 has been successfully obtained. The runaway electron (RE) generation, transport and mitigation experiments along with MHD activities and density enhancement with H2 gas puffing experiments have carried out in Phase-I, which is completed in March 2017. The Phase-II operation preparation in ADITYA-U includes calibration of magnetic diagnostics followed by commissioning of major diagnostics and installation of baking system. After repeated cycles of baking the vacuum vessel up to ~ 135°C, the Phase-II operations resumed from February 2018 and are continuing to achieve plasma parameters close to the design parameters of circular limiter plasmas using real time plasma position control. The plasma current has been raised to ~ 135 kA in phase-II with maximum chord averaged electron density of ~ 4 x 10^19 x m^-3. Hydrogen gas breakdown has been observed in more than ~ 2000 discharges including Phase-I and Phase-II operation without a single failure. Several experiments, including the control of REs with fuelling of Supersonic Molecular Beam Injection as well as sonic H2 gas puffing during current flat-top, MHD modes studies using multiple periodic gas puffs and radiative improved modes using Neon gas puff have been carried out. The experimental results from Phase-I and Phase-II operations of ADITYA-U tokamak are discussed in this paper.

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.