Peripheral fluctuations and particle transport during an L-H transition in the FT-2 tokamak

Experimental data on the processes in edge plasma that accompany the transition to an improved confinement regime during lower hybrid heating in the FT-2 tokamak are presented. The poloidal and radial distributions of the plasma parameters and drift particle fluxes were measured with the use of mobile mulitielectrode Langmuir probes and were found to be substantially nonuniform in the poloidal direction. The evolution of the plasma parameters in the course of heating and during an L-H transition is investigated. It is shown that, in FT-2 experiments, the drift of plasma particles in a slowly varying (quasi-steady) electric field and the fluctuation-induced particle fluxes make comparable contributions to the radial particle transport, whereas the contribution of fluctuations to poloidal plasma fluxes is negligibly small. The effective coefficient of radial diffusion is determined. The measurement results show that the L-H transition is accompanied by a substantial decrease in this coefficient.     

Numerical simulation of drift-resistive ballooning turbulence in the presence of the GA mode in the plasma edge tokamak

Drift-resistive ballooning turbulence is simulated numerically based on a quasi-three-dimensional computer code for solving nonlinear two-fluid MHD equations in the scrape-off layer plasma in a tokamak. It is shown that, when the toroidal geometry of the magnetic field is taken into account, additional (geodesic) flux terms associated with the first poloidal harmonic (∼sinθ) arise in the averaged equations for the momentum, density, and energy. Calculations show that the most important of these terms is the geodesic momentum flux (the Stringer-Windsor effect), which lowers the poloidal rotation velocity. It is also shown that accounting for the toroidal field geometry introduces experimentally observed, special low-frequency MHD harmonics—GA modes—in the Fourier spectra. GA modes are generated by the Reynolds turbulent force and also by the gradient of the poloidally nonuniform turbulent heat flux. Turbulent particle and heat fluxes are obtained as functions of the poloidal coordinate and are found to show that, in a tokamak, there is a “ballooning effect” associated with their maximum in the weak magnetic field region. The dependence of the density, temperature, and pressure on the poloidal coordinate is presented, as well as the dependence of turbulent fluxes on the toroidal magnetic field.     

Radial electric field during dynamic processes in a tokamak and L-H transitions

Expressions for the radial electric field in tokamaks are derived with allowance for an additional contribution of the longitudinal electron viscosity (or the associated Ware drift). It is shown that, in transient processes during which the toroidal electric field at the plasma edge increases, the additional electric field can become rather strong. An increase in the shear of the poloidal plasma rotation can trigger the L-H transition. That the experimentally observed transitions to an improved confinement mode can be ascribed to this effect is illustrated by simulating discharges in the current ramp-up experiments in the Tuman-3M tokamak.     

Characteristics of major plasma discharge disruption in the Globus-M spherical tokamak

The characteristics of the major disruption of plasma discharges in the Globus-M spherical tokamak are analyzed. The process of current quench is accompanied by the loss of the vertical stability of the plasma column. The plasma boundary during the disruption is reconstructed using the algorithm of movable filaments. The plasma current decay is preceded by thermal quench, during which the profiles of the temperature and electron density were measured. The data on the time of disruption, the plasma current quench rate, and the toroidal current induced in the tokamak vessel are compared for hydrogen and deuterium plasmas. It is shown that the disruption characteristics depend weakly on the ion mass and the current induced in the vessel increases with the disruption time. The decay rate of the plasma toroidal magnetic flux during the disruption is determined using diamagnetic measurements. Such a decay is a source of the poloidal current induced in the vessel; it may also cause poloidal halo currents.     

Investigation of statistical properties of peripheral fluctuations during an L-H transition in the FT-2 tokamak

The statistical properties of fluctuations of the plasma density and radial drift particle flux in the peripheral region of the FT-2 tokamak are analyzed using data from probe measurements. It is found that the probability distribution functions of the quantities under study vary over the radius and poloidal angle and change significantly after a transition to an improved confinement mode during auxiliary lower hybrid heating. Using experimental data and existing theoretical models, an analytic expression for the probability distribution function of the plasma density fluctuations is derived in a strongly nonlinear approximation. The expression is shown to agree well with experimental observations.     

Turbulent dynamics of helical perturbations in the edge plasma of the T-10 tokamak

Turbulent dynamics of the edge plasma in the T-10 tokamak is simulated numerically by solving nonlinear MHD equations in the framework of the four-field {?, n, p _e , p _i } reduced two-fluid Braginskii hydro-dynamics. It is shown that the transition from ohmic to electron-cyclotron heating is accompanied by a decrease in the amplitudes of turbulent fluctuations in plasma. This is caused by the enhancement of longitudinal dissipation due to the increase in the electron temperature. However, phase relations between potential fluctuations of different modes change in such a way that the Reynolds turbulent force increases, which leads to an increase in the poloidal velocity in the direction of ion diamagnetic drift. Since the poloidal and ion diamagnetic drift velocities enter into the equation of the radial force balance for ions with different signs, the radial electric field decreases. The simulation results agree qualitatively with the results of experiments in the T-10 tokamak. The dependence of the radial electric field on the plasma density, ion pressure, and neutral density is also calculated.     

Effect of density variations along ion drift trajectories on the poloidal and toroidal rotation velocities of a collisionless tokamak plasma

The effect of plasma density variations along ion drift trajectories on the ion velocity distribution function at a given point on a tokamak magnetic surface is studied. The observed distortion of the distribution function can be interpreted as a poloidal (or toroidal) plasma rotation that is additional to the neoclassical rotation. Due to this additional rotation, the velocity of the toroidal plasma rotation is different on the low-and high-field sides of the same magnetic surface. In the case of large ion density gradients, the poloidal rotation velocity on the same magnetic surface can have different signs at different poloidal angles.     

A study of low-frequency microturbulence by CO2-laser collective scattering in the FT-2 tokamak

Results are presented from studies of small-scale plasma density fluctuations in the FT-2 tokamak by the method of far-forward CO₂-laser collective scattering. The frequency and wavenumber spectra of fluctuations are measured using parallel k analysis at various positions of the scattering volume in the plane of the minor cross section of the torus. The data obtained are interpreted using numerical simulations. In phenomenological models, plasma fluctuations are substituted by a superposition of two-dimensional noninteracting cells with Gaussian profiles. A comparison of the calculated and experimental spectra shows that plasma fluctuations should be described based on the concept of strong microturbulence. The poloidal rotation velocity and the characteristic scale length of the scattering fluctuations, as well as the radial position of the region where they are located, are determined. The diffusion coefficient of the cells introduced in the model turns out to be close to the thermal diffusivity determined from the electron energy balance in the ohmic phase of the discharge.     

A possible mechanism for the generation and motion of blobs in tokamaks

A possible mechanism for the generation and motion of so-called blobs—peculiar perturbations that are observed in a tokamak edge plasma—is proposed. It is suggested that blobs are self-contracting plasma filaments generated either by the thermal-radiative instability of a plasma with impurities or by the nonradiative resonant charge-exchange instability resulting from the presence of neutral hydrogen atoms near the tokamak wall. Instability occurs in a narrow temperature range in which pressure is a decreasing function of density. Under these conditions, the most typical perturbations are the local ones that originate spontaneously in the form of separate growing hills and wells in the density. The temperature at the centers of the hills is lower than that in the surrounding plasma, but they are denser and, consequently, brighter than the background. The (denser) hills should move (“sink”) toward the separatrix, while the (less dense) wells should “rise” in the opposite direction, as is observed in experiments. It may even be said that they behave in accordance with a peculiar Archimedes' principle.     

Calculation of poloidal velocity in the tokamak plasma with allowance for density inhomogeneity and diamagnetic drift of ions

A one-dimensional evolution equation for the angle-averaged poloidal momentum of the tokamak plasma is derived in the framework of reduced magnetohydrodynamics with allowance for density inhomogeneity and diamagnetic drift of ions. In addition to fluctuations of the E × B drift velocity, the resulting turbulent Reynolds stress tensor includes fluctuations of the ion density and ion pressure, as well as turbulent radial fluxes of particles and heat. It is demonstrated numerically by using a particular example that the poloidal velocity calculated using the refined one-dimensional evolution equation differs substantially from that provided by the simplified model. When passing to the new model, both the turbulent Reynolds force and the Stringer-Winsor force increase, which leads to an increase in the amplitude of the ion poloidal velocity. This, in turn, leads to a decrease in turbulent fluxes of particles and heat due to the effect of shear decorrelation.     

Study of plasma fluctuations in the Tuman-3m tokamak using microwave reflectometry with an obliquely incident probing beam

Plasma fluctuations in the Tuman-3M tokamak are studied experimentally by analyzing backscattered radiation for different angles of incidence of the probing beam from the normal to the cut-off surface. The poloidal rotation velocity of the plasma fluctuations is determined from the Doppler shift of the reflected radiation spectrum measured on the edge of the tokamak during the transition to the H-mode. It is shown that, before the transition to the H-mode, the rotation velocity can be estimated quantitatively from the spectral shift or from the rate at which the phase of the reflected signal grows. The experimental data obtained during the transition to the H-mode provide evidence for the onset of a sheared poloidal flow. The shear makes it difficult to correctly estimate the poloidal rotation velocity in the improved confinement regime. The main mechanisms responsible for the broadening of the backscattered radiation spectra are considered. The turbulent diffusion coefficients determined under the assumption that the spectral broadening is diffusive in character are found to be close to those determined from the charged-particle balance.     

Effect of the radial electric field on the results of measurements of the poloidal rotation of a tokamak plasma by charge exchange recombination spectroscopy

Abtract The effect of the radial electric field E _r on the results of measurements of the poloidal rotation of a tokamak plasma by charge exchange recombination spectroscopy is considered. It is shown that the emission line shift arising from the finite lifetime of the excited state of the ions is proportional to E _r. For helium ions, the maximum shift corresponds to the poloidal rotation velocity, which is about one-third of the drift velocity in the crossed radial electric (E _r) and toroidal magnetic (B _t) fields. __________ Translated from Fizika Plazmy, Vol. 27, No. 11, 2001, pp. 1050–1052. Original Russian ? 2001 by Romannikov, Chernobai.     

Diagnostics of MHD instability in the Globus-M spherical tokamak

The paper describes a diagnostic system for studying MHD plasma perturbations in the Globus-M spherical tokamak (a major radius of 0.36 m, a minor radius of 0.24 m, and an aspect ratio of 1.5). The system includes a poloidal and a toroidal array consisting of 28 and 16 Mirnov probes, respectively, as well as a 32-channel proportional soft X-ray detector. Methods are described for calculating the poloidal and toroidal numbers of the dominant helical perturbations by using data from probe measurements. Results are presented of processing the experimental data from some tokamak discharges with a plasma current of 150–250 kA, an average electron density of up to 10²⁰ m^?3, and a toroidal magnetic field of 0.4 T. Specific features of MHD perturbations and their influence on the parameters of the plasma column in different stages of a discharge are briefly discussed.     

Microwave measurements of the time evolution of electron density in the T-11M tokamak

Unambiguous diagnostics intended for measuring the time behavior of the electron density and monitoring the line-averaged plasma density in the T-11M tokamak are described. The time behavior of the plasma density in the T-11M tokamak is measured by a multichannel phase-jump-free microwave polarization interferometer based on the Cotton-Mouton effect. After increasing the number of simultaneously operating interferometer channels and enhancing the sensitivity of measurements, it became possible to measure the time evolution of the plasma density profile in the T-11M tokamak. The first results from such measurements in various operating regimes of the T-11M tokamak are presented. The measurement and data processing techniques are described, the measurement errors are analyzed, and the results obtained are discussed. We propose using a pulsed time-of-flight refractometer to monitor the average plasma density in the T-11M tokamak. The refractometer emits nanosecond microwave probing pulses with a carrier frequency that is higher than the plasma frequency and, thus, operates in the transmission mode. A version of the instrument has been developed with a carrier frequency of 140 GHz, which allows one to measure the average density in regimes with a nominal T-11M plasma density of (3–5)×10¹³ cm^?3. Results are presented from the first measurements of the average density in the T-11M tokamak with the help of a pulsed time-of-flight refractometer by probing the plasma in the equatorial plane in a regime with the reflection of the probing radiation from the inner wall of the vacuum chamber.     

Numerical modeling of L-H transitions in the presence of a radial current at the edge of a tokamak plasma

Hydrodynamic equations describing wall plasma turbulence are analyzed numerically using a two-dimensional four-field model. Turbulent transport coefficients are calculated with consideration of the radial current. Numerical analysis revealed a possible scenario for L-H transitions that is associated with the radial current driven by nonambipolar processes. It is shown that the transition of a plasma to an improved confinement mode can also be triggered by other mechanisms.     

Impacts of pellets injected from the low-field side on plasma in ITER

Impacts of pellets injected from the low-field side (LFS) on plasma in ITER are investigated using the 1.5D BALDUR integrated predictive modeling code. In these simulations, the pellet ablation is described using the neutral gas shielding (NGS) model. The pellet ablation model is coupled with the plasma core transport model, which is a combination of the MMM95 anomalous transport model and NCLASS neoclassical transport model. The boundary conditions are assumed to be at the top of the pedestal, in which the pedestal parameters are predicted using a pedestal model based on the theoretical-based pedestal width scaling (either magnetic and flow shear stabilization width scaling, or flow shear stabilization width scaling, or normalized poloidal pressure width scaling) and the infinite-n ballooning mode pressure gradient limit. These pedestal models depend sensitively on the density at the top of the pedestal, which can be strongly influenced by the injection of pellets. The combination of the MMM95 and NCLASS models, together with the pedestal and NGS models, is used to simulate the time evolution of the plasma current, ion and electron temperatures, and density profiles for ITER standard type-I ELMy H-mode discharges during the injection of LFS pellets. It is found that the injection of pellets results in a complicated plasma scenario, especially in the outer region of the plasma and the plasma conditions at the boundary in which the pellet has an impact on increasing the plasma edge density, but reducing the plasma edge temperature. The LFS pellet has a stronger impact on the edge as compared to the center. For fusion performance, the pellet can result in either enhancement or degradation, depending sensitively on the pellet parameters; such as the pellet size, pellet velocity, and pellet frequency. For example, when a series of deuterium pellets with a size of 0.5 cm, velocity of 1 km/s, and frequency of 2 Hz are injected into the ITER plasma from the LFS, the plasma performance, evaluated in terms of Q _fusion, can increase to 72% of that before the use of pellets. It is also found that the injection of pellets results in an increase in the ion and electron densities, but does not enhance the central plasma density. On the other hand, it results in the formation of another peak of the plasma density in the outer region near the plasma edge. The formation of the density peak results in the reduction of plasma transports near the edge by decreasing the contributions of ion-temperature-gradient and trapped electron modes, as well as kinetic ballooning modes.     

Analysis of Dynamics of Plasma Current Quench in the Globus-M Spherical Tokamak

Data on the dynamics of the plasma current quench in the Globus-M tokamak are presented. The main current quench characteristics at different toroidal magnetic fields are compared. The distribution of the toroidal current induced in the vessel wall is determined from magnetic measurements, and the electromagnetic loads acting on the vessel wall during the current quench are calculated. By extrapolating the experimental data, the additional pressure on the vessel wall during the current quench in the upgraded Globus-M2 tokamak is estimated. It is shown that the current quench results in the appearance of bending stresses in the vessel domes. Using numerical simulations, it is shown that the best agreement between the measured and calculated plasma current dynamics during the current quench corresponds to the linear (in time) influx of the carbon impurity.     

Radial electric field and rotation of the ensemble of plasma particles in tokamak

The velocity of macroscopic rotation of an ensemble of charged particles in a tokamak in the presence of an electric field has been calculated in a collisionless approximation. It is shown that the velocity of toroidal rotation does not reduce to a local velocity of electric drift and has opposite directions on the inner and outer sides of the torus. This result is supplemented by an analysis of the trajectories of motion of individual particles in the ensemble, which shows that the passing and trapped particles of the ensemble acquire in the electric field, on the average, different toroidal velocities. For the trapped particles, this velocity is equal to that of electric drift in the poloidal magnetic field, while the velocity of passing particles is significantly different. It is shown that, although the electric-field-induced shift of the boundaries between trapped and passing particles in the phase space depends on the particle mass and charge and is, in the general case, asymmetric, this does not lead to current generation.     

Structure of the controlled halo-current magnetic field in the T-10 tokamak

In the T-10 tokamak, the magnetic field spatially resonant with a helical MHD perturbation is generated using the controlled halo current supplied using a contact method in the scrape-off-layer plasma. This paper is concerned with studying the spatial structure of the halo current and its magnetic field. For this purpose, the magnetic field of the halo current was measured in one of the cross sections of the torus near the tokamak vacuum vessel wall. The spatial distribution of the magnetic field as a function of the halo current configuration was calculated in the cylindrical approximation. The terms proportional to the plasma pressure were disregarded. The configuration of the halo current and the spatial structure of its magnetic field were determined by comparing the calculated and experimental results.     

Description of turbulent convection in a plasma with the help of interacting Lorentz oscillators

A four-field model is proposed that describes turbulent plasma convection inside the separatrix during the L-H transition. It is shown that the Braginskii four-field hydrodynamic equations, which describe fluctuations of the electron and ion temperatures, plasma density, and electrostatic potential in tokamak edge plasmas, can be reduced to three Lorentz-like systems of equations coupled through the equation for the kinetic energy of the fluctuations, i.e., to a four-field edge turbulent layer model describing the nonlinear dynamics of convective cells in the presence of a sheared flow. For three coupled oscillators, the critical pressure gradient corresponding to transitions to both L-and H-modes is found to be much lower than that for an individual oscillator, which describes turbulent convection driven by fluctuations of one type. The edge turbulent layer model makes it possible to describe the formation of a transport barrier inside the separatrix during the L-H transition; calculate heat and particle fluxes via ion and electron channels; and, in combination with the transport code for a core plasma, compute the auxiliary heating power required for a transition to the H-mode.