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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.

Tearing modes are the resistive magnetohydrodynamic (MHD) modes, which occur in all conventional tokamaks. They are known to degrade plasma confinement by creating magnetic islands which often lead to plasma disruption if their growth is not controlled. They have been extensively studied both theoretically and experimentally, as controlling them is foremost priority for every tokamak, including ITER and future large size tokamaks. In the high diamagnetic-drift frequency regime, the drift mode couple to tearing modes resulting in drift-tearing modes. The growth rate of Drift-Tearing modes decreases as its real frequency increases. In ADITYA as well as in ADITYA Upgrade tokamak, the frequency spectra of Mirnov signal show multiple frequency bands identified as drift tearing modes. Interestingly, the higher frequencies have precisely been found to be the integral multiples of the fundamental frequency. Further analysis reveals that these frequencies do not belong to different modes but harmonics of a single mode. These harmonics are also observed in the density as well as impurity emission. It has been observed that the harmonics generation is strongly correlated with the presence of high energetic electrons, known as runaway electrons, in the plasma. When the magnetic island grows in size, the runaway electrons get trap in these islands. And as the linear growth rate of tearing modes increases in presence of REs, the island size increases further due to the presence of runaway electrons, leading to harmonics generation.

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 effects of nitrogen gas seeding in the edge and scrape-off layer (SOL) regions of a tokamak plasma are studied through 2D fluid simulations using the BOUT++ code. Proper account is taken of the presence of multiple charged states of nitrogen ions due to ionization, recombination, and dissociation processes, and a self-consistent study of the interaction of these ions with the turbulent plasma in the edge and SOL regions is carried out. The self-consistent model includes the effects of polarization drifts of the main plasma and impurity ions for determining the plasma vorticity. Nitrogen seeding is found to modify the turbulence as well as to influence the profiles of the equilibrium plasma density and the electron temperature. The densities of N 3 + to N 5 + ions are found to be relatively higher than the other charged states. This is understood and further validated by a 0D simulation. The radial profiles of these impurity ions are mapped, and their radiation energy losses are estimated. The radial profile of the radiation losses is maximum near to the edge-to-SOL transition region and becomes broader in the edge region than the SOL region.

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.

The formation of a density blob in the boundary region of a tokamak plasma is widely believed to be due to the presence of poloidal velocity shear that differentially breaks off a radially elongated streamer structure. However, blobs have also been experimentally detected in the absence of poloidal velocity shear. We ascribe the formation of such blobs to the differential stretching of the streamer arising from a radial velocity shear and thereby present a unified picture of the breaking mechanism that takes into account both the processes in a synergistic manner. Our universal condition for blob formation is validated from detailed numerical simulations of the edge turbulence in a typical L-mode scenario.

Plasma flows and their profiles in edge and scrape-off layer (SOL) regions of a tokamak plasma are important as these can modify the interchange plasma turbulence. These flows have been investigated in the presence of neutral gas numerically using the two-dimensional model equations. A reduction of poloidal flows, radial particle, and energy fluxes has been observed in the edge and SOL regions by the presence of the neutral gas. The reduction of radial flux is due to the reduction in the radial velocity of the plasma blob. We have investigated Reynolds stress in the presence of the gas. It is found that in the presence of ion-neutral collisions, the magnitude of the stress decreases. The gas also reduces the diamagnetic drift frequency in the edge and SOL regions.

The modification of interchange plasma turbulence in the scrape-off layer (SOL) region by the presence of hot and cold neutral gas molecules has been studied. The nonlinear equations have been solved numerically using two different simulations (“uniform-Te” and “varying-Te”), and the results obtained from both of the models have been compared. The hot neutrals, responsible for the increase in the electron density in the SOL, also account for more ionization of the cold molecules. The effect of hot and cold neutrals on the interchange turbulence is almost similar in the “uniform-Te” model, but in the “varying-Te” model, the influence of the hot neutrals is very small, specifically in the far SOL region. The neutral gas in the “varying Te” model decreases the heat load on the material walls by about 7%. A reduction in the radial velocity by about 25% and effective diffusion coefficient of the plasma particles has been found by the influence of the neutral gas.

The formation of coherent structures by two-dimensional interchange turbulence in the scrape-off layer (SOL) of tokamak plasmas and their subsequent contribution to anomalous plasma transport has been studied in recent years using electron continuity and current balance equations. In this paper, it is demonstrated that the inclusion of electron energy equation in the simulations changes the nature of coherent structures in a significant manner and gives results which are in better agreement with experiments. Specifically, it is observed that radial potential gradients are established which give a poloidal elongation and movement to the structures. Only during the radial transport events do the structures get significantly extended in the radial direction giving radial velocities of order 1 km/s. Sometimes detachment of density structures from the main plasma is observed. These detached structures either decay into the background plasma or are transported out from the SOL. The simulated particle flux and its statistical properties also are discussed.

Major disruptions in Aditya tokamak are initiated by the growth and subsequent locking of m/n = 2/1 and 3/1 tearing modes, which leads to the thermal quench of the plasma. Thick filaments are seen to evolve at the low field side (LFS) of the plasma column following the thermal quench, and during the current quench. The number of filaments and inter filament spacing are observed to be related with the plasma stored energy just prior to the disruption. Rapid enhancement of the outward particle flux is seen during the thermal quench phase and the plasma conductivity reduces considerably. Interchange modes, with low poloidal wavenumber, are inferred to grow due to the reduced plasma conductivity and enhanced effective diffusivity. This may be a plausible explanation for the visualization of the thick filaments at the LFS.

The paper presents the complete engineering design of steady neutral beam injector, with emphasis on the deviation in the design from conventional short pulse injectors.

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° 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.

The nonlinear propagation of a superluminal, linearly polarized electromagnetic wave in the presence of a relativistic cold electron beam is investigated. At large amplitudes the wave couples to the electron-beam plasma mode owing to two important nonlinear effects, namely the relativistic variation of the electron mass and the excitation of longitudinal space charge fields by strong v × B forces. The nonlinear propagation equations for the coupled electromagnetic and longitudinal waves are derived within the context of a relativistic cold-fluid model. Nonlinear travelling-wave solutions are sought to describe the saturated state of the coupled system. Using Hamiltonian techniques, a wide variety of solutions are obtained and their characteristics discussed.

The role of neutral gas molecules in the Scrape-off Layer (SOL) region of tokamak plasma is important as it is expected to modify the plasma turbulence. Two-dimensional model has been used that consists of electron continuity, molecular ion continuity, quasi-neutrality, electron energy, and neutral molecular gas continuity equations in the presence of electron impact molecular ionizations and other non-ionizing collisions. The growth rate obtained from these equations has been presented using linear theory. It is observed that the growth rate increases with the neutral gas ionization coefficients. The nonlinear equations are solved numerically in the presence and absence of the neutral gas molecules. Radial profiles of plasma density, electron temperature, and electric field have been obtained. It is found that the neutral gas reduces electric fields. More significant reduction of the poloidal electric field has been found by the neutral gas. Time series obtained from the numerical data has been analyzed....

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

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.

Conceptual design of a fully solid state power supply system for 1000s ON/ 5000s OFF long pulse operation of a neutral beam injector is presented. A switched power supply with solid state crowbar is used for regulated HV power. A 400 Hz, 24 phase filament power supply is used in an integrated closed loop control with the 24 buck type arc power supplies. Temperature of each filament is controllable independent of each other. An air core transformer with a secondary RC load is used for passive snubber of the grid circuit. A power supply control system supervises the interlock, protection and safety system.

This article shows how drama is used to advocate change in sexual behavior among clients of sex workers in Papua New Guinea. This was done by incorporating a community based initiative, called the Seeds Performing Arts Theater, to the education programs already initiated by the PNG Institute of Medical Research (IMR) on truck drivers, sailors and dockside workers. The main function of the theater group is to present performances dramatizing the factors that lead to unsafe sex. The techniques that were used vary and include drama, dance, music, costumes, masks, and technical arts. During 1998, there were 14 performances and some 1900 condoms were distributed to the transport industry workers. Although there were problems with the education programs, the project is still ongoing and its success will be determined from the evaluation conducted in partnership with IMR.

This paper discusses the collaborative work between Papua New Guinea Institute of Medical Research, Lae, and the Seeds Performing Arts Theatre, which adopted the approach of using drama to target risky behaviors in the country. In a baseline study on knowledge, behavior and practice among target groups, results showed that general knowledge about sexually transmitted disease and HIV/AIDS is very poor in these groups. In addition, there is a continued practice of risky behaviors, which could lead to HIV infection among these groups. In view of such, drama presentations were undertaken to disseminate information to specific audiences. Scripts and songs were written and a flip chart was created to show the statistics on HIV/AIDS. In addition, role playing, which included references to factors facilitating the transmission of HIV was given. In 1998, a total of performances were given to over 3000 people: 12 for transport industry and sex workers, and 2 for general communities. Moreover, two other local theater groups were trained on HIV/AIDS topics.

Publisher Summary This chapter discusses the design of the neutral beam (NB) system for the SST-1 machine. The design of the NB is based on a single 5 MW source beamline. The physical arrangement of the neutral beam injector system (NBIS) with the SST-1 machine is presented in the chapter. The plasma generator is a standard multipole bucket type source that operates in the emission limited discharge regime. Filament temperature of each of the 24 filaments is independently controlled. The main design issues of the power supply system for the NBIS are: long pulse operation of 1000 S, stringent quality requirement, integrated control, effects on the source 132 kV line, reliability, maintability, and effective cooling of the system. For the positive ion plasma source, AC filament heating supply is used. For long pulse operation, uniform heating of the filament length helps in the precise control of filament temperature and increases filament life. An integrated control for arc and filament system maintains the filament temperature for a given gas feed rate.

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.

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.

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.

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.

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, the 2D model treats the two regions in a consolidated manner with a smooth transition region in between. The unified 2D model is simpler and less computer intensive than 3D models, but captures most features of the 3D edge and 2D SOL turbulence. It also illustrates the influence of tokamak edge turbulence on the SOL transport, something not captured by earlier 2D SOL simulations. Existence of an equilibrium radial electric field in the edge and SOL regions has been found. Two different plasma conductivity models have been used for the simulations. Turbulence in the edge is characterized by radially elongated streamers and zonal flows. The streamer structures occasionally break mainly in a region where the radial electric field changes sign. A phenomenological condition for the breaking has been obtained. Effective diffusion co-efficient and density front propagation speed from the simulation have been calculated. Statistical properties of the particle transport obtained from this simulation are compared with earlier flux-driven 2D SOL turbulence simulations and also with Aditya tokamak results.

This paper presents the design calculations for the ion deflection system for the SST-1 NBI system. The main design driver for engineering this system has been long pulse compatibility with the heat load.