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We consider a visco-resistive magnetohydrodynamic modelling of a steady-state incompressible tokamak plasma with a prescribed toroidal current drive, featuring constant resistivity η and viscosity ν. It is shown that the plasma velocity root-mean-square behaves as ηf(H) as long as the inertial term remains negligible, where H stands for the Hartmann number H≡(ην)−1/2, and that f(H) exhibits power-law behaviours in the limits H≪1 and H≫1. In the latter limit, we establish that f(H) scales as H1/4, which is consistent with numerical results.

We report on the observation of spatially asymmetric turbulent structures with a long radial correlation length in the core of high-collisionality H -mode plasmas on DIII-D tokamak. These turbulent structures develop from shorter wavelength turbulence and have a radially elongated structure. The envelope of turbulence spans a broad radial range in the mid-radius region, leading to streamer-like transport events. The underlying turbulence is featured by intermittency, long-term memory effect, and the characteristic spectrum of self-organized criticality. The amplitude and the radial scale increase substantially when the shearing rate of the mean flow is reduced below the turbulent scattering rate. The enhanced long-radial-range-correlated (LRRC) transport events are accompanied by apparent degradation of normalized energy confinement time. The emergence of such LRRC transport events may serve as a candidate explanation for the degrading nature of H-mode core plasma confinement at high collisionality on DIII-D tokamak. [ABSTRACT FROM AUTHOR]

Coupled plasma-neutral transport simulations are performed on ADITYA-Upgrade tokamak scrape-off layer (SOL) plasma, where flows in the core and SOL were measured to reverse signs with density variation. The simulations performed using the EMC3-Eirene plasma-neutral code combination incorporate the toroidally continuous high-field-side belt limiter placed in a moderate circular tokamak equilibrium. The development of mutually counter-propagating toroidal plasma flows in the top and bottom regions of both the SOL and core is recovered for relatively high upstream density cases with high input power (300 kW and 3 m ^2 s ^−1 ). The origin of the flows is traced to the poloidal density variation introduced by high recycling on the inboard localized belt limiter. The results are compared with similar observations, for example, in Doppler-shifted passive charge exchange line emission on the ADITYA-Upgrade (ADITYA-U) tokamak, highlighting the role played by residual stress in the total Reynolds stress. The external stimuli, such as a localized gas puff, are discussed as potential drivers of flow, via residual stress, based on the existing resonant model of the tokamak plasma rotation.

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• The new XICS diagnostic, which has a poloidal angle for the vertical direction and a tangential angle for the toroidal direction, can simultaneously measure both poloidal velocity and toroidal velocity through data processing. • An XICS system is developed with high spatiotemporal resolution and wide spatial coverage to measure parameters of a large volume and strong shaping plasma through limited diagnostic window conditions. • A series of auxiliary systems for optimizing XICS diagnostic systems. The construction of an X-ray imaging crystal spectrometer (XICS) diagnostic system on the HL-3 tokamak plays a crucial role in measuring core plasma parameter profiles, including ion temperature, electron temperature, rotational velocity, and impurity radiation profiles. This diagnostic system has been specifically designed to provide detailed and accurate data essential for understanding the behavior and characteristics of tokamak plasma. The resonance spectral line (w line of Ar XVII at 3.9494 Å) and its satellites of helium-like argon ions were chosen on the basis of the HL-3 parameter range. A spherically bent quartz crystal (1012) with a lattice constant of 2d = 4.562 Å, a curvature radius of Rc = 3.0 m, and a size of 10 cm (high) × 5 cm (wide) was mounted on an adjustable displacement platform in the XICS system. The position was adjusted in three dimensions (vertical, inclined, rotation) to effectively record the spectra of the helium-like argon ions. The XICS has a tangential angle of 49° for the toroidal direction in the magnetic axis. This layout accounts for 65.6 % of the toroidal rotation velocity component. The XICS system was set at an 11° poloidal angle in the mid-plane to cover a plasma range of 10 cm above to 50 cm below the mid-plane, which corresponds to the q = 1 surface of the HL-3 plasma (elongation κ > 1.8). This XICS layout produced a spectrum with poloidal and toroidal rotational contributions, requiring decoupling through data processing. In general, because the contribution of the poloidal rotation velocity is much smaller than that of the toroidal rotation velocity, it can be ignored. The XICS system offers a spatial resolution of ∼1.5 cm and a temporal resolution of 5–10 ms on the basis of a high-performance PILATUS3 × 900 K detector. The spectral resolution is λ / Δ λ ∼ 4 × 10 4 , which satisfies the experimental measurement requirements. Preliminary results of ion and electron temperature profiles were obtained for the HL-3 tokamak. [ABSTRACT FROM AUTHOR]

Encapsulated payload pellet injection (EPPI) has been demonstrated to be an effective method of disruption mitigation system (DMS) through a number of experiments and simulations in DIII-D and JET. This article investigates the effects of plasma rotation on impurity spreading under neon EPPI. Our analysis of simulations shows that the magnetic surface stochastization has a decisive influence on the core-oriented spread of impurity. The MHD instabilities increase with impurity ionization, especially of the $n = 1$ component, leading to the enhancement of the local transport of impurities along the stochastic field lines. In the rotating plasma, the toroidal rotation can suppress the magnetic perturbation, and then, the boundary-oriented spreading of impurities is reduced. When the pellet source is toroidally localized at the magnetic axis, the rotation can quickly flatten the toroidal distribution of impurities, resulting in a smaller maximum radiated power and a lower toroidal peek factor (TPF). [ABSTRACT FROM AUTHOR]

We report on the observation of spatially asymmetric turbulent structures with a long radial correlation length in the core of high-collisionality H -mode plasmas on DIII-D tokamak. These turbulent structures develop from shorter wavelength turbulence and have a radially elongated structure. The envelope of turbulence spans a broad radial range in the mid-radius region, leading to streamer-like transport events. The underlying turbulence is featured by intermittency, long-term memory effect, and the characteristic spectrum of self-organized criticality. The amplitude and the radial scale increase substantially when the shearing rate of the mean flow is reduced below the turbulent scattering rate. The enhanced long-radial-range-correlated (LRRC) transport events are accompanied by apparent degradation of normalized energy confinement time. The emergence of such LRRC transport events may serve as a candidate explanation for the degrading nature of H-mode core plasma confinement at high collisionality on DIII-D tokamak.

Advanced momentum transport analysis is used to study matched hydrogen (H) and deuterium (D) plasmas in the core of ASDEX Upgrade. The aim is to validate gyrokinetic theory and assess a possible isotope dependence. The methodology extracts momentum diffusion, convection, and intrinsic torque as a function of time from experiments employing neutral beam injection (NBI) modulation. H and D plasma scenarios with comparable ion heat fluxes, NBI torque, electron densities, and several dimensionless parameters were designed to highlight any mass dependency. Linear gyrokinetic simulations predict that, for similar background gradients, the Prandtl and pinch numbers should be similar for H and D. This was confirmed by the experimental momentum transport analyses. The assessed intrinsic torques were found to be similar between H and D, co-current directed and located near the outermost region of the plasma core. The strength of the intrinsic torque is correlated with the amplitude of the plasma pressure gradient in the pedestal. Finally, a robust error analysis demonstrates the uniqueness of the parameters obtained together with their uncertainties. Neglecting the intrinsic torque, or its time dependence, systematically distorts the assessed momentum diffusion and convection. This is the first method to separate all three transport mechanisms from experimental data by retaining their time dependencies, that is found to match, quantitatively, the gyrokinetic predictions for Prandtl and pinch numbers, within experimental uncertainties.

The SMOLA open magnetic trap was created at the Budker Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences (BINP SB RAS) for studying the physics of suppression of plasma longitudinal losses from the system in which rotating plasma is confined in a magnetic field with helicoidal symmetry. The possibility of controlling the plasma rotation velocity is crucial in this concept. Methods of spectroscopic and magnetic diagnostics that allowed obtaining parameters of plasma rotation in the experiments are described. The angular plasma rotation velocity was found to be (0.5–1) × 106 s–1 in different regimes of system operation. The dependences of angular plasma rotation velocity on various parameters of the experiment are discussed. [ABSTRACT FROM AUTHOR]

The impurity ion poloidal rotation and ion temperature from the Aditya-U tokamak plasma have been measured using a high-resolution spectroscopic diagnostic. It comprises of a high resolution, 1 m, f/8.7, Czerny-Turner configuration spectrometer along with charge coupled device (CCD) detector. The system monitors the spectral line emission of C2+ impurity ions at 464.74 nm from the top port of the Aditya-U vacuum vessel with the lines of sight covering the plasma minor radius from r = 11.55 cm to 21.55 cm. The impurity ion poloidal rotation velocity and temperature have been estimated using the Doppler shift and Doppler broadening of the spectral lines respectively. The maximum poloidal rotation at a radial location of 21.55 cm in the edge of the plasma during the plasma current flat top was observed to be ~4 km/s for the analyzed discharges and the ion temperatures measured in the edge were in the range of 32−40 eV.

Novel concept of the longitudinal plasma flow suppression by active plasma pumping in helicoidal magnetic field in mirror traps was proposed recently. Concept exploration device SMOLA is now being constructed to prove the possibility of the suppression and determine basic scalings of ist effectiveness. Technical solutions of the SMOLA device are described in the article. Vacuum system, magnetic system, plasma source and the source of the radial electric field are discussed. Estimations of the biasing required for optimal confinement are given. Physical program fort he first experiments is also proposed. [ABSTRACT FROM AUTHOR]

A novel concept of helical mirror confinement with active axial and radial plasma flow control is discussed. The idea relies on the retarding force that appears when biased plasma experiences rotation in crossed electrical and helical mirror magnetic fields. Preparations for its experimental study started in Budker Institute of Nuclear Physics, Novosibirsk. Possible parameter space for a concept-exploration-class device SMOLA is identified. Details of numerical optimization of the magnetic structure of the device are presented. The main physical task for the SMOLA experiment will be direct demonstration of the helical mirror performance in simple experimental conditions at reasonably low plasma parameters. [ABSTRACT FROM AUTHOR]

Neutral beam injection (NBI) based on a negative ion source is one of the basic heating and current drive systems designed for ITER required to reach its goals of the operation with high fusion power, P _fus ∼ 500 MW with fusion gain, Q = 10 for 400 s in a baseline scenario, and P _fus > 250 MW, Q = 5 operation for 3600 s in an advanced scenario. A total power of 33 MW from the two heating neutral beam (HNB) injectors is envisaged in the present scenario. The scope of the present paper is to provide an overview of the main aspects of the interaction of the HNBs with the ITER plasma. Various operational scenarios with different mixtures of the main ion species, He, H, DD and DT, foreseen at different phases of the ITER operation are considered.

High-resolution spectroscopy is a powerful tool for the measurement of plasma rotation as well as ion temperature using the Doppler shift of the emitted spectral lines and their Doppler broadening, respectively. Both passive and active diagnostic variants for the COMPASS tokamak are introduced. The passive diagnostic focused on the C III lines at about 465 nm is utilized for the observation of the poloidal plasma rotation. The current set-up of the measuring system is described, including the intended high-throughput optics upgrade. Different options to increase the fiber collection area are mentioned, including a flower-like fiber bundle, and the use of micro-lenses or tapered fibers. Recent measurements of poloidal plasma rotation of the order of 0–6 km/s are shown. The design of the new active diagnostic using a deuterium heating beam and based on charge exchange recombination spectroscopy (C VI line at 529 nm) is introduced. The tool will provide both space (0.5–5 cm) and time (10 ms) resolved toroidal plasma rotation and ion temperature profiles. The results of the Simulation of Spectra code used to examine the feasibility of charge exchange measurements on COMPASS are shown and connected with a selection of the spectrometer coupled with the CCD camera. [ABSTRACT FROM AUTHOR]

In modern tokamaks, the toroidal plasma rotation has been demonstrated to play a beneficial role in the fusion plasmas. In this paper, the simulation of the toroidal momentum sources, momentum diffusivity χ and plasma rotation with neutral beam injection (NBI) on EAST tokamak have been carried out by using MMM95 and GLF23 anomalous transport models in ONETWO and NUBEAM codes. The physical characteristics of the toroidal momentum transport for the different plasma density and temperature are analyzed. According to the simulation results, the main momentum sources for NBI are from the collision and $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {J} \times \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {B}$$ ( $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {J}$$ is the plasma current and $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {B}$$ is the magnetic field). The former is most in the center and the latter is primary in the middle of the plasma. Also, for EAST, χ is about twice of the thermal diffusivity χ according to the MMM95 model in the like H-mode. Moreover, because of the enhancement of collision and deposition, the density tends to improve the total beam torque. Nevertheless, to keep the neutrality of the plasma, the density reduces the plasma rotation by increasing the ion density and the momentum diffusivity according to the MMM95 and GLF23 anomalous transport models. In addition, in the L-mode plasma, although the increasing temperature displays little influence on the beam torque, it improves the momentum diffusivity because of the ion temperature gradient (ITG) mode in the anomalous transport and reduces the plasma rotation. The results will be valuable for the experimental research of the momentum transport on EAST or ITER. [ABSTRACT FROM AUTHOR]

Recently the possibility of resonant excitation of pressure perturbation by external helical magnetic perturbations near the rotating plasma edge was shown taking into consideration the finite plasma conductivity. In present paper the influence of the small safety factor variation on the pressure perturbation and on the plasma current response is studied. This phenomenon may explain the existence of the small window of safety factor values where ELMs were completely eliminated. The possibility to control the plasma current response to penetration of external helical resonant magnetic perturbations into the edge plasmas is shown. The investigation is carried out in the frame of one-fluid MHD.

For a rapidly rotating plasma, the effects of the resulting Doppler shift have to be included in the neoclassical theory of neutral beam heating, current drive, and plasma transport. In this paper, an improved simulation of neutral beam injection (NBI) and current drive in rotating plasmas is introduced. NBI is simulated using the Monte Carlo code NUBEAM along with the transport code ONETWO. The physical characteristics of heating and current drive for co- and counter-NBI are investigated for non-rotating, co-rotating, and counter-rotating plasmas, all of which can take place in the experiments. In general, it is found that rotation of the plasma can increase the NBI power deposition on the plasma electrons but has little effect on the ions. Moreover, plasma heating by co-NBI is more efficient than that by counter-NBI. For neutral beam current drive, because of the Doppler shift, co-rotation (counter-rotation) of the bulk plasma tends to decrease the co-NBI (counter-NBI) driven current. On the other hand, due to trapping and orbit loss of the fast ions, co-rotation (counter-rotation) has little effect on the counter-NBI (co-NBI) driven current. The results are applied to the forthcoming NBI heating and current drive experiments of the EAST tokamak and should also be useful in the design of experiments in ITER. [ABSTRACT FROM AUTHOR]

With a quasi-steady 3-D MHD model, time-dependent axial magnetic field vacuum arcs (AMFVAs) characteristics in commercial vacuum interrupters for half-wave interruption of power-frequency (50 Hz) current are simulated. In a current time-dependent simulation, the influence of AMF lagged from arc current is considered in the arc model. 3-D spatial distributions of many important plasma parameters and electrical characteristics in AMFVAs can be obtained, such as ion number density, ion temperature, electron temperature, plasma pressure, current densities along different directions (x, y, and z), electric fields strength along different directions, and so on. Simulation results show that there exists a significant spiral-shaped rotational phenomena in the AMFVAs and the rotational velocity at the moments (9, 8, 7, and 6 ms) after 5 ms (the current peak value moment) is always larger than that before 5 ms (1, 2, 3, and 4 ms) because of the lagged AMF. This kind of plasma rotational phenomena also can be observed by high-speed arc photographs. In the future, the asymmetric appearance in high-current vacuum arc will be simulated based on a 3-D model. [ABSTRACT FROM AUTHOR]

Abstract: Effects of compressibility on the double tearing modes (DTMs) in rotating plasmas are numerically investigated by using a compressible magnetohydrodynamics (MHD) model. It is found that due to the compressibility effects, the threshold of the interlocking magnetic island width in the slow and intermediate rotation regimes is larger than the counterpart in the incompressible plasmas. In the fast rotation regime, the compressible effect makes the DTM islands interlock more easily and faster. Moreover, in the very fast rotation regime, the plasma rotation can more effectively suppress the DTM islands. The scalings of the interlocking threshold in the different rotation regimes are obtained. Effects of plasma viscosity and beta on the DTM interlocking in the compressible plasmas are also discussed. [Copyright &y& Elsevier]

Abstract: In current designs for the Conceptual Demonstration Fusion Power Plant (DEMO), a large proportion of the auxiliary power is consumed by the neutral beam systems used for heating and current drive. It is known that if the plasma is rotating in the same direction as the fast ions produced by the beams, then the efficiency of the current drive is reduced. However, if the plasma could be induced to rotate against the fast ions, the efficiency of the current drive would be increased, reducing the auxiliary power required by the neutral beam systems and/or the necessary beam acceleration energy. In this paper we estimate what power savings could be achieved, and examine possible methods for driving the counter-rotation. Keeping the plasma density (〈n e〉=0.96×1020)m−3, D–T) and temperature (T i0 =40keV) constant, if a counter-rotation of M =−0.2 could be achieved, it is estimated that this technique could lead to power savings of up to 15% of the wallplug power requirements of the deuterium neutral beams, reducing the injected power by 30MW. Conversely, with the same counter-rotation and keeping the original injected power, beam energies could be reduced from 1.5MeV to 1.0MeV whilst driving the same current. Numerical simulations using the Monte Carlo TRANSP/NUBEAM code confirm these calculations. From the back electron current term, it is possible that a counter-injected high-Z beam using neon or argon would not only effectively induce rotation counter to the main beams, but also drive additional co-current. DEMO is expected to have a high radiative fraction to protect the divertor and consequently some method of seeding the core with radiating impurities is required in any case. Alternatively, rotation against the torque of the main beams could be induced by charging the plasma and generating a radial electric field. This could be achieved through the use of ICRH to encourage ion loss from the edge of the plasma, or by tangential beam injection at the edge of the plasma to suffer prompt ion orbit losses onto a recovery target. The power requirements of these methods and corresponding reductions in the main beam injector energy will be discussed. [Copyright &y& Elsevier]

Abstract: A new one-dimensional transport code named TASK/TX, which is able to describe dynamic behavior of tokamak plasmas, has been developed. It solves simultaneously a set of flux-surface averaged equations composed of Maxwell’s equations, continuity equations, equations of motion, heat transport equations, fast-particle slowing-down equations and two-group neutral diffusion equations. The set of equations describes plasma rotations in both toroidal and poloidal directions through momentum transfer and evaluates the radial electric field self-consistently. The finite element method with a piecewise linear interpolation function is employed with a fine radial mesh near the plasma surface. The Streamline Upwind Petrov–Galerkin method is also used for robust calculation. We have confirmed that the neoclassical properties are well described by the poloidal neoclassical viscous force. The modification of density profile during neutral beam injection is presented. In the presence of ion orbit loss, the generation of the inward radial electric field and torque due to radial current is self-consistently calculated. [Copyright &y& Elsevier]

Abstract: This paper will discuss alterations and enhancements of the real-time digital plasma control system (PCS) which were recently developed at DIII-D. These enhancements greatly increased the performance and scope of the control system after a successful upgrade from the previous VME-based version. A specific goal of these enhancements was to incorporate the charge exchange recombination spectrometry diagnostic for plasma rotation control. Enlargement of the PCS includes additional motional Stark effect channels for improved current profile control and a series of real-time data displays. Performance improvements include increased computing resources, multi-threaded parallel processing, and 11μs cycle times for the resistive wall mode component feedback. [Copyright &y& Elsevier]

Abstract: A major modification made recently to the DIII-D tokamak was the rotation of one of the four neutral beam systems to allow injection of power in the opposite direction of the usual plasma current (counter-injection). Mixing the usual co-injection beams with the counter-injection beams provides a new capability that allows, for the first time, a partial decoupling of the injected energy and momentum during neutral beam heating experiments. To implement this capability, we have developed within the plasma control system (PCS) [B.G. Penaflor, et al., Fusion Eng. Design 71 (2004) 47] a model-based control algorithm for simultaneous regulation of plasma rotation and beta. The present work describes the development of the model, discusses its validation using actual experimental data, and presents the details of the implementation of the model within the PCS to allow simultaneous control of both plasma rotation and stored energy. [Copyright &y& Elsevier]

Coaxial plasma guns are commonly used to inject magnetic helicity in innovative confinement concepts (ICC’s) for magnetic fusion. One of the key issues in magnetic helicity injection is to maximize the magnetic helicity injection rate. We have identified experimentally an alternative way to increase the magnetic helicity injection rate through rotating plasmas by extending the length of the inner electrode of a coaxial plasma gun so that an additional E × B region interweaves the standard J// B configuration. In the so-called “enhanced helicity injection” mode, the gun voltage is larger compared with the “normal” mode and decays more slowly. Another signature of the enhanced mode is increased edge magnetic field in conjunction with larger edge rotation. The results indicate that tuning plasma rotation is another way to enhance magnetic helicity injection using coaxial plasma guns. An alternative ICC is proposed based on the experimental observations. [ABSTRACT FROM AUTHOR]